variable pump speed for CH system

7 Mar 2012
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United Kingdom
Hi all....I've read some excellent info on this site, so I decided to join and ask for a bit of help myself.

CH system in our house (2005 construction) is fully pumped, inc HW. Glow worm 18HXi condensing modulating boiler, 16 rads, all with TRV apart from one in the hall used as bypass. Main flow/return is 22mm copper, with 10mm copper drops to rads, with plastic push fit tails. Original problem was that system wasn't balanced correctly, which it is now. Pump is a wilo 60 (6M head) and needs to be set on speed 3 to get correct flow rate through boiler when system is demanding heating & water. I've measued flow & return temps at boiler at full whack and its approx 20C diff, which is correct.

My problem is that the pump is in an airing cupboard next to the bedroom, it pretty quiet when the heating is demanded, but as the system closes the 3 port valve to HW only, the pump is running much too fast for the required pressure and becomes noisy. I know from measurement that speed 1 on the pump is fine for minimum boiler flow for HW only, but obviously not when CH is needed.

I tried a Wilo smart pump, but the control adaptation is really slow, so in the winter time the speed control would not speed up quickly enough to react to the opening of the 3 port valve for CH demand, leading to slow a flow rate through the boiler (which caused it to modulate the heat back)

My only solution seems to be to re-connect the pump through a couple of relays, set by the 3 port valve, so that I can set speed 1 for HW only, speed 2 for CH only and speed 3 for HW+CH demand..this is quite simple but I just wondered if there's an easy soultion for what must be a common problem ??

Anyone know if the Grunfos Alpha 2 is a possibility ?

Cheers and thanks

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Do you have a balancing/gate valve on the cylinder return(bottom pipe) that may be closed too much, and making it noisy?
Yes, there is a gate valve on the HW return to the seems to be set about half way.. To be honest I haven't moved it because I guess if it's too far open then when the system call for both HW & CH then the flow will all go through the cylinder if that loop is a lower pressure drop than the Rad loop ? I guess ideally that valve should be set to make the HW loop pressure drop equal to the Rad circuit pressure drop ???
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You could fit a spring bypass to the hw if the noise is a problem.

Adaptive pumps are a great idea but are less durable than the non modulating types.

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