I'm in the UK with a small modern, well insulated, double glazed home heated by a traditional gas boiler.

My heating had been achieving room stat temperature within 30 minutes but is now taking at least 3hours. After the boiler has run continually for about 2hours I think it probably cuts out on the boiler stat (ie it is not up to temperature on the room stat - also confirmed with seperate thermometer). It switches off for a few minutes and then comes back on again and continually cycles the boiler on and off for at least another hour before the room stat temperature is reached. I have monitored the rise in temperature closely this morning and find that the inside temperature does not even keep up with the rise in the external temperature!

If I only run the "hot water" part of the system (ie no central heating) the hot water gets to temp within about 45minutes. The problem appears to only be with the central heating side of the system.

New room & boiler stat have been fitted. The pump "sounds" to be running fine and all radiators are uniformly hot across their entire surface.

Does anyone have any suggestions as to what the problem might be please?

  • Duplicate of: diy.stackexchange.com/q/27213/22. Please don't post the same question multiple times; see the tour page for a quick overview of how the site works and the faq for more detail.
    – Niall C.
    Apr 28, 2013 at 15:21
  • I have to ask a serious question. Is it possible that before the "inspection" the boiler was operating to way hotter temperatures than it should have? And then somehow during the inspection that situation got corrected?
    – Michael Karas
    Apr 28, 2013 at 17:43
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    Please consider registering your account. Then you'll be able to edit your question more easily rather than reposting it.
    – ChrisF
    Apr 28, 2013 at 21:44
  • is this radiant heat or forced air?
    – amphibient
    Nov 11, 2013 at 18:28

4 Answers 4


A very blind shot: Air in the heating system? You have to de-air the central heating radiators, there should be a special vent on it, which might look like this:



You better put some bowl on the floor before you open the vent. If water starts to run out of it immediately, eveything is ok. If not and you hear the sound of leaking air, there's air in the radiator which has to be let out. Be careful, the air is usually quite smelly and water which starts to run out of the vent is usually very dirty and dark at first.


As an answer I have to ask a serious question.

Is it possible that before the "inspection" the boiler was operating to way hotter temperatures than it should have? And then somehow during the inspection that situation got corrected?

Such a situation could have resulted from the boiler thermostat getting stuck in the on state and never shutting off. I could see this happening with the older thermostats that got extra dust, dirt or insects inside. During the inspection maybe the thing simply got un-stuck or was opened and cleaned out. As a result your system returned to operating like "normal".

If the above idea is correct it would certainly explain why replacing the thermostats with new ones did not change the current situation.

  • I think this is probably unlikely as prior to this issue the boiler never got anywhere near as hot as it does now.
    – user12776
    Apr 30, 2013 at 19:48
  • Then maybe there is a flow rate blockage in the line going from the boiler to the heater units.
    – Michael Karas
    Apr 30, 2013 at 20:07
  • The flow rating has been done but only with hot water rather than central heating running (does this make a difference?) and the flow rate is good. The radiators are all getting consistently hot across their entire surface - this heat just doesnt seem to get into the rooms.
    – user12787
    May 1, 2013 at 16:45

You claim "all radiators are uniformly hot across their entire surface." If this is so, then your heating system is performing its function.

Try arranging a small fan to blow on a radiator to see if that helps with heat distribution.


Another possibility I have experienced - the heat exchanger in the boiler may be clogging, reducing the flow through it. This would carry less heat around to the radiators and cause the boiler to get hotter.

Do you ever hear anything clink very faintly in the pipes (not expansion/contraction from the changes in temperature)? In my case I had limescale in the system, I think in part due to the previous owners not using inhibitors; small bits would sometimes get through and make noise when pumped around.

Or does it ever sound like the flow changes (like a water pipe bend or tap will whoosh at a certain flow rate and change if you turn the tap)? This could happen if there is a part blockage, which dislodges a bit.

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