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We recently bought a house that had been renovated in the 90's. It is a 4 zone hydroponic water baseboard system. We've noticed that the thermostats do not seem to control the heat very well. I replaced all the thermostats and the problem persisted. After some careful testing, I think I know what is going on.
When zone 1 is firing up, all the other zones' radiators seem to get hot too. In the basement, the control panel shows only zone 1 lit up and only the zone 1 pipe that is connected to the control panel gets hot. So far, so good. However, I have noticed that there are another four pipes behind those going back into the boiler. All four of those pipes get hot.
So, it seems like the thermostats are working and the control panels/valves are ok, but somewhere in the house, the zones get merged together. Is there any way to fix this? A diagram of how I think it is supposed to work is attached. All the pipes represented in the lower right corner get hot when only one zone is on.