mysteries of HVAC
Oct. 19th, 2010 10:35 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
eubWe went to replace the old thermostat with a new-style Skynet model that plays Farmville and dances. The old thermostat is a T18H-200 Cam-stat, dated to about 30+ years old by paint strata, and if the stamped "16-9-65" is a date, well, it's older than I am.
There are three wires coming out of the wall: blue, white, and red. The blue and white were connected to the old thermostat, and the old thermostat had no obvious contact points for anything more. The red was crimped as if it might have been attached to something in the past, but not now.
So how do these correspond to the seven lettered contacts on the new thermostat? Why do its diagrams range down only to three-wire, and can't I think of a thermostat as a temperature-controlled SPST switch these days? The Internet suggests that basically I still can, heating transformer means "the hot wire", and heating control means "the furnace wire", and all the rest is modern complication.
Okay, so which is the hot wire? I turned the furnace back on, and the multimeter says the blue wire is 18V AC relative to ground, where by ground I mean Katy holding the other prong to an electrical box screw across the hall. The white wire... is 4.5V to ground. Maybe this is not a good ground reference. Blue and white are some oddball voltages relative to the red wire, if you were also wondering if that's a ground. And blue and white are 27V from each other.
There is no triangle with edges 18, 4.5, and 27.
We're just going to figure that blue is hotter, and wire that way. *Probably* the worst that can happen is we blow up the new thermostat. And the day of reckoning is postponed 'til tomorrow since the patch behind the old one needed painting.
There are three wires coming out of the wall: blue, white, and red. The blue and white were connected to the old thermostat, and the old thermostat had no obvious contact points for anything more. The red was crimped as if it might have been attached to something in the past, but not now.
So how do these correspond to the seven lettered contacts on the new thermostat? Why do its diagrams range down only to three-wire, and can't I think of a thermostat as a temperature-controlled SPST switch these days? The Internet suggests that basically I still can, heating transformer means "the hot wire", and heating control means "the furnace wire", and all the rest is modern complication.
Okay, so which is the hot wire? I turned the furnace back on, and the multimeter says the blue wire is 18V AC relative to ground, where by ground I mean Katy holding the other prong to an electrical box screw across the hall. The white wire... is 4.5V to ground. Maybe this is not a good ground reference. Blue and white are some oddball voltages relative to the red wire, if you were also wondering if that's a ground. And blue and white are 27V from each other.
There is no triangle with edges 18, 4.5, and 27.
We're just going to figure that blue is hotter, and wire that way. *Probably* the worst that can happen is we blow up the new thermostat. And the day of reckoning is postponed 'til tomorrow since the patch behind the old one needed painting.
no subject
Date: 2010-10-21 02:38 pm (UTC)