Forced hot water baseboard 3 zones (1st floor, 2nd floot & basement) + Boilermate Hot Water heater.

1st floor, basement & HW work.

No heat on 2nd floor.

I twisted wires together at t-stat
Got 24vac at the electric control box & at zone valve.

"Taco" brand zone valve, has lever for "auto" or "open" tried moving it to open, still no water flowing thru zone. (if I shut the manual valve on the other side of the system I can hear the water stop on the other zones. When I opened the drain for that zone, water came out.

What do you think? Frozen pipe? (It was 0 this morning). We let heat go down to 50 when we aren't there (Auto thermostats), but I do fire up the wood stove (on 1st floor) before I leave for work, usually puts out heat for 7-8 hours. Or maybe the zone valve is sticking? I guess I should call the plumber either way. (both are beyond my abilities to fix).

I closed the manual valve & disconnected the T-stat in hopes of isolating the zone in case it is a frozen pipe & decides to burst & flood the house.

Please help!