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.