thermosyphoning

Kevin Seiler

I have a single story home that just had a three room addition. The crews installed a new 3/4" run to the new addition while the original system had a parallel 1 1/4" run. The new zone has a single 3/4" W/R 1361 zone valve while the original house also has one W/R 1361 to serve the two 1 1/4" runs. The zone valves were put on the return side just before the pump. The zone valves also have isolation valves just before them for service reasons. When we wired up the new 3/4" zone and fired it up we got hot water finding it's way out of one of the 1 1/4" supply lines of the other zone, even with the isolation valve closed on the return line of this 1 1/4" loop. (The isolation valve serves both 1 1/4" supply loops on this original house piping.) The guys say there is no cross piping. The original house is now overheating. Can thermosyphoning be the issue even though it is only a single story house. All areas are served by baseboard, old house is iron while the new is finned tube.