Central heating pump sizing

[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.