I just bought a ranch with a finished basement. The basement was finished in the last 15 years, and the previous owner had baseboard heat added to the finished basement during the renovation. The house is currently served with hydronic Heat, radiators upstairs and baseboard heat in the basement. The basement heat is separately zoned through a second circulator pump which I assume was installed during the plumbing rough-in when the basement was finished. The issue that I am experiencing is if the heat is off in the entire house, and I turn the basement thermostat on, I get zoned heat in the basement, as expected. The heat is on in the basement and off upstairs, as it should be. Here is the problem. When I perform the same experiment upstairs, the upstairs thermostat seems to override the basement thermostat, and turn heat on in the basement as well even though that stat is OFF. Clearly, I have zoned heat considering I can isolate the basement heat from that stat when the upstairs stat is off. Why is my upstairs thermostat overriding the basement thermostat and providing heat to the entire house as if I have one zone? One thing worth mentioning, I noticed that I have a single zone switching relay. Could that possibly be the reason? I don't think this is a plumbing or check valve issue, this is a controls issue. Any help is appreciated at my basement gets up to 80 degrees when I turn the upstairs stat to 70!