Help Please - CH Only Comes On When Water Is Heating?

29 Dec 2009
Reaction score
United Kingdom
I've got a problem with my central heating .... it will only stay on while the hot water is heating up. If the tank water is hot the heating won't come on at all but if the hot water is being used I'll get heat. The CH won't work independently.

System is a Potterton Suprima Boiler with Honeywell wall thermostat, timer (ST6400C), Silver Box (V1073A 1039) & Tank Thermostat (L641A1039) controls. Pump is a Grundfos Selectric (59525600) set to III, Tank has a Sadia Water Heater on top.

So far I undid the large pump centre screw and turned the smaller inner screw left/right. The lever on the back of the silver box doesn't seem to have any resistance so I undid that to check if a spring had come loose but all looked ok. Moving the lever left to right is loose but it did click back once as if it was on a cog?

Can anyone offer any advice ?

Sponsored Links
Are you sure the 'silver box' is not a 4073A?

It will be this box that needs replacing. The microswitch has broken in it.
Thanks...probably is 4037A, the first part is scratched over.

What is it that this box does, or doesn't do in my case?

Are they easy to change for a novice & are they expensive to buy?
Sponsored Links
Cheers...all sorted.

Got a replacement 4037A (Prices quoted between £35-£95) and fitted it myself. Was told that motor could be changed for £12.50 but was too late.

Simple job....turn off mains, drain down tanks & tie up floats then it's just three like for like bolts and five wires to replace. Turn it all back on and bleed the rads. Worth treating the system to some descaler at the same time....I'd forgotton how warm it should be.

Definately worth a go yourselves.

Thanks for the help

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