Hot Topics: Picking the Right Thermostat

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Original Post: Picking the Right Thermostat

TJMas - Member

I just moved into a new house and am looking at getting a smart thermostat. I grew up in a house with just oil heating and AC's in the rooms so a swap was easy, but now I have a CA system as well as oil. I just want to make sure I buy the right thermostat and know if I need to run a C cable. Here's what the guts of the older one look like.

old thermostat circuit board

wiring inside an old thermostat

PJmax - Group Moderator

Welcome to the forums.

You have two systems controlled by that thermostat. Two distinct cables.

Cable 1: The heat is on red (Rh) and white (W).

Cable 2: The AC is on red (Rc), green (G) and white (W).

You absolutely need a C connection and it comes from the AC air handler.

Check and see if there are any cut off wires in the AC cable.

TJMas - Thread Starter

Thanks for clearing that up for me. As far as wires around the AC unit I couldn't find anything that looked cut or out of place. It seems everything is hooked up right.

thermostat wiring

PJmax - Group Moderator

You have two options:

1) Run a new cable down to the stat. (recommended)

2) Lose control of the blower at the thermostat and use the fan wire as the C wire.

With this method there is no more "FAN ON" mode.

TJMas - Thread Starter

Sounds good, sorry if the info I was giving wasn't nearly helpful enough. By oil heating I mean that I have furnace and baseboard heating. And the cooling is a Goodman A24-00. You wouldn't happen to have a link to a wiring diagram so I can put that C wire in properly?

Houston204 - Forum Topic Moderator

C is the big light gray wire under your thumb in the picture that you posted.

The AC condenser connects to stat Y and air handler C so you will end up with two wires connected to the air handler C.

PJmax - Group Moderator

You have a boiler and baseboard heat—a furnace is hot air. A boiler is hot water.

Grady - Forum Topic Moderator

You have just one thermostat for heat? No others in other parts of the house?

TJMas - Thread Starter

It's a ranch so there's just the one.

PJmax - Group Moderator

So have we helped you make a decision? Any more questions?

The Ecobee 3 is a good choice. Honeywell T6 is another.

TJMas - Thread Starter

Yeah you guys have been a great help thanks a ton. I was looking at the Ecobee 3 before I took to the forums so I'll go with that and hopefully not blow up the house in the process.