Central heating pump sizing

[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.