Radiators come on when hot water is on?

17 Jun 2012
Reaction score
United Kingdom
I've just bought a house and am trying to figure out what's wrong with it. The radiators come on even though the heating is 'off' and just the hot water is 'on'.
A bit about the system: We have a Potterton Promax FSB HE boiler, and a hot water tank on the first floor. There are three 'honeywell' valves by the hot water cylinder: one for the hot water, one for the 1st and ground floor heating and one for the 2nd floor heating. There is also a pump for the heating system by the hot water cylinder.
I have temporarily solved the issue by closing off all the radiators, but I can't close the 2nd floor underfloor heating, so the top floor gets hot!
Any ideas?
Sponsored Links
Two possibilities,
1 Zone valve is not shutting off CH circuit.
2 Reverse circulation due to incorrect pipework connections.
First one can be checked by allowing the system to cool; and then turn on the hot water only and see which valve (other than the HW valve) is allowing water to pass.
Second one is probably unlikely and would require a site visit to assess.
Probably reverse circulation in the heating system. This is caused by incorrect connection of the HW return from the cylinder. There must not be any other connections between it and the boiler.

Quick check

Turn heating only on. Check each rad to see which pipe gets hot first (left or right).

Turn heating off and let it cool down.

Turn HW only on. Check each rad again. If the other pipe gets hot first, you have reverse circulation.
Sponsored Links
Ah, I suspect I might understand what the problem is: there are three Honeywell motorised zone valves, each feeding off the same pipe from the boiler; one for the hot water going into the tank, one for the first floor heating and one for the second floor heating. They are on 'auto' but you can move a lever on them to push it to 'manual'. In doing so, on two of them (the hot water and 2nd floor heating one) I feel some resistance if the systems are 'off' - so I presume I'm working against the motor to open them. The one for the first floor heating gives no resistance even if the heating is 'off', so it might be permanently open (broken?). The one for the second floor heating resists, but it looks like it's been fitted the wrong way: the arrow indicating the flow direction on the main pipe fitting is pointing back towards the feeder pipe from the boiler.
Could it be I have two problems?


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