Central heating pump sizing

[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.....🤞