Why your gas condensing boiler may stop working in very cold weather

There has been a very short but very cold spell of weather in theUnited Kingdomrecently.Londonhas even seen a little snowfall, but the temperatures, which in some parts have fallen to -10 Celsius, are very unusual for this part of Europe, which is warmed by the Gulf Stream, which prevents the British Isles having the same weather as say,Nova Scotia.

So, if you live in Britain your heating system has been working overtime to cope with the weather. Around 80% of Britons heat their homes with natural gas boilers and for the past few years (since April 2005, actually) the only gas boilers that have been permitted by law are condensing boilers. These effectively and efficiently recycle waste heat and gases and increase efficiency when it comes to space heating, however they do not make much difference when it comes to their efficiency of water heating, compared with the old conventional boiler. However, they can stop working in very cold weather if they are not set up correctly or not correctly installed, and if this happens to you, possibly this article will help you have the problem sorted out.Condensing boilers work on the principle of a cooler return temp 50-55 deg Celsius. This return temperature coming into the boiler is designed to meet the hotter exhaust flue from the boiler in the heat exchanger, when they meet the heat is extracted from the exhaust flue by the heat exchanger by the cooler temperature, this causes condensation to form on the heat exchanger; it is this by-product of condensation that is taken away by the condense pipe. It is this pipe that can freeze and cause problems.

 The flue does, however, expel water vapour (a plume of it) as steam and the boiler will regularly dump waste water into the soil drain through its condense pipe.

Most condensing boilers are set up to operate in normal local conditions. In very cold weather, especially overnight, you might find that your boiler does not work because the condense water has frozen across some of the lower condensate pipe, or else frozen into the boiler and in either case this will prevent the boiler from working.

Sometimes If the boiler attempts to work and finds its condense pipe blocked it will often be programmed to retry later and this constant re-trying can adversely affect the boiler.

If the condense waste water has backed into the boiler or home and frozen serious damage could result, which will involve a very costly repair

I do not recommend that you try de-icing the pipe. You might cause more damage and any condensing boiler should only be serviced or mended by a trained and qualified plumber having experience in that type of condensing boiler. Good plumbers use a wide condense pipe (which reduces the risk of freezing) and insulate them outside the home.

One solution adopted in Germany, where they regularly get much colder winter temperatures is to program the heating system to operate throughout the day and night, but at  lower night time temperatures than the daytime temperatures. If you find 20 degrees Celsius comfortable for the day time in exceptionally cold weather it might be worth programming the boiler to operate at 14 degrees during the time that you normally sleep when it would otherwise be turned off.

If the boiler has been properly programmed it will probably save on gas because the home would not have be heated from a very lower temperature in the morning when heating normally comes on.

With the boiler operating throughout the night, when temperatures are normally at their lowest, the condense pipe will be kept clear of ice and you will have no nasty surprises when you wake up.

Some very sophisticated boilers have the facility to do this automatically, so check with your instruction book first and if you are unsure it is worth hiring a plumber expert in your kind of condensing boiler to check the control system to make sure that it is correctly set up for this when you have your annual service.


89 Responses

  1. Get a ubend with a blank cap fitted to your condensate pipe outside so that if it freezes up at ground level you can drain the pipe of into a bucket etc This works like a charm and is simple and inexpensive. If the condensate in the pipe has frozen pour some hot water oevr it and it will quickly thaw out.Leave your screw on cap off for the duration of the cold snap

  2. Dear Mr K,

    I wish I had seen your comments before purchasing a Worcester-Bosch condensing boiler through British Gas. My previous boiler (with cold water tank) worked fine for 15 years. But because I wanted to get rid of the tank and immersion heater (both located in attic) I went for the new boiler.

    The W-B has never worked properly even when still under guarantee but because the problems would only arise when weather temperatures were freezing and BG engineers would only turn up after the thaw, they would say that there was nothing wrong with the boiler or the installation. At no time did any engineer point out the main flaw as you have described. In fact on one occasion the engineer behaved as if I was just imagining the fault (of course, being female I could not be expected to understand the workings of these machines) and did absolutely nothing. Even when I had the first annual check, the BG engineer (a hugely obese, smelly man) did no checks and didn’t even empty the magnetic gizmo attached to remove sludge.

    From the first week of installation in July 2009, I used to have to adjust the water pressure at least 3 times a week. When I mentioned this to the engineer (condescending one), he said that I would have to purchase some sort of pump and have it attached to the boiler to increase the pressure. My next door neighbour has the exact same boiler and she never had to adjust the water pressure.

    My neighbour advised me to pour boiling water on outlet pipe as this works on hers when it freezes. However, my outlet pipe goes down 1 metre, then along the outside wall for 1 metre, then round another corner for 20cm then down into the drain. So pouring hot water has no effect. All pipes are lagged but clearly this has no insulating effect. I was thinking of cutting the first downward pipe to 30cm length so that the frozen moisture does not have so far to go but am not sure if I should carry out this DIY.

    Needless to say, these last two weeks of extreme cold, my boiler works intermittently – basically when it feels like it (honestly, I’m now giving it human negative qualities) and today (Friday 25 Jan) when it is not even that cold the blasted thing just won’t switch on so the thermostat shows 12.5 degrees. Even when it switches on, the temperature hardly goes above 16 degrees.

    If it weren’t for the fact that I am in a terraced house and being sandwiched in makes it slightly warmer and also I have purchased several oil electric radiators for the really bad days, I would be wrapped in a blanket for the remainder of winter.

    Sorry for the long moan. In a nutshell, if the individual boiler was faulty then I wasted my money, but at least I can buy a new one (if I can afford it – though never again from British Gas and never again a Worcester-Bosch). If the system of condensing boilers are not what we have been made to believe, then whichever boiler I purchase as replacement I could potentially have the same problems.

    • You are entitled to moan because the service you have experienced has been rotten.

      If your neighbour’s condensing boiler works well off mains pressure so should yours, unless there is a complicated route that your mains takes in the house before it reaches the boiler. Loss of pressure may mean a small weep near a radiator valve, which you might not spot because the heat may evaporate it too quickly. You alos should ensure that the system is bled of air regualrly (if you have to top up the pressure there is dissolved air in the fresh water coming in), and then balance the radiators.
      You might change the route of the condensate pipe and perhaps increase its diameter.
      If you buy I new boiler I found that ATAG are really reliable and very efficient.

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