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Discuss Hot water pouring into the central heating expansion cistern in the Central Heating Forum area at

  1. Paul Taylor

    Paul Taylor New Member

    Homeowner here trying to solve central heating problems. Just realized hot water pouring into the central heating expansion cistern from CH supply pipe vent, am I right in thinking that essentially the boiler is producing more hot water than it knows what do with and this is causing some to go up vent pipe into cistern.

    And this could occur if there is a blockage somewhere else in the system or if all radiators were turned off preventing flow through them, or if pump was circulating water too powerfully?

    Are these reasons correct are they any other reasons ?

    Some more details, I did not have this problem a few days ago. But two radiators sharing 15mm plastic pipe (in loft but 1.5m underneath expansion tank) were refusing to heat up, though the supply pipe to both was warm and I had tried rebleeding.

    So I turned off every other radiator in the system went out, left it for a few hours and found the two radiators were still only just lukewarm (the radiators are brand new so don't have sludge problem) but hot water pouring into expansion tank, and return pipe into boiler not much cooler than supply pipe

    I immediately reopened all the other radiators and left for a few minutes but water still gushing into expansion tank, maybe it takes longer than that. Because I had the radiators on earlier and the house was already warm I thought maybe the thermostatic valves were preventing water going in anyway since rooms already warm temperature. So I turned system off and will retry when cooler to see if water goes into tank when all radiators being used, having tuned off system gushing stopped.

    Also there is a T-valve controlling supply pipe and whether it does central heating, hot water or both. Even though set to Central heating( on both programmer and valve itself) the hot cylinder supply pipe was hot, it is not usually.

    My boiler is Worcester Bosh 24R, I have Grundfos selectic UPS 15-50 circulating pump, set to speed 2

    I have access to the problem radiator piping (goes up wall into eaves and then radiators, teed in the eaves) but I don't know how to establish there is indeed a blockage and if so how to resolve it, since I have already tried the approach of forcing the supply to those radiators and of bleeding a pint of water to each radiators first with supply closed, return open and then vice versa.

    The water in the cistern was a bit bubbly on top, is that normal or does that indicate an air problem ?

    Related, in normal operation if you have central heating on how does the boiler know that theres plenty of hot water in system and that it doesn't need to heat anymore for a bit. Does it look for sufficient temperature differential between supply and return or does it just keep heating regardless ?

    Does it rely on a room thermostat, I don't have one in my system but I do have thermostatic valves on all radaitors.
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