Furnace wiring - Easier than you think! (Preparing for the "Nest")

Hi all,

In a previous post, I let you know that my Thermostat had died on me - It was getting stuck off, or stuck on... I took a look at my Energy bill... and could see exactly how much it cost me - A/C got stuck on for 3 hours, for a grand total of  3KwH of juice every hour it was stuck on (about 20 cents during non-peak/weekend time)

What I've also realized is that I can control the Furnace/AC by shorting out some connections.

Its all a 24 volt system... which may tingle a little bit if your fingers are wet... but generally one should be afraid of touching exposed wires in a building, let alone doing it with wet fingers. (Darwinism, Preservation instinct, are a few topics that come to mind)  

However, these wires (provided you have a "low voltage" 24volt system) are completely safe to handle.

So Depending on your setup, you'll have anywhere from 2 to 6 or even 8 wires. MOST places with A/C will have 4 or 5 wires. Heat only might just have 2.

Basically, There's a bunch of wires going to your thermostat.
In my case, "There. Are. Four. Wires!"  (we'll see who gets that reference)

I have a fairly standard setup.  (yay! something simple and straight forward for once!)

The Colors:

Red: 24 Volts - basically the power source to switch the other wires "on"

Green: Controls the fan/blower in the furnace - used for both Heat and Cooling!

White: Controls the Heating.

Black: (NORMALLY called the "Common" wire, also normally supplies 24 volts... but because my house is older, I can tell its been taken over to signal the A/C to engage, hense why its hooked into "Y" on my broken thermostat.

Yellow: (if you have it) is dedicated for Cooling. my "yellow" is being run through a repurposed Black "common" wire, which is fairly "common" practice (har har...)

More wires may exist if you have a new house with a fan-dangled high-end modern HVAC system thingofabob. (Like geothermal, Heat Pump, 2 stage, Variable Speed DC Fans, etc.)


Bottom line is, after looking at these wires, I have learned is that there is a 99% chance that the NEST Thermostat will work fine for me.

It is a mini computer, and leaches power from that 24 volt line, so ideally, That black line should have been "common" so it wouldn't weaken the red wires "signal" to the furnace - However the nest has a built in battery to help isolate itself and avoid problems - Here's hoping it works... or I'll be standing in line at the returns at Home depot in a few days!

Another trick looking into all this has taught me is that if you are ever in a bind, and your thermostat breaks down in the middle of winter - Just tie the red and white wires together, and your furnace will turn on. (or use a paperclip to bridge the connectors if you have a removeable faceplate like I do (pictured earlier on)

Because I'm a nerd, I'm actually super excited to get my hands on this Nest.

Next post: Installing the Nest!