I have a condensing boiler system where we keep some rads off or low using TRVs (upstairs) and others on (downstairs). As far as I can work out it is a 2 pipe system. It worked fine until last November when the rads that were on downstairs suddenly lost heat to varying degrees. When I turned the upstairs rads on, the downstairs ones got hot again. When the rads lose heat, it is not due simply to reaching the room stat temp as the CH motorised valve remains on.
An engineer came to look and rebalanced the system and it worked fine until the other day when exactly the same thing happened. I can't see the point of rebalancing it but something odd is clearly going on.
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