Two Zone Boiler System. One calls for heat, both zone return lines get hot

Question

I have a newly replaced two zone boiler system and am having issues. One zone is the basement only, the other zone is first and second floor. I have basement zone at 70 and it functions as expected. However, even with the upstairs zone thermostat at 62 the temp reads 73-75, some radiators in the upper zone are HOT as are the return pipes from all zones near the boiler. Basement pump is warm or hot, other zone pump is room temp. Air whooshing is ongoing despite bleeding. Water pressure goes between 12-17 poounds. Contractor seems stumped so open to feedback/ideas. Thanks!

Answer 1

Probably thermo-syphoning (heat delivery without pumping. Hot water rises and cold water falls. Might be aggravated by the comically large iron lines (old steam system, converted to hot water? Probably even an old gravity steam system, so everything is designed to thermo-syphon?)

You can verify by turning off both thermostats for a while while the boiler remains hot (unless you have a "rapid-start" or "cold-start" boiler that's only hot when there's a call for heat.)

Check valves that permit pumped flow but won't open on thermo-syphoning micro-pressures, or deep U-bend heat traps are typical approaches to limit that.

It's sadly common for HVAC folks to be blithely ignorant of the problem and solutions. They shouldn't be, given how common it is, but since they never install gravity systems these days they don't think about it, as far as I can tell.