Hot water thermal syphoning when off.

[corkee]

Revamped my sealed heating system recently and noticed that when system is off, the new pipe layout allows the hot water from the vented cylinder to thermal syphon around the circuit, through the boiler and back into the cylinder coil.

Question is, is it OK to add a single check valve to the coil outlet near the cylinder to stop this reverse cycle?

System comprises a Baxi solo HE condensing boiler, sealed circuit, Y plan (3 way valve - moves to DHW when off) and stored hot water (vented hot water cylinder).
TRV's on all but one radiator where room stat is.

Cheers,
Col

 

[Onetap]

Question is, is it OK to add a single check valve to the coil outlet near the cylinder to stop this reverse cycle?

Yes.
I'd change the 3-port to an S-plan, i.e., two 2-port zone valves.

 

[corkee]

Thanks for the confirmation.
Think I'll stick with Y plan and just add the check valve - considerably cheaper!
Col

 

[Onetap]

Thanks for the confirmation.
Think I'll stick with Y plan and just add the check valve - considerably cheaper!
Col

It is, which is why I once used one for this purpose. It didn't work, which is why I wouldn't recommend one.

However, my experience was with a thermal store, which had a large thermal mass driving the gravity circulation; it overheated the HWS storage cylinder when the boiler was off. Your installation probably has much less force behind the gravity circulation, so it may work. Spring check (as against swing check NRVs) are apparently used sucessfully for this. I much prefer the S-plan to Y-plan, so that also influences my choice.

There shouldn't be any significant heat loss from a condensing boiler, so you shouldn't get gravity circulation. Are you sure it's not something else, e.g., heat loss from uninsulated pipework, driving the thermo-syphon?

 

[corkee]

Just to let you know - Fitting the single check valve on the outlet from the coil worked perfectly.

Thanks for your help and advice.
Col