I have a standard British heating configuration at home - there is one gas heater that provides hot water both for the boiler (and bathroom etc.) as well as for central heating. There is a small electronic device where you can set if you want to heat the water or run the central heating (it has a timer, etc). The central heating works based on a thermostat in the living room, which shows green when the actual temperature is lower that the set value (hence the heating is working), or red otherwise.

Recently we switched the central heating on for the first time this year, but it didn't work as expected. When the hot water is off everything seems fine. But when the system heats the water up, all the radiators got boiling hot, despite the thermostat showing red all the time. What might have gone wrong? Is it something I could fix myself?

EDIT: To be more precise I did a test and the results are as follows:(CH - central heating, HW - how water)

  • HW off, CH off - everything cold [OK]
  • HW on, CH off - everything is hot [WRONG]
  • HW off, CH on, temp > thermostat - everything cold [OK]
  • HW off, CH on, temp < thermostat - everything cold [WRONG]
  • HW on, CH on, temp > thermostat - everything hot [WRONG]
  • HW on, CH on, temp < thermostat - everything hot [OK]

So it seems that the hot water switch now operates both HW and CH, and the central heating switch does nothing at all (and thermostat is likewise ignored). Is this something I could fix myself?

  • Does the tap water in your bathroom or kitchen get as hot as the radiators?
    – Niall C.
    Oct 23, 2011 at 22:00
  • Hi @NiallC., yes, the water is hot as well. This is expected though - I actually wanted to warm up the water, but I didn't want to use central heating!
    – Grzenio
    Oct 24, 2011 at 8:09
  • temp > thermostat - I think you're using the wrong symbol there - right now you're saying that when it's hotter in the room than the thermostat is set for, that everything is hot, and that's ok. (IE the room is 80, the stat is set to 70, and the radiator is hot) remember > is greater than, < is less than :) Oct 24, 2011 at 13:40
  • @TheEvilGreebo, sorry, this is a typo of course - fixed.
    – Grzenio
    Oct 24, 2011 at 14:14
  • Not sure how much help I can be with this, but have a look at this question to see if you can identify which type of system you have. This may help others narrow down the location of your issue.
    – BMitch
    Oct 24, 2011 at 15:47

2 Answers 2


A lot depends on the type of system you have. If you have a system or combi boiler, it could be the diverter valve which is faulty. This will be in the boiler itself. If you have a Y-Plan system with a three port valve, it could be the three port valve which is faulty. These are usually found in your airing cupboard or in your loft.

  • 3
    Yeah, I called a British Gas engineer and he changed the diverter valve.
    – Grzenio
    Dec 18, 2011 at 19:53

One of two immediate problems come to mind.

1) Bad thermostat, causing the heating system to move water through the system constantly despite the current temperature

2) Failed circuit in the boiler system itself, causing it to THINK the thermostat is calling for heat.

A quick test would be to remove the thermostat from its mounting (usually they just snap in place) and see if the heat shuts off. If that works, replacing the thermostat is a fairly easy process if you follow the directions carefully and make sure you tag all the wires properly.

Otherwise you'll need a pro to come check your system.

  • Rule out not roll out. ;) Check your > and < sign usage (see other comment) and then we'll see ;) Oct 24, 2011 at 13:41
  • You are right, fixed.
    – Grzenio
    Oct 24, 2011 at 14:14
  • Ok I'm confused - how many sources of hot water are there in your house? I mean - how many different devices are actually heating water - is there one burner / heater for both hot water AND central heat or are there two? Oct 24, 2011 at 14:21
  • There is only one burner, which handles both hot water AND central heating.
    – Grzenio
    Oct 24, 2011 at 14:40
  • Then how can HW off CH on result in hot radiators? Oct 24, 2011 at 14:44

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