It was about -15 here in Fairbanks and two nights ago someone sleepwalked and turned the thermostat all the way down for the upstairs living room zone. It's maybe 25-30 feet of finned copper tubing total. When we turned the thermostat up again in the morning there was no flow of hot water to the zone. I've been trying to learn about the system to DIY and ended up replacing the White-Rodgers zone valve innards (1311 3-wire) and power head today. With it demonstrably fully open the problem remains. I apparently replaced it for nothing. The zone right next to it, to the upstairs bathroom, has normal flow and heat, as does the ground floor zone. The boiler has 28 lbs. pressure at 205 degrees F. The upstairs zone has one air vent valve I can find, in a closet. I guess I can't attach photos of the furnace room and air vent. The air vent is a green cylinder about 5" tall and 1 1/2" or so in diameter that tapers to a silver 7/16" nut at the top that has a hole drilled in it near the base. My understanding is that the air vent is supposed to work automatically to bleed the system, is that right? I loosened up the silver nut with the hole in it (clogged by dust, it appears) about a turn and nothing happened. I was afraid to turn it farther. Last year we had the system glycol checked and replaced and there had been a bit of creaking when the system came online. Otherwise it has worked flawlessly since 2000 when we bought the place. The only other thing I can think of is that the pipes run through the wall under a door to the second story deck; the section in the wall couldn't have frozen, could it? We have had -50 degrees here without an outage previously. Any advice regarding further bleeding procedures to follow or anything else would be greatly appreciated. I have searched the forum and couldn't find a specific enough thread to get the answer I need.

 

It sounds like it might be frozen. Those temps are way beyond anything I'm familiar with. You might try these guys, http://forums.invision.net/Main.cfm?CFApp=2 They have some pros from your neck of the woods on there. It's not a DIY site though.