Central heating pump sizing

[Reddwarf4ever]

The calc for KW is, KW= m3/hr*16.66*60*deltaT/860. = 0.85*16.66*60*12/860, 11.86KW.

This the ref I have to D.18. (1 is the default)

View attachment 47439

Does that mean the pump will run 24/7 ?

 

[John.g]

Only applies to pump in overrun and for the overrun period I would think but don't know how it (if on eco mode 3) can differentiate between overrun after a low heat demand vs a high heat demand, there seems to be no simple settings on these boilers, surely it's really not that difficult to heat a few litres of water.

 

[Reddwarf4ever]

Changed D17 to 1, set the temperature by the dial to 55oC ( should be the inlet temperature) took some readings while it was firing, but abandoned for now, as not sure quite what was happening....D41=37oC D40=47oC D05=36oC wasn’t firing very hard 🤔

I thought D05 was the target temperature, so if I am controlling the inlet temperature, I thought it would read 55oC ???

will try again tomorrow....

some of the descriptions leave a lot to be desired, especially as you say the Eco mode 3

 

[John.g]

One would think that it should control as you say, above.

Another often discussed item is the power output and duration during ignition sequence, some say its 80% for 60 secs, others say 80% for 20 secs. You can get a good feel for this by noting the (pump) flowrate, the deltaT and the fan speed, d.34, during normal operation, then sometime when you next start up the boiler just watch and note the fan speed d.34 at start up and the time it remains at this speed before changing, can then calculate the output as fan speed is almost directly proportional to power output.

 

[Reddwarf4ever]

One would think that it should control as you say, above.

Another often discussed item is the power output and duration during ignition sequence, some say its 80% for 60 secs, others say 80% for 20 secs. You can get a good feel for this by noting the (pump) flowrate, the deltaT and the fan speed, d.34, during normal operation, then sometime when you next start up the boiler just watch and note the fan speed d.34 at start up and the time it remains at this speed before changing, can then calculate the output as fan speed is almost directly proportional to power output.

Hi
did a single observation ( Wife was pressuring me to do lunch 😂 )
Fan speed initially 400, quickly dropped and stabilised at 277, stayed there for 1 minute exactly, then dropped to 150 for around 20 seconds then climbed back to 277 for 30 seconds then cut out.....not sure what that means, if anything 🤔

also have been running steadily @62oC (10oC DT) for past few days, very stable, although boiler does modulate up & down a lot....tried increasing to 64oC to help warm up the house quicker, but it then it increased firing, expected but continued to 67oC and cut out 🤯, seems unacceptable that a boiler can’t fire up just enough to give a 2oC raise in temperatur.e, of course a 5oC tolerance would, likely have made this possible.

UPDATE
on normal running@ DT 10oC 0.7m3/Hr target 64oC the fan speed modulates between 143 & 192, doesn’t sound a lot, but can be heard. With proper tuning, this differential should slowly decrease & the modulation should be minimal.

I wondered if the PP setting on the pump could be responsible for varying the pump flow, chasing the continual boiler modulation 🤔 so switched the pump to CP2, 0.6m3/Hr and lower Watts 👍 and immediately the modulation stopped 👍 the fan is now constant at 150, occasionally it goes up to 154, then falls back to 150......so much better than the constant noise of the boiler firing up and down, although being in the kitchen it cant really be heard in the lounge, although the modulation could be heard in the spare bedroom, which is immediately above the boiler, but not any longer 👍

wonder if this pump behaviour is normal, or just ‘ one of those things’ 🤔

Any thoughts on the room stat question ???

thanks

 

[John.g]

Will be able to put a few numbers together once you (if) can get a steady reading, even briefly, the fan/boiler output should be more or less rock steady normally, changing from PP setting may help but normally this also stays steady unless TRVs etc are operating.

 

[John.g]

Hi
did a single observation ( Wife was pressuring me to do lunch 😂 )
Fan speed initially 400, quickly dropped and stabilised at 277, stayed there for 1 minute exactly, then dropped to 150 for around 20 seconds then climbed back to 277 for 30 seconds then cut out.***.not sure what that means, if anything 🤔

also have been running steadily @62oC (10oC DT) for past few days, very stable, although boiler does modulate up & down a lot....tried increasing to 64oC to help warm up the house quicker, but it then it increased firing, expected but continued to 67oC and cut out 🤯, seems unacceptable that a boiler can’t fire up just enough to give a 2oC raise in temperatur.e, of course a 5oC tolerance would, likely have made this possible.

UPDATE
on normal running@ DT 10oC 0.7m3/Hr target 64oC the fan speed modulates between 143 & 192, doesn’t sound a lot, but can be heard. With proper tuning, this differential should slowly decrease & the modulation should be minimal.

I wondered if the PP setting on the pump could be responsible for varying the pump flow, chasing the continual boiler modulation 🤔 so switched the pump to CP2, 0.6m3/Hr and lower Watts 👍 and immediately the modulation stopped 👍 the fan is now constant at 150, occasionally it goes up to 154, then falls back to 150.***..so much better than the constant noise of the boiler firing up and down, although being in the kitchen it cant really be heard in the lounge, although the modulation could be heard in the spare bedroom, which is immediately above the boiler, but not any longer 👍

wonder if this pump behaviour is normal, or just ‘ one of those things’ 🤔

Any thoughts on the room stat question ???

thanks

No real thoughts on room stat, its switching in at SP-1C (should be SP-0.5C) and off at SP-0.3 but should be SP so 0.3C error at this end, maybe faulty controller but not a huge problem?.

PP mode would certainly seem to be a bit erratic, I wouldn't have thought it should behave like this.
Now that its settled down at 0.6 ms/hr and 1500 rpm, what are the flow/return temps??, I can then give some numbers.

 

[John.g]

At the moment the best I can come up with is that 5200 rpm = 18kw.
So at startup the boiler did a pre purge at 4000 rpm, probably followed by ignition at 77% output = 13.85kw but fell very rapidly to 2770 RPM = 53.35% output or 9.6kw for 60 secs and then reduced further to 1500 rpm or 5.2kw. The lowest (erratic) fan speed was 1430 rpm = 4.95kw.

 

[Reddwarf4ever]

No real thoughts on room stat, its switching in at SP-1C (should be SP-0.5C) and off at SP-0.3 but should be SP so 0.3C error at this end, maybe faulty controller but not a huge problem?.

PP mode would certainly seem to be a bit erratic, I wouldn't have thought it should behave like this.
Now that its settled down at 0.6 ms/hr and 1500 rpm, what are the flow/return temps??, I can then give some numbers.

re room stat, is the 0.5oC offset setting, ‘ shared’ either side of the target temperature?

i.e. 22.5oC +-0.5oC.....or is it meant to be only below the target temperature ? Presently, it doesn’t come back on again until it’s fallen to 21.5oC which is a Whole degree below the target temperature....which is too far really 🥶

will ask DAB about the PP mode causing the boiler to continually modulate And CP mode keeping it steady. The boiler ran virtually all day without cutting out, then in the evening it cut out, no obvious reason and continued starting and stopping until 1030pm when it shut down for the night....wish I could find out what causes this.🤔 have boiler set at 14KW to allow it to heat up quicker first thing in the morning, but feel of I reduced this to say 11KW maybe it would cope in the evening, but wouldn’t be suitable, if the cut out allowed +-5oC tolerance maybe this would prevent it cycling in this way.....can’t monitor the boiler to see why it cuts out......
DT 10oC return 54oC target 64oC.flow 0.6m3/Hr.....thanks

 

[John.g]

That changes things so, it might mean that the boiler is outputting 12.9 kw for 60 secs which is closer to what some users are finding, if correct it means that the deltaT for that 60 secs or so is 18.5C at a flow rate of 0.6m3/hr and means that the flow temp at start up or re firing is < 45.5C to avoid exceeding a target temp of 64C. I feel that a flow rate of 0.8/0.9 m3/hr is the optimum, unfortunately PP3 which gives that causes modulating, if the pump watts are not changing then it points to a very/over sensitive boiler controller bearing in mind that a change of 1 watt in PP3 mode only results in a flow change of 0.08 m3/hr.

Might also be worth talking to DAB re a replacement pump as it was stopping on full speed originally which it certainly shouldn't have been so faulty?.

Your old pump I think gives almost your exact requirement (will look at pump curves again), so maybe take the top off it and see if its sludged up.

 

[Reddwarf4ever]

That changes things so, it might mean that the boiler is outputting 12.9 kw for 60 secs which is closer to what some users are finding, if correct it means that the deltaT for that 60 secs or so is 18.5C at a flow rate of 0.6m3/hr and means that the flow temp at start up or re firing is < 45.5C to avoid exceeding a target temp of 64C. I feel that a flow rate of 0.8/0.9 m3/hr is the optimum, unfortunately PP3 which gives that causes modulating, if the pump watts are not changing then it points to a very/over sensitive boiler controller bearing in mind that a change of 1 watt in PP3 mode only results in a flow change of 0.08 m3/hr.

Might also be worth talking to DAB re a replacement pump as it was stopping on full speed originally which it certainly shouldn't have been so faulty?.

Your old pump I think gives almost your exact requirement (will look at pump curves again), so maybe take the top off it and see if its sludged up.

Hi
don't remember it stopping in high speed, assume CP3 ?...the inlet temperature was excessive, but that’s all I remember ....will see what DAB say, thanks

 

[John.g]

The pump was running at its maximum setting of constant curve 3 which shouldn't have caused the below.
See post 126 on page 11, also post 123 on page 11.
I would watch the watts on PP3 again and if no deviation then difficult not to conclude that you have a hyper sensitive (or faulty) boiler PID controller.
But if the DAB is displaying LOW FLOW again on full speed 3, I wouldn't be too happy and would contact DAB.

"UPDATE
1- I have set the boiler to 18KW, first time ever....its modulating ok, but have had to reduce the target temperature to 68oC to keep the inlet below 55oC.***.so quite a low DT
2-occasionally the pump declares ‘LO FLOW’ then shortly after shuts off, restarts with the same message, then after a few minutes reverts to the previous readings I.e. 0.7m3/Hr etc....set it to heating only for now to establish that works ok."

 

[Reddwarf4ever]

The pump was running at its maximum setting of constant curve 3 which shouldn't have caused the below.
See post 126 on page 11, also post 123 on page 11.
I would watch the watts on PP3 again and if no deviation then difficult not to conclude that you have a hyper sensitive (or faulty) boiler PID controller.
But if the DAB is displaying LOW FLOW again on full speed 3, I wouldn't be too happy and would contact DAB.

"UPDATE
1- I have set the boiler to 18KW, first time ever....its modulating ok, but have had to reduce the target temperature to 68oC to keep the inlet below 55oC.***.so quite a low DT
2-occasionally the pump declares ‘LO FLOW’ then shortly after shuts off, restarts with the same message, then after a few minutes reverts to the previous readings I.e. 0.7m3/Hr etc....set it to heating only for now to establish that works ok."

Thanks, completely forgot about that, your memory is better than mine 😂 have emailed DAB & will check other settings again......

 

[John.g]

Maybe no harm to check that full speed setting again as there may have been some air in the pump after installation, if "no flow "not annunciating now then pump OK IMO.

 

[Reddwarf4ever]

Maybe no harm to check that full speed setting again as there may have been some air in the pump after installation, if "no flow "not annunciating now then pump OK IMO.

Exactly my thoughts 😎CP3 30W 4.5m 0.9m3/Hr....will leave as is amd see how the boiler performs

 

[John.g]

CP3 wasn't the problem No Flow, it was fixed speed 3 @ 36W AFAIK, even if you never use this mode, the pump should run at this setting with no problems.

 

[Reddwarf4ever]

CP3 wasn't the problem No Flow, it was fixed speed 3 @ 36W AFAIK, even if you never use this mode, the pump should run at this setting with no problems.

I thought CP3 is fixed speed 3 🤔 there is CP PP and the other one which AFAIK is for underfloor heating, does that make sense ?

had a reply from DAB, they said the pump can’t be faulty as no error codes are shown....this is crazy, I explained that although internal diagnostics will capture virtually all faults, this should not be relied on 100% as there could be a fault , not actually recognised as a designated fault.....seems the way of the world, relying 100% on technology, without any understanding of science principals and common sense.

Update, 2nd reply from DAB

“Hi,
There appears to be something within your system which is causing the pump to ramp up and down due to high resistance , which the pump is designed to do in PP setting, we suggest then that in regards to your system to be set and operated in CP setting to overcome this issue”

 

[John.g]

CP3 is fixed pressure, the pump changes speed to maintain a constant pressure, the one you had it on was constant curve or fixed speed (3) where the pressure will decrease with increasing flow and increase with reducing flow, this is the way all the old circ pumps ran, so c/o to CC 3 and ensure its not flagging up NO FLOW.

 

[Reddwarf4ever]

Trying CP3 28W 4.5m 0.8m3/Hr 62oC target DT 19oC Boiler started modulating fan speed 145<>177 consistent. Then later when I checked, it was 150 and steady, no modulation ??? Then within seconds of me looking at it, it cut out ??? Can’t see why though 🤔 took 3 restarts and stayed running, started modulating again 145<>180

it seems, sometimes at least, that the boiler decreases it’s firing with each successive restart, which allows it to eventually stay running, is that how it works, or am I imagining it ???

 

[Reddwarf4ever]

After only 1/2 Hr it stopped ‘ continually modulating‘ and is steady at a fan speed of 150

 

[John.g]

The whole thing is baffling, even though its highly unlikely that the pump would fluctuate on both CP and PP, it might be worth trying it on a "true" fixed speed or constant curve setting ie CC3 as shown above, if the boiler is still fluctuating then I think you can rule out the pump as the cause.

 

[John.g]

Trying CP3 28W 4.5m 0.8m3/Hr 62oC target DT 19oC Boiler started modulating fan speed 145<>177 consistent. Then later when I checked, it was 150 and steady, no modulation ??? Then within seconds of me looking at it, it cut out ??? Can’t see why though 🤔 took 3 restarts and stayed running, started modulating again 145<>180

it seems, sometimes at least, that the boiler decreases it’s firing with each successive restart, which allows it to eventually stay running, is that how it works, or am I imagining it ???

Nothing really adds up from the above numbers, wonder if the thermistors are faulty or improperly attached to the flow/return pipes.
A deltaT of 19C&0.8m3/hr = boiler output of 17.67kw, clearly not correct. Fan speed of 145/177 = ~ 5.57kw output = deltaT of 6C, more believable but seems too low except restricted by d.00?.

 

[Reddwarf4ever]

Running very well today CC3 35W 5.0m 1.0m3/Hr boiler 63oC DT 19oC fan 150 running steady....been off a couple of times as room stat satisfied, so thats good, but been milder today.....only cut out once today for no apparent reason, but normalised fairly quickly, but wasn’t able to observe. Obviously CC3 uses a little more power than PP3, but maybe there are benefits.....(it did show lo flow just once after reinstalling the pump, but think it was due to being on hot water only and a lot of air in the system) tried to fit the insulation shroud, but typist for a 180 version and doesn’t fit mine, so it’s back to hessian wrap....dont Need the shroud now 😩

Air noise has disappeared now completely, hiding in a radiator somewhere 🤯 wonder if I should connect a valve to the vent, ( currently return and feed linked together ) to occasionally vent to the F&E tank ???

 

[John.g]

Something not adding up there, you have a deltaT of 19C ie flow of 63c & return of 44C? which at a flow rate of 1M3/hr equals to a boiler output of 22.1kw, no way.
The fan speed would indicate a boiler output of ~ 5.2kw so something has changed dramatically from your original readings. If you keep slowly reducing d.00 until the flow or/and return temps start falling then you will get a good feel for the real boiler/rad output, can then start trouble shooting.

Can you give a sketch of your combined vent & cold feed, mine has the vent carrying on up as per normal with the cold feed teed into it.

 

[Reddwarf4ever]

Sorry I meant DT 9oC......hope that makes more sense....🤞will have. look at the F&E tank, a plumber did this for me and I insulated it all, so can’t see where / how it joins.....will jot down how it’s connected.

 

[John.g]

A deltaT of 9c = 10.47kw, more like it but the fan speed is a mystery as IMO it should be ~ 3024, as I said above, progressively reducing the output will tell alot, ie if deltaT still remains the same at a d.00 of 6kw as at 12kw then the boiler isn't outputting 10.47kw.

 

[Reddwarf4ever]

A deltaT of 9c = 10.47kw, more like it but the fan speed is a mystery as IMO it should be ~ 3024, as I said above, progressively reducing the output will tell alot, ie if deltaT still remains the same at a d.00 of 6kw as at 12kw then the boiler isn't outputting 10.47kw.

Could it be the f,ow isn’t 1.0m3/Hr ? As this is a calculated flow, not sure how accurate that is....

 

[John.g]

Will answer that shortly, what is the output clamped at just now, d.00?.

 

[John.g]

The calculated flow with a clean pump is quite reasonably accurate IMO, its another story if the pump is fouled as the efficiency falls off and as the pump uses power to calculate the flow rate then it can show a very inaccurate number. There was a case here where a UPS3 was suggesting no flow in all its different modes so suggested completely blocked boiler Hx etc but when BG opened the pump and cleaned it, everything returned to normal, the system was pretty dirty.
Its unlikely IMO that you have a problem like that because if the flow rate is for example 50% of the apparent then a 5.2kw boiler output would result in a 20/25C deltaT. However I would use all the tools at my disposal to establish the boiler output and one of these is by reducing d.00 until balance is achieved and you will then know the output. Once you have done this I would ask Vaillant why the fan speed is apparently so low, next time you look at it have a look at the target speed d.33 as well as its value, d.34.

 

[Reddwarf4ever]

D33 & D34 always identical 👍 did a test this morning, target 62oC DT 8oC Fan 167 ( 150-167 ) set D0 to 6KW ( 5KW resulted in output dropping to 60oC, but held steady while under observation )
so how does that sound with pump on CC3 ?

 

[John.g]

Will do the calcs in a minute, is the flowrate still indicating 1.0M3/hr?, this is important.

 

[Reddwarf4ever]

Will do the calcs in a minute, is the flowrate still indicating 1.0M3/hr?, this is important.

DT 6oC fan steady 150 assume boiler limited to 5KW Target 62oC, actual 62-63oC
Pump 35W 5.0m 1.0m3/Hr ( maybe CC2 would help reduce the return temperature ? )

be good to make up some calculations to put in an excel sheet....with various unknowns......have to look through the posts and set this up 😎

UPDATE
boiler outlet fell to 54oC with D0 5KW, increased to 7KW fan now 170 outlet climbing slowly now at 59oC.....think it will settle around 6KW with the fan around 160-170...DT currently changing during heat up

 

[John.g]

Ok then, if I assume that the heating demand is 5.5kw from the fan speed, a deltaT of 8C means the flowrate has to be 0.59m3/hr, Also, it means that the total rads output at max (50 deg) output is 7.9kw, which implies that you only have maybe 4/5 rads in service just now?.
If I assume the heating demand based on a flowrate of 1 m3/hr and the same deltaT of 8c then the output is 9.3kw and implies that the max output of the rads in service (at 50C) is 13.3kw.

A different approach is to monitor your gas meter.
Your gas flowmeter probably measures the flowrate in m3/hr, the older type (here anyway) measure in 1/10ths of a m3 which means a very quick/accurate spot check can be made in a short time, if the meter only turns over every m3 then you would have to monitor it over a hour, a m3 of gas around here is ~ 10.8 kwh so easy to calculate the real boiler output from this assuming a boiler efficiency of ~ 88% (or whatever).
Have a look at your meter and see its scale.

"UPDATE
boiler outlet fell to 54oC with D0 5KW, increased to 7KW fan now 170 outlet climbing slowly now at 59oC.***.think it will settle around 6KW with the fan around 160-170...DT currently changing during heat up"

I think the fan speed is a pretty good indication of your boiler output, from the above, fan speed 1650 rpm so output = 18*1650/5200, 5.7kw.

So, how many rads are currently turned on or in service?.

 

[Reddwarf4ever]

All 12rads, 3 without TRVs fully open, all hot, house warming up well.

D0 now at 7KW DT 8oC outlet stable 62oC fan constant 1500rpm

“18*1650/5200, 5.7kw.” Are the 18 and 5200 constants ?

 

[John.g]

Yes, I'm assuming that when the boiler is running at 18kw that the fan speed is 5200 RPM.
The gas meter will/would provide a good check especially if its turning in 1/10ths of a m3.

 

[Reddwarf4ever]

Been looking back at previous posts. Not sure if I have a definitive solution 🤔 currently on CC3 today was particularly mild....watched the boiler as it went down to the lowest firing ‘ Fan 150’ outlet temperature was 63oC ( Target 62oC ) ...but slowly climbed and the boiler could not reduce firing any further so cut out when it reached 65oC. Couldn’t see what i could do about, other than raise the target temperature. Before I bought the new pump, I was mistakingly running the outlet at 72-78oC amd the only problem I had, I believe was the pump, but at that temperature I was certainly not in condensing mode. If I currently run the outlet higher than 64oC the inlet will be above 55oC.....I assume a higher flow will allow the rads to radiate more heat, as they are constantly kept closer to the boiler outlet temperature, but the return temperature will rise also. Maybe I should have bought an ultra low firing boiler, which goes down to 3.5KW, although these seem to be much more expensive and can’t justify the cost to change it now, even though it’s 11 years old.

during colder days or in the evening, it manages ok, but as we get closer to the warmer weather, the boiler will struggle more. I wonder if a lower flow would suit my boiler better, won’t release so much heat from the rads, but the return temperature should be a lower and help it fire and very low KW......

not sure now which way to go 🤯

 

[John.g]

I would suggest not reducing the flow rate below ~ 0.6/0.7 m3/hr.

Is the room stat switching in/out during these low boiler demands?.

Also, a boiler should be reasonably able to cycle without any real problems, I would consider a 5.2 kw minimum output as quite acceptable.

You might try return temp control again some time even though it didn't control to the expected (return) target temp, it would be interesting to see then what parameter shuts the boiler down on minimum output, ie the target return temp+3C or the actual return temp+3C or whatever.
From post #203
Changed D17 to 1, set the temperature by the dial to 55oC ( should be the inlet temperature) took some readings while it was firing, but abandoned for now, as not sure quite what was happening....D41=37oC D40=47oC D05=36oC wasn’t firing very hard 🤔

Edit: "d.05 Determined heating flow set target temperature" maybe this is why your setting doesn't influence it?, it implies that the heating curve does but if so, without outside temperature sensing one would think it should follow the target temperature, vaillant will know, maybe.

 

[Reddwarf4ever]

Will have a try again......so difficult emailing vailant, they take forever to reply & tend not to answer all the questions, just the first one....but will try and ask....thanks

 

[John.g]

Before c/o, see if d.05 is the same as your set flow target temperature, also after c/o, see if altering your (return) target temperature also alters d.05.

 

[Reddwarf4ever]

Will have a look at what you suggest.
But the simple science is that I need a boiler that will go below 5KW, or find a way to dissipate more heat into the house, can change two downstairs rads to double fins, possible 30% extra heat from each, maybe enough. Currently target 64oC inlet 55oC, fan 150-155 modulating. Have tried CC2 to see if it helps, not sure if it has or not, trying to determine best pump setting to give a condensing return temperature and allowing the boiler to have some control over the outlet temperature. When the weather was colder, at the start of this post, it was a lot easier, however as the weather warms, it will get more difficult.

Trying to work out if a higher or lower pump flow is better in my circumstances. Also is it safe to assume that if I see steam from the boiler vent it’s in condensing mode ? Or would I see a little steam anyway ?

Thanks

 

[John.g]

Even though I am reasonably happy that a fan speed of 1500 RPM is min output of ~ 5.2kw, this is not borne out by using the flowrate of 1M3/hr and a deltaT of 9C which gives a output of 10.46kw, if the deltaT is correct then the actual flowrate is only 0.5 m3/hr for a output of 5.2kw, if the flowrate is correct then the deltaT is only 4.5C. Would like to monitor the gas meter.

Condensing: Achieving the utopia of 100% condensing requires a flue gas temperature of down around 30C which requires a return water temp still lower of ~ 20/25C, so you will always see a plume from the boiler unless these conditions are met, IMO to achieve ~ 3/4% condensing benefit requires a return temp of ~ 45C, the big advantage of condensing vs non condensing is that even when not condensing there is a big benefit, a non condensing boiler might have a flue gas temp of 230C (like my own) but a condensing boiler will easily achieve ~ 80C (non condensing) with a gain of ~ 7%,
I would definitely try and run on the return target temp of say 40/45C, you can then adjust flow rates etc to give optimum comfort and performance.

 

[Reddwarf4ever]

Even though I am reasonably happy that a fan speed of 1500 RPM is min output of ~ 5.2kw, this is not borne out by using the flowrate of 1M3/hr and a deltaT of 9C which gives a output of 10.46kw, if the deltaT is correct then the actual flowrate is only 0.5 m3/hr for a output of 5.2kw, if the flowrate is correct then the deltaT is only 4.5C. Would like to monitor the gas meter.

Condensing: Achieving the utopia of 100% condensing requires a flue gas temperature of down around 30C which requires a return water temp still lower of ~ 20/25C, so you will always see a plume from the boiler unless these conditions are met, IMO to achieve ~ 3/4% condensing benefit requires a return temp of ~ 45C, the big advantage of condensing vs non condensing is that even when not condensing there is a big benefit, a non condensing boiler might have a flue gas temp of 230C (like my own) but a condensing boiler will easily achieve ~ 80C (non condensing) with a gain of ~ 7%,
I would definitely try and run on the return target temp of say 40/45C, you can then adjust flow rates etc to give optimum comfort and performance.

Will take 24 hr gas readings over next 3 days, last week I used exactly 3 units in 24 hours, but will compare in a few days. Talking about plumes, so is it possible to see a plume if no condensing is taking place ?, hoped that was a sign that it was 🤔

tried various pump settings today, settled on CP2, for now at least. Trying boiler on 66oC target, will give it ‘more to work on’ return temperature 53-55oC....but as it’s milder the room stat is switching it off for hours at a time....so have to catch it 🤨 decided to be more sensible with room temperature, now set at 21.5oC....only increases to 23oC 0800-1100 & 2200-2230 as I heat the hot water during these periods also. Read another post that said Rads should be developed with a delta of 40oC & not the traditional 50 or 60oC.....will defo change my two lounge rads this summer for double fin....have others upstairs, but as most in bedrooms the TRV’s not set very high anyway, could do two more upstairs,, will see how changing the two on the ground floor helps Gas today 7pm 4759.17 does that look correct ? Gas is only used With the boiler thankfully

 

[John.g]

You will always see a plume from a high efficiency boiler even if its never in condensing mode as generally the flue gas temperature is never more than ~ 65/100C, the relatively cool gases then condense very soon after hitting the much colder air, a standard efficiency boiler with a flue gas temperature of ~ 230C will never show a plume because by the time the relatively hot gases cool to dew point all the water vapour (steam) has been diluted/absorbed and will never condense.

That meter looks to me to be reading 475,917 FT3 and when the counter on the right reaches "1" will read 475,920 FT3, I also base this on a meter life of 35 years which is a average gas consumption of 13598 FT3/annum or 13598x0.3, 4079 kwh/year, a very modest consumption if house in occupation during this 35 year period.
What were your exact readings to give you these 3 units?

 

[John.g]

Think you are correct re the reading, the silent red 0 at the end confused me.

I also think there is a little rotating mechanical device (outlined with red oval) that you can get a very accurate boiler consumption number from, time it exactly for say 5 revolutions, if it takes say 70secs then the boiler consumption is 5.5 kw.
Or maybe time the red counter for exactly one revolution (1 FT3?)

 

[John.g]

deleted, double post.

 

[Reddwarf4ever]

Think you are correct re the reading, the silent red 0 at the end confused me.

I also think there is a little rotating mechanical device (outlined with red oval) that you can get a very accurate boiler consumption number from, time it exactly for say 5 revolutions, if it takes say 70secs then the boiler consumption is 5.5 kw.
Or maybe time the red counter for exactly one revolution (1 FT3?)

Think you are correct re the reading, the silent red 0 at the end confused me.

I also think there is a little rotating mechanical device (outlined with red oval) that you can get a very accurate boiler consumption number from, time it exactly for say 5 revolutions, if it takes say 70secs then the boiler consumption is 5.5 kw.
Or maybe time the red counter for exactly one revolution (1 FT3?)

Thanks, at least that dispels what I was told about the plume being a sign that the boiler is in condensing mode.....would measure the condensate, but it’s all piped in quite secure and insulated, so won’t disturb that.

Re the gas meter, I think the dial marked in orange, is 1/10th of a unit, so each full rotation of the dial circled in green would increase the digit ‘ circled in orange’ by One, IMO

 

[ShaunCorbs]

Yep

 

[John.g]

Thanks, at least that dispels what I was told about the plume being a sign that the boiler is in condensing mode.***.would measure the condensate, but it’s all piped in quite secure and insulated, so won’t disturb that.

Re the gas meter, I think the dial marked in orange, is 1/10th of a unit, so each full rotation of the dial circled in green would increase the digit ‘ circled in orange’ by One, IMO

View attachment 47612

That's fantastic, you can now use this to either get a new DAB pump or a new boiler or something in between!.
When you think your boiler is at minimum output get a stop watch and time in secs one complete revolution of the dial which is 0.1FT3?, calc then is
0.1x0.3X0.85X3600/(time in secs), KW. will be very interesting to see your findings.

Those 3 units you got for 24 hour period don't make sense as it means a consumption of less than 1 kwh.

 

[Reddwarf4ever]

what did you mean about getting a new DAB pump or boiler ? Was that a joke 😂

I actually removed the radiator from the kitchen in 2008, the year before I got the valiant, so considering reinstating that, but with a 100x50cm double fin, together with replacing the two lounge rads with double fins should hopefully give the boiler more to work on & help the return temperature ......so frustrating this morning, trying to heat the house, but at 64oC target, inlet 54oC, it was only consuming 5KW...with the pump on CP3....just not enough heat being dissipated.....increased outlet to 68oC but now the inlet 58oC and not condensing.....looks like won’t win as things stand.

just put back to outlet 64oC ( inlet 54oC ) & see if it eventually warms the house up, fan 1600rpm 🤯

Gas Units ( ft3 ) conversion to KWh
units × 2.83 × 1.02264 × 38.9 ÷ 3.6 = kWh

therefore 3 units ( 24 Hrs ) = 94KWh

will do the consumption reading shortly

P.S. used 2845 KWh in January 2021 which is 92KWh / Day which ties up.

 

[John.g]

I can certainly assure you that 3 FT3 of natural gas does not contain 94 kwh but 300 FT3 does.
Just carry out that test I suggested and that will do a rough meter check as well.
Have you got before/after meter readings for any billing period or before/after for that 24 hour period?.

I wouldn't get too carried away with this condensing, the extra saving in running at a return temp of 58C as against 54C is the sq. root of nothing as IMO you have to get down to a water temperature of ~ 50C to even begin to condense.

 

[Reddwarf4ever]

Guess having paid for a condensing boiler, gives an incentive to actually get it to work as it should ( & save money )
but back to Gas use, I now notice a Gas ‘Unit’ is actually a 100 F3 of gas....learn something new every day 🥳 so the meter window with the orange circle is actually x 10, which is what the little red zero is for ( advised by my Wife that she told me this but I didn’t listen, as if 😂😂😂 ) so this afternoon will get the readings

included some gas bills.......

 

[John.g]

Might be easier so to time 5 revolutions of that rotating gazebo (post 244) that I initially suggested and/or time just 1 division of the counter which is 1 FT3 as it would take ~ 35 minutes to time one complete revolution in which time the boiler output may have changed.

 

[Reddwarf4ever]

set the boiler at 5KW, (1500rpm fan) took 2min 47 secs for 1.0 ft3 of gas

from your calculation 0.1x0.3X0.85X3600/secs

= 0.5497 ?

 

[John.g]

No, its 5.497kw because the "0.1"Ft3 is now 1Ft3. Looking good as from the fan speed calc the boiler output is 1500/5200*18, 5.2kw.

So not joking about the DAB pump, I assume it was/is reading 1.0 M3/hr and the deltaT was 9C?, so IF the deltaT is correct then the flow must be ~ 0.5M3/hr so a error of 100% or else there is a problem with the flow or return thermistors as the deltaT should be 4.7C if the flow is correct.
I would be slightly concerned that magnetite/sludge build up is affecting the pump, in which case it won't be covered by warranty, I would suggest inspecting the Wilo by removing the top and (take a photo) cleaning it up with special attention to the impeller vanes and re installing it. You can then inspect the DAB before making any decisions.

 

[Reddwarf4ever]

Was so happy about the early performance of the DAB pump & the silent operation compared to the Wilo, I am afraid I disposed of it. Couldn’t see any point in keeping it. So will have to work with what I have......today has been a good day, using CC3, 64oC outlet and 54oC inlet......kept going all day.....house warming up nicely......will leave as is for a few days and try very hard not to adjust anything.......

thanks

 

[John.g]

Re "condensing", it would be interesting if you could break the condensate line and measure it (if any), from my basic calc you should get around 0.16 Ltr/kw of gas burned if fully condensing, so at minimum output of 5.2Kw ~ 0.8 litres of water/hour. If you can run on return target temp control then you will be able to see the exact effect the return temp has on the condensate rate, or adjust the target flow temp when boiler firing harder to reduce the return temp but not below the minimum boiler output.

 

[Reddwarf4ever]

Not easy to break the condensate line, as fully insulated and glued fittings.
during the recent cold snap, the boiler worked ok all day, pretty much at its minimum of 5KW, when the weather became milder, it actually satisfied the room stat several times in the day, flow temperature still set at 64oC and return around 55oC. Going to increase two rads this summer to double fin & as high an output as I can find. Hot water working well, set room stat to 23oC a couple of times a day for an hour, so hopefully the hot water will heat efficiently 🤞 The boiler still cuts out at +3oC above target and it does fire up a bit too vigorously, often overshooting......but learning to live with these limitations. Wish the radiators had been designed around a D 40oC wouldn’t have so many problems achieving a boiler DT of 20oC & a lower return temperature.

just phoned a ‘recommended‘ Gas safe engineer. Had no idea if the 3oC trip or firing rate from startup could be changed, thought my idea about increasing radiator sizes is wrong 🤔 suggested I get valiant out as they should be able to fix my problems. But I am not convinced that I wouldn’t just be wasting my money, if I get told either “it can’t be fixed “ or “ that’s going to be expensive “, a quick look at the valiant website gives a ‘one off service‘ as £285, apparently as my boiler is 11 years old it can no longer have a contract cover ( Max 8 years old ) even if for the £285 my problems were fixed, it was would takes years to payback this with any ‘possible’ fuel savings. Keep emailing Vailant with the questions can the 3oC cut out be adjusted or fixed to 5oC and can the fire up rate on start up be adjusted, ‘ reduced’, maybe it is the P.I.D. Controller.....maybe should just wait till it packs up & replace with a non valiant boiler.....one that does more what I need......wish I knew a friendly gas safe engineer with a good knowledge of valiant boilers to give me some answers

 

[John.g]

Welcome back.

You still have a lot of control over your return temperatures but the boiler may not be happy.
Taking the above.....flow/return/deltaT, 23C room temp and a TRV...
64/55/9 if you increased the flow temp to 70C then the exact same rad output will be achieved.....70/48/22 so you are now condensing nicely with a deltaT of 22C .
If you increased the flow temp to 75C then the exact same output will again be achieved ....75/44/31, better condensing but at the boiler deltaT limit of 30C. (I run my boiler all the time at ~ 70/75 and see these return temps of 43/46C regularly). I'm sure I mentioned these numbers previously??.

Now, if you doubled the size of the rads to give the same output as above then, for the same flow temp of 64C......flow/return/deltaT 64/24/40, deltaT 10C greater than boiler limit of 30C.
60/28/32 excellent condensing but marginal deltat for boiler
55/33/22 again outstanding condensing and only 22 deltaT.

SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C
If you double the rad size, you can have a flow temp of 55C with a return of 33C and a deltaT of 22C.

 

[Reddwarf4ever]

Welcome back.

You still have a lot of control over your return temperatures but the boiler may not be happy.
Taking the above.***.flow/return/deltaT, 23C room temp and a TRV...
64/55/9 if you increased the flow temp to 70C then the exact same rad output will be achieved.***.70/48/22 so you are now condensing nicely with a deltaT of 22C .
If you increased the flow temp to 75C then the exact same output will again be achieved ***.75/44/31, better condensing but at the boiler deltaT limit of 30C. (I run my boiler all the time at ~ 70/75 and see these return temps of 43/46C regularly). I'm sure I mentioned these numbers previously??.

Now, if you doubled the size of the rads to give the same output as above then, for the same flow temp of 64C.***..flow/return/deltaT 64/24/40, deltaT 10C greater than boiler limit of 30C.
60/28/32 excellent condensing but marginal deltat for boiler
55/33/22 again outstanding condensing and only 22 deltaT.

SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C
If you double the rad size, you can have a flow temp of 55C with a return of 33C and a deltaT of 22C.

 

[Reddwarf4ever]

So basically what you are saying is there is no need for me to keep driving the flow temperature down 🤔 I realised that radiator BTU released would suffer, but assumed ‘wrongly it seems’ that that was the way to go. I have read elsewhere ‘ but shouldn’t believe everything on the internet’ that keeping the flow temperature as low as possible was the best solution to ensuring a condenser boiler is in condensing mode.

reading your post, I conclude that raising the flow temperature would increase the DT across the radiators, and not adversely affect the return temperature.

will certainly give this a try, ( SO, to sum up, with existing rads (output) then with a flow temp of 70C you will have a return of 48C and a deltaT of 22C ) if successful, will result in a faster house warm up, a more efficient boiler & allowing the boiler to cycle frequently as it’s meant to, plus no need to replace the lounge radiators.

will update soon.....🙏 set flow temperature to 72oC.....🤞

 

[John.g]

Yes, as long as the rads are controlled by TRVs, if not controlled by TRVs then reducing the flow temp is the only way to reduce the return temp.

 

[Reddwarf4ever]

Yes, as long as the rads are controlled by TRVs, if not controlled by TRVs then reducing the flow temp is the only way to reduce the return temp.

if a TRV shut back, wouldn’t the heat released from the associated radiator fall ?, or would the flow increase through the remaining radiators, if not wouldn‘t this result in an increased return temperature To the boiler ? If the TRV was fully open, then this would be the same as not having a TRV ???

8/12 have TRVs

 

[John.g]

Yes, the rad output decreases with TRV throttling, that is the whole point of a TRV, it maintains a room temperature by controlliing the flow through the rad and changing its mean temperature which changes its output.
Yes, the flow through the remaining rads will increase leading to increased output and room temp tends to rise so the TRVs on these rads will throttle in more to return the room temperature to normal.
Yes, having a TRV fully open is exactly the same as having no TRV.
Remember my comments above are based on the assumption that TRVs are used throughout and its pointless increasing the flow temp to 70C as the higher return from the non TRVd rads will mix with the lower returns from the TRV rads leading to a increased boiler return temp.

If you are controlling the return temps on the non TRV rads by reducing the flow temp then this is fine and if you want to maintain the same output but with a reduced return temp then you will have to throttle the lockshield valves to give a return of 48C with a flow temp of 70C or a return of 44C with a flow temp of 75C, as you are aware, in all cases including your own of 64C/55C that the rad(s) output is reduced to 65% of its rated output.

 

[Reddwarf4ever]

Set the boiler flow temperature to 72oC and the return is 62oC, so clearly not suitable for my system.....I don’t have TRVs on the lounge radiators, because that’s what I have been told, you never fit TRVs to all rads, unless that philosophy has changed.

so back to 64oC for now...

 

[John.g]

Throttle the lock shield valves until you get 46/48C , remember its recommended anyway to throttle to get a 20C deltaT with a condensing boiler.

 

[John.g]

Forgot that you have a "smart" circ pump so a much easier method of achieving your output of 5.3kw with reduced return temp.
Your present settings are flow/return/deltaT/Kw, 64C/59C/9C.
If you increase the flow temp to 67C, change from CC3 to CP3, observe the return temp, then change to CP2 and finally CP1, if the pump curves are as I think they are then you should/might get 67C/50C/17C. Downside is you would probably have to return to CC3 to enable the boiler to refire after shutdown. Maybe worth a try?.

 

[Reddwarf4ever]

Forgot that you have a "smart" circ pump so a much easier method of achieving your output of 5.3kw with reduced return temp.
Your present settings are flow/return/deltaT/Kw, 64C/59C/9C.
If you increase the flow temp to 67C, change from CC3 to CP3, observe the return temp, then change to CP2 and finally CP1, if the pump curves are as I think they are then you should/might get 67C/50C/17C. Downside is you would probably have to return to CC3 to enable the boiler to refire after shutdown. Maybe worth a try?.

Will certainly give that a try, Friday 🤞....running 62C/52-55C/7-10C/5-6KW working well, heating cycles 2-3 times a day when the room stat is satisfied, Hot water heated at same time as heating is on, so not a problem so far. With the two lounge radiators increased from 2784 to 5439 BTUs ( based on 50oC DT, [although I realise mine are 40oC DT]) Each, hopefully will be enough to improve things next winter.

just wish Vailant would actually give me an answer about the +3oC cut out Vs +5oC, which it’s meant to be.

 

[king of pipes]

These maybe of interest to you gives a bit more info on the pump settings. Kop

 

[Reddwarf4ever]

Haven’t had a chance to try your ideas out yet, on my list.
But........

I Compared February 2020 average temperature, 7.44oC with February 2021, 7.75oC
Gas use in Feb 2020 was 71.16 units and Feb 2021 71.0 Units

On the face of it, doesn’t seem a lot of point running my boiler at 62-64oC instead of 75oC. Was considering upgrading two lounge radiators to double fin to increase heat output on each by 30%, but with almost zero efficiency gain, seems it may not be a viable idea. Will try Johns suggestions. Wish these condensing boilers had built in efficiency calculators, would just need a condensate flow meter to achieve this I think🤔. Or even a light to say in condensing mode.....be a selling point maybe 🤔 assuming of course customers understand what it means.

will also look at the pump you tube video(s) but the post only contained an image not links, but will look for them thanks.

looking at this chart, my boiler is around 87% efficient if I increases the size of my two lounge rads to double fin, I am not sure if I will achieve even a 45oC return temperatur, even then the efficiency will be 90% which is only 3% more than I have now. To get the best from a condensing boiler IMO, it needs to be installed the same time as the rads etc. To size everything to get the most out of the condensing boiler. Not so easy, as things stand now.

the graph shows 3 flow rates, so if I read it correctly, using the lower flow rate ( in Red ) would achieve a better efficiency 🤔

graph obtained from here

Monitoring energy efficiency of condensing boilers via hybrid first-principle modelling and estimation

The operating principle of condensing boilers is based on exploiting heat from flue gases to pre-heat cold water at the inlet of the boiler: by conden…

www.sciencedirect.com

most in depth explanation / discussion on condensing boilers I have ever seen

 

[ShaunCorbs]

Need to keep it condensing else it will break the boiler in a year or two as the heat exchanger will get blocked as there’s nothing to move the waste

 

[Reddwarf4ever]

Hello, until this February it has not been in condensing mode since new 2009, only just realised how it should operate. Can’t see how the heat exchanger will get blocked ? It’s just a case of efficiency 🤔

 

[Reddwarf4ever]

Last week, I had a phone conversation with a Vailant engineer. I asked why my boiler cuts out at a DT of only 3oC between flow and target temperature & not 5oC as I have been told it should be, he replied it varies, depends on how hard the boiler is firing, I replied that doesn’t make sense, the cut out temperature is a specific value or % of range, either way a definite value, he just repeated it again, then said it controls on DT of inlet and outlet temperatures D41-D40, I said that doesn’t make sense either, it can only have one control parameter, which is D5.....he kept on about things not being specific and depends on this or that.......I completely gave up in the end, none the wiser......send one FINAL long email to Vailant re that phone call and asking for clarification......don’t suppose I will get any....🤯🤬😡

 

[John.g]

Overall, I think you are doing pretty well, you can get the return temp down to ~48C by increasing the flow temp to ~ 75C and reducing the flow rate but then the boiler will exceed the 30c max deltaT on fire up due to the reduced flow, and trip, if you keep reducing the flow temp then the rad outputs will fall below the minimum that you require and below the minimum output of the boiler.
Also, in non condensing mode, 10C difference in flue gas temperature (~ same difference in return temp) will only result in ~ 0.45% difference in boiler efficiency.

 

[John.g]

Just had another look at those numbers and I can see now why you can't get into condensing mode at a demand of 5.2kw.

If all your rads were in service and assuming a rated output of 18kw then with flow/return of 75/58 & 0.75m3/hr flow the actual output will be
15kw. IF the heat demand now fell to 5.2kw and you reduced the flow temp to 45C then you will get a return of 39C and 5.2 kw output.

Now you only need to heat 4 or 5 rooms so these rads which I reckon have a rated output of 7.2 kw are required to output 5.2kw so for the same flow rate of 0.75m3/hr then flow/return of 65/59 will give you the required 5.2kw but the return is way above condensing and there isn't much you can do about that IMO.
If you reduce the flowrate to the minimum required of 0.33m3/hr to maintain a boiler deltaT of 25C for the required 9.6kw boiler output for 1 minute on fire up then to give the required min output of 5/5.2kw requires a flow temp of 68C which results in a return of 54C, still no condensing.
You would have to oversize the rads by a factor of 1.25 (9kw) to even begin condensing at 50c (flow of 66C) and by a factor of 1.8 (13kw) to get meaningful condensing at 42C (flow of 55C)

It seems very apparent that no matter what the combination of flow temp and flow rate is that it is thermodynamically impossible to have a boiler in condensing mode with the rads outputting their rated output and have to be outputting only ~ 56% of their rated output to get a return of ~ 42C and real condensing effect.

 

[Reddwarf4ever]

I agree John, tried all your ideas and no matter what I do, either the boiler continually cuts out due to over firing / over temperature, or the inlet temperatures continues to be around 55-58oC. Think condensing boilers work best when the heating system is designed from new for a condensing boiler.

looking at the condensing efficiency chart, I am at 86-87% with the boiler in Non condensing mode, if I change 2 lounge radiators to increase heat output to 30% for Each, not sure how much difference this would make to the return temperature, even if I could achieve 45oC the efficiency would only be 4-5% higher, so wonder if it justifies the expense......

thanks

 

[John.g]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

 

[Reddwarf4ever]

I don't think it's worth the hassle/expense, I have always been skeptical of the actual condensing benefit achieved.
On oil fired boilers, the flue gas temp on a SE boiler is around 230/250C and 80/100c on a HE so a real gain of ~ 6 to8%, I would like to see typical flue gas temperatures for SE/HE gas boilers, I wouldn't expect a SE gas boiler temperature to be as high as that of a oil fired boiler.

But there are lots of ways of skinning a cat, I ran and maintained relatively large 50MW, 45 bar superheated steam boilers and because of the high water temps involved (259C) regenerative rotary air heaters were used to heat the incoming combustion air with the outgoing flue gas and the boilers had ~ 85% efficiency....your next project.

I worked most of my life on refineries, where heat recovery was paramount, especially producing 600# steam from furnaces to drive turbines. I love thinking of ways to be more efffcient, but sometimes the odds are weighed against me. I envisaged a system, where a separate cold water tank ( feeding the DHW cylinder, could be preheated, using an other boiler heat exchanger, just before it vents outside, to extract the remaining latent heat, would only need a very small pump, only running when the boiler is on......the ‘heated’ DHW storage tank would be the size needed for a days use of DHW, if it fell low, it would be made up from the main DHW storage tank, would need to be foam insulated also......just a dream......

the way Energy is going, won’t be long before burning gas is a thing of the past, we will all be on electric heating, so much simpler. There is so much more better ways to use the gas, or at least the oil it was produced from, than just burning it. Burning fossil fuel for energy should be the very last thing we do, when it can’t be re used or recycled anymore IMO

take care

 

[king of pipes]

Products do exist but they are costly. Kop

 

[Reddwarf4ever]

Shaun corbs said “Need to keep it condensing else it will break the boiler in a year or two as the heat exchanger will get blocked as there’s nothing to move the waste”

but dont see why this should be or how 🤔

 

[ShaunCorbs]

You will get particles that will build up in the burner chamber that won’t get flushed through with the condense and will build up to a point where it just blocks the hex and could cause it to split

 

[king of pipes]

Pm him I am sure he will have a explanation

You will get particles that will build up in the burner chamber that won’t get flushed through with the condense and will build up to a point where it just blocks the hex and could cause it to split

Like this

 

[ShaunCorbs]

Pm him I am sure he will have a explanation

Like this

forgot to reply yesterday 😀

 

[Reddwarf4ever]

That does make sense, but wonder how many condensing boilers are actually running in condensing mode ? Maybe apart from new installs 🤔 mine not been condensing since new in 2009, only since January 2021 have I been trying, but still not a great success, minimum return temperature 53-55oC, only thing I can do is replace two lounge radiators with double fin, each would then give approx 30% extra heat output ( 100cm x 50 cm )

 

[king of pipes]

I am sure it will be condensing at certain times of the day, I by no means have John G' s grasp of the figures but I have 80 + boilers on my books which I service and maintain, and where possible I advise my customers to keep the flow temperature setting at 65° c by keeping the heat exchanger clean serviced correctly and that means a full strip and clean every year flushed though electrodes cleaned condense trap removed and fully cleaned or replaced any small leaks addressed gas valve adjusted , every 5 years the system drained flushed and a good quality inhibitor added I rarely get a breakdown from doing a flue gas analysis which we do on each service you can determine efficiency at different stages of firing , modulation, and flue gas temperature I personally think you have probably got your system running the best you can without changing things replace radiators for larger ones by all means this allows you to run the system at a lower temperature and still be comfortable in your living space. Regards kop

 

[John.g]

The boiler will certainly condense very heavily on start up in the morning where it might take a 18kw boiler ~ 10 minutes to heat say 75 litres of water from 20c to 50C, also if the house/room temp is very cold then a certain amount of condensing will also take place.

Its interesting though that if you have a fixed flow temp of 65C and a flow of 0.75m3/hr (fairly normal for a 18/20 boiler) then no matter how oversized the rads are that the return temp cannot be lower than ~ 51C once the rooms are up to temperature (I use 23C) the room temp and the return temp will then start rising.
I spoke to a (long retired) acquaintance of mine earlier on and he said he never remembers measuring a gas fired boiler flue gas temp below 55C, what kind of flue gas temps are you seeing KOP?.

 

[king of pipes]

I am working on one now as we speak

 

[John.g]

Boiler NO 81? should now be be running at 65C/0.39M3/hr (flow) and rad(s) output of 59% OR
55C/0.77M3/hr (flow) and rad(s) output of 48%.
and Shaun's condensate flow at roughly 0.7 lit/hr @ 20 kw boiler output or pro rata.

 

[king of pipes]

I am working on one now as we speak

Bit cleaner now 😉

 

[John.g]

That's some service.

 

[Reddwarf4ever]

I thought the same thing as John mentioned, when the boiler first fires up in the morning, return temperature is typically 20-25oC & after it cuts out on the room stat, may not restart for 1-2 hours, then again very low return temperature for some time ( will try and monitor return temperature next time it restarts, be interesting to see the climb rate and how long it ‘ should be’ condensing for. So going back to deposits on the heat exchanger wouldn't these periods of heavy condensing keep the exchanger clean ? I am running my boiler outlet at 64oC, i tried 62oC, but even 2oC seemed to make noticeable difference to radiator heat output. The return temperature when steady and modulating is 53-56oC. As I said previously I was considering replacing 2 lounge radiators with double fin, but looks like the best I might get is a 45oC return temperature.....only an educated guess.....which would give a 3% saving, would take around 20-25 years to pay back the cost..so very likely going to abandon that idea.

I know I may get shouted at, but I dont suppose I could clean the heat exchanger myself, I think it will need a Gas Safe person most likely.......

there is a move to make manufacturers add an efficiency display to their boilers and show when it’s in condensing mode, I think they will fight hard against this, as people will soon realise they are not getting the 97% efficiency that they expect..

take care all & keep safe 😎

 

[Reddwarf4ever]

Products do exist but they are costly. Kop

Says “cost effective “ at £630 I wonder how long it would take to pay back, also for combi boilers only, so no good for me...🤯

 

[John.g]

Not shouting but not whispering either, NO you cannot clean the heat exchanger yourself.

 

[ShaunCorbs]

I maybe thinking wrong but how can 2dc make a room feel colder when it’s well above the room temperature

I could understand if the flow temp was 40 but not 60dc you sure you have a flow temp at 62dc at all your trvs ?

 

[Reddwarf4ever]

I maybe thinking wrong but how can 2dc make a room feel colder when it’s well above the room temperature

I could understand if the flow temp was 40 but not 60dc you sure you have a flow temp at 62dc at all your trvs ?

Hello, there must come a flow temperature that just doesn’t release enough heat to be comfortable, not sure if flow temperature and radiator heat release is linear. Just seeemed to make a difference when I lowered it to 62oC.....no idea if its the same temperature at the TRVs all rads do get hot, so assume that must be the case.....

also at 62oC the boiler often cuts out at 65oC as it can’t reduce below 5KW......

very happy with how it’s running now, have had some excellent advice and learnt lots in the process.....

thanks

 

[John.g]

This thread has thrown up some extraordinary "facts" re flowtemps/flowrates and their effect on return temps etc.

Any temperature change in either flow or room temp will have a effect on rad output but its not linear, it works with a exponent of 1.3 where the rad output is based on the ((meanradtemp)-requiredroomtemp))^1.3.

Using your observations above, initially I calculated that the 4/5 rads that you have running at a fixed flow of 0.76M3/hr......
flow/return/rad output 65C/59C/5.21kw. Now reduce the flowtemp to 63C results in........
63C/57.4C/4.9kw (which is why your boiler is cycling), you are running right at the switch point because you require the rads to run at ~ 68% of their rated output so running very close to the minimum boiler output.
If you require a rad output of 5.21kw with that 63C flowtemp then the flowrate would have to be increased from 0.76M3/hr to 2.24M3/hr, (impossible) and you would achieve 63C/61C/5.21kw.

If you had TRVs on these rads and increased the flowtemp to 75C then they would have throttled in to give 75C/48.8C/5.21kw at a very reduced flowrate of 0.17M3/hr. rad&boiler deltaT of 26.2C, OK if boiler is running but flow too low for the 1 minute fire up requirement of 9.6kw.

The above might look a bit complicated but very easy to calculate in a spreadsheet I made.

 

[John.g]

Should also add another bit of info.
If your room(s) should suddenly fall to 10C then the flow/return/output will be
65C/56.6C/7.35kw. 5C roomtemp = 65C/55.6C/8.21kw & 0C = 65C/54.4C/10.6kw.
The most interesting part of all this is that the return temp doesn't change much in any of the above so it looks like any system with fixed flow temp and fixed flowrate will never condense under any conditions except at start up

 

[Reddwarf4ever]

Very interesting John, would love to see the spreadsheet, always making these for all sorts of things......thanks

Should also add another bit of info.
If your room(s) should suddenly fall to 10C then the flow/return/output will be
65C/56.6C/7.35kw. 5C roomtemp = 65C/55.6C/8.21kw & 0C = 65C/54.4C/10.6kw.
The most interesting part of all this is that the return temp doesn't change much in any of the above so it looks like any system with fixed flow temp and fixed flowrate will never condense under any conditions except at start up

interesting stuff 👍 would love to se your spreadsheet, can you attach ?

 

[John.g]

I will try once again but for the past few months havn't been able to attach any, they used to attach no problem previously in zipped form.

 

[Reddwarf4ever]

I will try once again but for the past few months havn't been able to attach any, they used to attach no problem previously in zipped form.

Can give you a dummy email address that I keep for such purposes.......👍

 

[John.g]

Yes, do that, no problem.