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Central Heating

Re:central heating comes on when hot water is switched on only

I have a Megaflow system with potterton profile boiler and Honeywell 9400 programmer.Up to now it was working fine .After draining the system to add fresh inhibitor , the central heating now only works when I turn on the hot water switch.The system was plumbed with separate motorised valve for hot water and separate for central heating,If I switch on the central heating switch the central heating does not work but if I switch on hot water switch the central heating works until the water reaches the required temperature then the heating goes off also.Any idea of what to do ,any advice will be highly appreciated. Once the water temperature goes lower from required temperature, the heating comes back on from hot water switch.

3 Answers from MyBuilder Heating Engineers

Best Answer

If your description of symptoms is acurate it sounds like the microswitch in the motorised valve for the central heating is not passing the switch feed to the boiler. If this is the case then the way to go is replace the motorised valve you may be able to get it working but it is likely to be problematic.
The valve itself is actually opening hence the heating system is getting hot when the hot water is on but the boiler is only fired up when the hot water motorised valve is open, the boiler should fire up if either valve is open.
Hope this helps-Kevin.


Answered 16th Dec 2012

Hi. Whilst I agree with the last 2 answers, it seems rather odd this should happen as a result of a system drain & refill. This said & as it is a peculiar coincidence, I would make sure that you prove the fault beyond doubt before ordering or installing a new MV as the fault may be elsewhere.


Answered 16th Jul 2016

it sound like motorised valve faulty need replace
best to get heating engineer to change it, because ciruit tests will need to be carried out before and after the valve change

I hope this is helpful
mr gurdial
Hayre property Services


Answered 9th Mar 2013

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