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