A month or two ago my ~25 yr old basic programmable thermostat suddenly died and new batteries wouldn’t wake it up.
But not needing heat since March and having no A/C I put off replacing it for a while. But today I got 20% off on a $25 Honeywell so I got it.
Note that I don’t need or want a “smart” thermostat, so please don’t suggest this. My home is partially heated via a sunroom and we often go a week or two in a row with the heat shut off in the winter when it is sunny enough…so each day we may decide to turn on the heat or not. And on a few cold nights a year we may leave the heat on at night with the programmable features, but this is rare.
So I removed the old one and it had just 3 wires connected (R, G and W…24v, fan and heat).
When I look inside the furnace I see Hum, Com, W, Y, R and G with only W, R, G connected and the others wrapped up unused. All the online install videos show that the Blue (Common) is used.
(I don’t have a humidifier, so I don’t need Hum. Y is for A/C…don’t need that)
Questions: should I connect this? Also, if needed how was it working before without this connected?
I am currently waiting for a big spackle spot to dry so I can paint it since the new thermostat is smaller than the old one, otherwise I would have just connected the 3 wires and tried it already.
Right now, we have a thermostat hanging from a hole in the wall with only 3 wires connected. But it is just to run the AC to dry and cool the house. The heat would not be connected since we do not have the propane installed yet.
Questions: should I connect this? Also, if needed how was it working before without this connected?
The basic thermostat interface to a furnace is very simple.
Red supplies power, if that is connected to white, the thermostat is saying “turn on the heat”. If that is connected to green, the thermostat is saying “run the fan”.
If your thermostat runs on batteries, (or is an old mercury-switch one that doesn’t need batteries) then there isn’t a need for “common” to be connected.
If your thermostat uses power supplied by the furnace, it (probably) needs common connected. The power and common wires will provide the energy to the thermostat so that it can run it’s electronics and connect/disconnect the red-to-white and red-to-green connection.
If it were me - I’d connect common (black or blue) in the furnace and at the thermostat, and stop buying batteries for the thermostat.
But I’d also buy a “smart” thermostat so that I can turn up/down the thermostat from my phone. That way if I’m away for a week I can turn the heat off/down, then turn it back on/up as I’m driving home.
(And I do own an ecobee thermostat so I can do that as well as set schedules, etc.)
If your thermostat runs on batteries, (or is an old mercury-switch one that doesn’t need batteries) then there isn’t a need for “common” to be connected.