Underfloor Downstairs Hot, Radiators Upstairs Luke Warm: Any Ideas?

12 Nov 2019
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United Kingdom
We have underfloor heating on the ground floor of our house (wet underfloor heating system). The upstairs is radiators. Both work off the boiler. We are finding that when the thermastats downstairs and upstairs are both set to heat the whole house, only the ground floor underfloor heating gets warm and the radiators upstairs hardly get warm at all. A few radiators are barely luke warm and the rest stone cold. If we turn off the downstairs underfloor heating by turning the downstairs thermastats to off and leave upstairs thermastats turned on, then radiators upstairs start to get hot. It is as if the underfloor heating is robbing the upstairs radiators of heat and the two don't want to work together. Odd as never used to be an issue. Any ideas of what could be causing this? We have bled the radiators upstairs and all fine. When the downstairs underfloor heating is off and we just set the upstairs radiators to come on they heat up nice and hot so doesn't appear to be a bleed issue? They just won't come on properly when downstairs heating running?
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A balancing issue would cause that, but unless someone has been tampering with the system and caused the system to go out of balance, then it could be circulating pump not working fully or part blockage in pump or in pipes somewhere.
There probably is a zone valve (motorised valve) for your upstairs radiators circuit and it could be seized nearly off or blocked, so another thing to check.
Make sure someone checks any bypass valve that it is set and functioning correctly.
Hi ReJect, thanks for the reply and list of suggestions. I did think it could be the pump. However, looks like it could be any number of issues. Not a DIY task for me. Will call in a plumber. Thanks for the advice.

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