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Combi Boiler Heating Cycling on/off Whilst Constant Call for Heat

Discussion in 'Plumbing and Central Heating' started by kevkdg, 27 Mar 2018.

  1. kevkdg


    27 Jul 2007
    Thanks Received:
    United Kingdom

    Vokera Compaxt 24.

    Engineer changed Heat Exchanger today as old one had crack causing pressure in CH system to rise.

    However, the Thermostat in the Hallway is constantly calling for heat as I set the target temp to 23 degrees, and it's not getting over 19.5 degrees. Plus there is a little triangle on the display to signal it is calling for heat.

    The boiler on the other hand seems to cycle on and off, NOT really very frequently, such as every 2 mins, it seems to be a good 20 mins to half an hour before it turns off for a few minutes, then comes back on. However, in this period of downtime the radiators cool off a little, and the thermostat is struggling to rise above 20 degrees. Heating has been on for a few hours now. All radiators get nice and hot whilst the boiler is running.

    My question is should the boiler cycle on and off if there is a constant call for heat from the thermostat? Or does this suggest a potential issue.

    The engineer who changed the Heat Exchanger seems to think that it is normal behaviour.

    Hot water seems fine, behaves as it always did. When you first turn shower on it gets hot then goes cold for a few seconds before going hot again and staying hot.... although I'm not sure that is right either!

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  3. chrisw99


    5 Apr 2004
    Thanks Received:
    United Kingdom
    There are two thermostats in play here, your room one which calls for heat and also the thermostat on the boiler itself which controls how high the system water can get (i.e. the water inside the radiators). Once all your radiators are nice and hot, if the boiler kept burning they would get hotter and hotter and the boiler would overheat, so it has to cut out for a bit. The radiators should still stay hot, and once the return water drops below a certain temp it fires up again.

    It's usually a setting you can change on your boiler, not to be confused with the setting for the hot water temperature from your taps.

    I noticed my boiler was set at 74C recently and the radiators were too hot to touch. I've dropped this to 67 and the radiators are still too hot to touch and warming the rooms nicely, so that extra gas to reach 74 was probably a waste of money :(
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