Wiring a Thermostat – For a Furnace

Ok — I had originally written a post on this at furnacebook.com but feel the need to ad a tutorial here

We are going to wire a basic furnace without air conditioning . I will add photos to try and make it easy for you.

There are usually only 3 wires needed to wire in a new furnace or an old one.

The colors are usually Red (24 volt power into thermostat) Green (fan on off power) and white (24volt back to the  furnace)

This is all fine and dandy if your living in the 60′s 70′s and 80′s, things have changed since then.

Now dont go throwing in the towel yet this is very easy when we toss in diagrams, so stick with me.

If you have two or three wires coming  into your thermostat and you have any kinda project (changing the thermostat) I highly recommend that you change the thermostat wire. I would buy a 12 strand #18 wire to adjust for the future (maybe an air conditioner or humidifier or something else)

You know as i started this it was going to be a simple 1 2 3 and your out of here tutorial but its really not that simple.

I am a big fan of simple. If your thermostat is working (dont care how old it is) keep it on the wall and keep it simple. However if its old and needs a few simple little adjustments , keep the ding dong thermostat and dont get a new one. If you need to work on your current thermostat then check out my DIY thermostat repair page. Again i do not advocate changing a working 20 year old thermostat for one that is electronic and will last you two years before you have to buy a new one.


Thermostat wiring

Thermostat wiring


In this diagram I have drawn the normal wire colors. The arrows indicate the direction the electricity flows to and from the thermostat. The power comes into the furnace (thick black line). There is a transformer that changes it from 110v or 220v to 24volts (or control voltage). The red wire sends 24 volts to the furnace. Then the thermostat acts like a switch and decides to either send 24 volts back to the furnace (if its cold in the home) or not send the voltage back to the furnace.  The green wire is for the fan on/off switch only and will turn the fan on or off from the thermostat. The fan will turn on or off automatically when the thermostat is calling for heat. There you have it. Dont run off yet we have one more item to clear before you are a professional.


One other thing that should be noted here. The older thermostats have anticipators that have to be set on the thermostat.  A lot of folks dont know about these but your about to get educated.

What an anticipator tries to do is guess when the themostat will reach temperature, then shuts the burners off at the right time so that the furnace will not overheat the home.

The newer thermostats (electronic)  have this automatically built into them but the older ones do not. Heres a photo

A heat anticipator

Heat Anticipator

The anticipator is the dial in the middle with an adjustment arm. This can be turned clockwise or counter clockwise. The device is a tool to allow the furnace burners to run longer or shorter in the furnace/heater on cycle. In fact is you look closely the dial will say “longer” and “shorter” on it.

To properly and easily adjust this you should have a multi meter with the ability to read amperes (simple $10 clamp on multimeter will accomplish this) .

Take a look at the next photo and see how to put on the multimeter to get a proper reading.





Setting anticipator

How to test for anticipator setting


Here we have a multimeter set to read amperes. I have taken the wire (white wire) next to the thermostat and coiled it up three times. I will then turn the thermostat swtich to heat and start the furnace. Take the reading on the ampprobe. Divided that by the number of coils you made in the wire. Then set your anticipator to that number.

HINT: it is usually bettween .5 and .9

In fact if you dont have an amprobe and cannot afford one or just plain ole dont want to do this I would start out my anticipator at .9 and work from there.

If you notice the furnace overshoots the setting on the thermostat set it to shorter like .8 , if you notice it short cycles and does not reach temp with out having to restart up again set it up a little bit longer.

So there it is . At this point you should be as competent as a service technician with simple thermostats.

However I will be doing a tutorial on how to repair a thermostat in the near future.

All comments welcome – except for bad comments about the artistry

Category: Heating

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