Firstly, I should probably mention that I'm in the UK...

My wife and I bought a house new from a builder over 9 years ago and have had no problems whatsoever with our central heating system in that time. It is a typical modern system with a gas boiler in the kitchen feeding radiators upstairs and downstairs and a pressurised hot water system. The radiators have thermostatic valves on them (except for one in the entrance hall).

Earlier this year, I contracted a local plumber to do 4 things:

  1. Replace the original ("bimetallic") thermostat with a more modern programmable thermostat so that we could keep the heating on over night but at a lower temperature
  2. Service the boiler (for no real reason other than that it was 9+ years old and had never been looked at)
  3. Re-balance the radiators
  4. Re-pressurise the hot water system as the showers had lost their original mains water pressure

The system has never been quite right since but of course we haven't really been able to pin anything down until the recent cold weather. We are now seeing the following:

  1. For a while, the thermostat would call for heat, but the boiler wouldn't fire up. The boiler casing was very, very hot, so I suspect overheat protection had kicked in.
  2. When the boiler does fire, it doesn't stay on for more than a minute and then goes of for 2-3 minutes before firing up again. Again, suspicious that it is overheating.
  3. As a result of the above, it can take all day to get the house up to temperature (20C)

The plumber came to investigate last week and made a number of noises about the pump overrun not working, and the possibility of a pump failure, but ruled both of those out. He made some adjustments and now:

  1. The boiler appears to fire up reliably, although it does cut out periodically. He assures me this is normal and that it switches itself off while the water is circulated. I can't honestly say I was aware of it doing this before the summer.
  2. The radiators get too hot to touch, but I'd swear you used to be able to feel the heat radiating from them up to 12+ inches away which you no longer can.
  3. But still the house is very slow to heat (it appears to heat up at a bit under 1C per hour).

The house is a fairly typical British 4 bed detached (c. 1800 sqft) and is relatively modern (being newly built 9 years ago) so has pretty good thermal characteristics.

So my questions are:

  1. Does taking 8+ hours to get the house warm sound right? I'd swear it used to be warm within an hour or so of the heating coming on.
  2. What further diagnoses can I perform to help (the plumber and/or you good folks) identify the problem.
  3. Does it sound reasonable to assume (having had 9+ years of faultless service) that my problems stem from the work done in the summer? Perhaps a valve was closed while the plumber worked that he subsequently forgot to open, or something like that?

Taken together, all the symptoms sound to me like the hot water is not being circulated around the system quickly enough (boiler over temperature, house not heating up), and the plumber would appear to agree. He seems rather keen to fit a bigger pump. But I'm reluctant given that the system has worked fine for so long previously.

Sorry for the rambling post, but I'm pretty much at my wits' end with this and we have two small children that we'd really like to keep warm over the winter.

  • 2
    As mentioned a second professional option would be best due to the complex number of problems that could be causing it. Having said that from your description the first thing I'd be doing if I came to your house is re-bleeding the whole system.
    – UNECS
    Dec 18, 2012 at 20:26
  • Have you bled the air out of the radiators? Dec 20, 2012 at 21:19
  • 1
    Yes. They get uniformly hot.
    – dty
    Dec 21, 2012 at 14:33
  • Did you manage to get this solved? I seem to have the exact same problem and no one came seem to find a resolution. I'm despairing! Would appreciate knowing what if anything solved it for you.
    – user12614
    Apr 18, 2013 at 18:49

6 Answers 6


So, nearly a year later I think I've got this solved! It appears that the bypass valve was left almost fully open. This explains a lot.

The "path of least resistance" is for the water to use the bypass circuit to return to the boiler instead of going around the radiators, which explains why it was taking forever for the house to heat up.

Now, the water returning to the boiler is almost as hot as the water leaving, having lost very little of its heat in the brief trip to the bypass circuit and back. This explains why the boiler shuts down very quickly. It is expecting a much larger temperature difference between the output and the return and hence gets very hot very quickly. In the mean time, the pump continues to circulate hot water around the bypass circuit.


You've bought a programmable thermostat so you can lower the temperature of the house at night, however by doing that you are making it much harder work for the boiler to bring the house back up to temperature. Before your house was at a constant temperature so you boiler would turn on for a bit, off for a bit, on a bit, off a bit over the whole day. Now it goes off for a long time and your house gets cold, then it has to run a very long time to get it back up to temperature again, leading it to overheat and shut down.

It may be your boiler simply doesn't have the grunt to heat your house for that long. Try setting your night-time temperature a bit higher and see if that improves the situation.

It could also be that your system is gunked up, leading to poor water circulation in your system. Did you have your heating engineer put a cleaner in the system? If not, have him try it.

  • Thanks for your suggestions. Before we had our boys, the heating would be off overnight, so it would have to heat up from "cold" (whatever was lost overnight) in the morning. Now, it should actually be starting from a warmer place than before! It's only the last couple of years we've left it on overnight and felt the need for better night time control.
    – dty
    Dec 18, 2012 at 19:43
  • Also, when the plumber came, he commented at how clean the system was considering it was 9 years old. His original quotation had included adding an inhibitor to the system, but when he bled the radiators he said the water was almost completely clear and didn't need inhibitor, so I saved some money.
    – dty
    Dec 18, 2012 at 19:44
  • I'm not a heating expert, however my understanding is that there should always be inhibitor added even if the system is clean. One of the reasons it is clean is probably that inhibitor was added last time! I'd get a different heating engineer in @dty as I'm not sure you're getting good answers from the one you've got.
    – GdD
    Dec 19, 2012 at 9:34
  • Hmm... you may be right :-(
    – dty
    Dec 19, 2012 at 13:56

To help you compare, I'm in NY and my 2400 sqft home takes approx. 30-60 minutes to reach 20C (68F) when outdoors its 4C (40F). My house was built in 1960, but the windows are double-pane and were replaced ~8 years ago. The roof was replaced at the same time.

I'm far from an expert, but I would suggest getting a different professional to take a look. Here in NY, I have a contract with the Oil company to perform service was needed. They take care of any maintenance related to heating.

The scenario you presented, specifically that your house would heat up in a timely manner before, leads me to believe something was either changed or broken since the last maintenance. Note, I do not mean to imply it was the cause, just that it was since then.

A second professional opinion looks to be worth it.

  • I think I agree with you. What temperature do you keep your house at overnight? (i.e. how hard does the heating have to work to heat it in the morning) Or do you switch it off overnight? And what kind of heating do you have?
    – dty
    Dec 18, 2012 at 19:45
  • The temp in my house is programmed to 68F from 6am to 9:30pm. Then I allow it to drop to 62F overnight. This works best for my house. I know it since I'm using the Nest thermostat and clearly tells me when it's working and how long. I highly recommend it! I use oil heating in my house. With this schedule my heat runs for about 1 hr in total per day. When I set it to keep a constant temp of 68F the heat runs about 3hrs. I know this contradicts a post here, but I've tested this more than once.
    – Jose Leon
    Dec 18, 2012 at 23:57
  • I've seen the Nest controllers and they look awesome, but they're not available in the UK because they're incompatible with they way our heating systems work. (I believe all the wiring in the US uses 12v, whereas in the UK it all runs at "mains" voltage - 240V AC!)
    – dty
    Dec 19, 2012 at 13:55

The new digital room stats start shutting down in pulses before they reach the set temperature these pulses or gaps get longer the nearer it gets to the set temperature. This works great with a highly insulated house but feels like it never gets hot in an old house. You will have to turn up the desired temperature an extra degree to overcome it.

  • Thanks for your answer, Dave. As you can see, this post is over 5 years old. We don’t even live in that house any more!
    – dty
    Nov 8, 2017 at 23:32

Check your furnace. Every time I have this issue, it is due to the furnace. Sometimes it needs a new filter, sometimes it needs the flame sensor cleaned. I have never had an issue with the thermostat, always the furnace.

  • Hello, and welcome to Home Improvement. This doesn't add to the existing answers. You should probably take our tour so you'll know how best to contribute here. Nov 15, 2020 at 13:47

Do you leave an overnight temp on at all? Also, when this happened to me, my boiler would start firing off and on even when it was 2 degrees less than what I’d asked it to get to. I started being really ridiculous and putting it up to 29 degrees just to ensure it kept firing non stop

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