'C' blue wire on furnace thermostat

[pepe]

Any of you guys know about furnaces.

I've got a Tempstar F9MVT0801716A1 furnace. The new thermostat I got needs a 'C' (blue wire). I've got a blue wire at the thermostat and furnace but they are not hooked to anything. My control board screw 4 has a COM 24V terminal. The red wire from the outside compressor is hooked to it. Is this were the blue wire would be hooked to? Sure don't want to trash out a 500 dollar control board. Thanks guys

[I'MAPUNKROCKERDAMNIT]

I'm only familiar with old school, two wire thermostats that simply make and break the circuit..
My last house had 120v two wire. I had to test the voltage before buying a new one because the old thermostats didn't say if they were 24v or 120v.

Even if your's is 3 or 4 wire, shouldn't the new thermostat be hooked up the same way as the old thermostat?
If the blue wire wasn't hooked up on the old thermostat, why do you need the blue wire on the new thermostat?

[angrytortoise]

Because a lot of new thermostats have battery power, like the Google Nest. It will run without that blue common wire but it constantly drains said battery unless that wire is hooked up. It constantly recharges the battery from the air handler.

[Landshark]

ran into this problem with my mom's Nest T-stat ... if i remember right, there are two ways to fix it ...

- adapter that plugs into an outlet near the T-stat. you would probably want to snake this wire down the wall and have it exit at the bottom, hopefully near an AC outlet so you don't have a wire hanging from the T-stat to an outlet.

https://www.amazon.com/dp/B09Y1XP2GR/re ... GhlbWF0aWM


- adapter that you wire near the furnace control board and uses the existing wires run to the T-stat. i went with this method.

https://www.amazon.com/Wire-Accessory-B ... afb5986a15

just remember to take good pictures of the T-stat and furnace control board wiring before removing anything. i did not research either of these products - just grabbed these two as examples. check the descriptions and reviews before buying ...

[linthat22]

I'd try what Landshark said. Our last house was built in the 40's and had to drop new wire to the furnace, so I'm of no use on this one.

[pepe]

Yep ... Just hook the blue wire to the COM 24v terminal and I'm in business. The unit is to new to not have a 'C' connection. The main board is marked weird compared to other units. Thanks guys.

[IndyWS6]

^ Ha ha - fuck off Ruskie

[Landshark]

ban hammer, aisle 2 ....