Heating only coming on when hot water is on


6 Dec 2009
Reaction score
United Kingdom

Santon unvented hot water cylinder (170L), Potterton Suprima (40 ???)conventional boiler with frost thermostat, no room thermostat, radiator controls/thermostats. Landis and Staefa RWB9 electronic control panel in ground floor kitchen, boiler and hot water tank in loft above 1st floor (makes testing a pain).


Hot water tank (Megaflow) sprung a leak back in June, replaced with Santon. All seemed to work fine.

Now find that heating only comes on when hot water is on (which is how we have the timer set up anyway). Heating also seems to shut down when hot water tank is full. If hot water is switched off with heating still on, heating will stay on if hot water tank not full.

When just hot water is on, radiators are heating up. Towel rail radiator and small radiator always seem to be on, regardless of setting when hot water and/or heating on.

I have had a look at the two Honeywell valves, and the wiring/junction box (please note I havse zero pluming/wiring knowledge). The heating valve seems to be constantly on even when switched off (slider moves full range), whereas the heating valve goes from half range of slider (when off) to full range (when off). But this was when I was testing switching these on and off, so not sure if heating slider goes to half range some time after heating is switched off. I can hear hot water valve click on, not sure about hearing the click for the heatinv valve.

I opened up the junction box/connection board. All wiring seemd to be secure, no loose wires. I did notice that where the hot water valve was wired in, there was a port for a white wire, but no white wire was connected nor was there a white wire from the cable connected to the hot water valve. Whereas for the heating valve, there was a white wire that had been terminated with a connector block.

It is only now in the cold weatehr that we are noticing that the heating is switching off when the hot water tank is full, so it is possible that it has behaved like this since the hot water tank was fitted in June. Is it worth having the original plumbers back and is it likely they made a mistake when fitting/wiring ? Or is it an age thing with the valves? We mvoed in 2.5 years ao, so don't know how old the valves, bolier, original pipes, wiring etc. are

I nede to have the bolier serviced soon anyway, so will have them in to service in the next week or so, but if I can sort this out myself even better.

Any ideas/questions/comments ?

Sponsored Links
Thanks for that.

Fortunately the junction/connection box is labelled with wire colours etc. but I will check out the diagram anyway.

If it is a wiring fault any idea what it could be ? Could the heating and hot water valves be wired into the wrong/each other's connections (hence the missing/extra white wire) ? is it worth swapping the wiring over to the other's connectors ??

the 'heating' valve seems to on a vertical pipe, which seems to be the cold water supply, the 'hot water valve' is on a vertical pipe, leading to the hot water cyclinder after a junction with the cold water supply pipe. Does this seem to be the right way around for those two valves ?

Sponsored Links
Position of valves is unlikely to be an issue, it sounds more like an electrical issue. If I have interpreted your description correctly it may be possible the heating valve isnt opening fully to activate the microswitch, which in turn then supplies the power to the boiler and pump in order for them to operate and provide heat.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local