Replacing an old thermostat. Energy Savings:

Replacing A Plain Thermostat
With A Set-Back Thermostat

In This Article:

An existing thermostat is removed and replaced with a new energy-saving set-back thermostat. Please read the mercury warning at the end of this article.

Related Articles:
Skill Level: 2 (Basic) Time Taken: About ½ Hour

By , Editor



Perhaps the easiest way to reduce residential heating and cooling energy usage is to turn back the thermostat when people are away from home or sleeping. By "turn back" I mean two possible things:

  • Lower the temperature during heating season;
  • Raise the temperature during air conditioning season.


Why Does This Save Energy?

One of the fundamental laws of the science of Heat Transfer is that the rate of heat transfer between an object and its surroundings is proportional to the difference in temperature between the object and its surrounding environment.

If there is a big difference in temperatures, your house will lose more heat per hour than if the temperature difference were small. This is intuitive: your furnace runs more on cold winter nights than during milder spring and fall weather.

A couple of years ago I wrote a little article about the science of heat transfer, and it explains this in more detail.


The basic Honeywell thermostat.

This device controls both heating and air conditioning. It also has a fan control switch, which you can see in the photo, that lets you run the fan all the time.


These Honeywell thermostats have a plastic ring that just pulls straight off.


Thermostat without the cover.

There are some exposed electrical connections here, but this is only 24 volts AC.


In this house the furnace is in the crawl space.


Turning off power to furnace heating system. I turned off the power to the furnace.


I also could have simply turned off the breaker for the furnace, if I hadn't wanted to go into the crawl space. Not all houses have a separate circuit breaker for the furnace.

All furnaces are supposed to have a disconnect switch (which may be a simple light switch) within reach of the furnace. On some very old houses there may not be a disconnect switch.

There are 3 small screws that hold the Honeywell thermostat to the base plate.


Removing round Honeywell thermostat. The thermostat came right off, with no wires attached.


The base plate has all the wiring connections.

The blue wire was connected to the terminal for the yellow wire, which is marked "Y".

All the other wires were connected to their respective terminals.


I unscrewed four connectors.


I found it necessary to use pliers to carefully unwrap the wires from the screw terminals.

This is why you need to turn off the power to the furnace. There will be 24 volts at some of these wires, and if you touch the wires together you will cause the furnace (or air conditioning) to momentarily turn on. That's bad for equipment.

Removing thermostat base from wall. I removed 2 screws that held the base plate to the wall.


I removed the base plate, and pulled the 4-wire cable through the hole.

At this point remember one thing: Don't let that wire fall back inside the wall cavity, or else you'll have to fish it out, which can be a real chore.


The Replacement:

This is the Ace Hardware ATX500 Series programmable thermostat.


There is a release tab on the bottom of the thermostat to separate the unit from the base plate.


This thermostat also employs the base plate as a connection point to the control wiring.

There are 5 metal pins that provide electrical connections to the actual thermostat.

This thermostat has 5 connections, but two connectors are joined with a jumper wire, which you can see in the photographs.

A small problem: If I simply mounted the new base plate, there would be some unpainted drywall exposed.


So I enlarged the hole with a knife.

This will allow the wire to move over.


I held the base plate against the wall and marked the screw holes. I was able to reuse one of the old holes.


Fastening new setback thermostat to wall. I drove in the two mounting screws. I was surprised to find wood behind the drywall, so I didn't need any drywall anchors.

I used my cordless impact driver here, but using a simple screwdriver might be a better idea because it's easy to over-tighten small screws and ruin something.


The instructions showed which wires to connect.

There are several different wiring schemes out there, and I'm not familiar with all of them. This 4-wire scheme (for heating and air conditioning) is quite common.


Connecting wires to new setback thermostat. I connected the 4 wires to their proper terminals. I didn't need to curve the wire into a hook, just slide the wire under a metal piece and tighten the screw.


I installed the batteries in the new thermostat.


I snapped the new thermostat onto the base plate.

Then I turned on the power to the furnace.


I cranked up the temperature to see if everything worked.


The finished installation.

The programming for this thermostat is explained on the fold-down cover.

This thermostat has 4 set points per day, which is meant for all you workin' fools that spend all your time away from home. The temperature can rise in the morning, then fall back during the day when your gone. When you get home late in the day the temperature has risen again, and then falls back at night while you sleep.

This product has default time and temperature settings, but of course you can re-program the thermostat to your own preferred settings.


Warning: Old Thermostats May Contain Mercury!

Many old thermostats, such as the Honeywell unit that I removed in this article, use a mercury switch to turn the furnace on and off. Mercury switches are very effective and are part of an excellent design, but with one dark side: if that mercury ever gets out of its little glass bulb it becomes a serious health risk and an evil environmental pollutant. A recent article in Discover magazine illustrates how truly wicked mercury can be. The liquid metallic form, which is inside the mercury switch in a thermostat, isn't so bad, but the vapors given off by the liquid can be lethal in amounts measured in micrograms... that's millionths of a gram! When mercury gets into the environment, whether it be the soil, air, or water, it just wreaks havoc. Forever.

DO NOT throw away an old thermostat in your household trash.

Call a local heating and cooling supply shop (the kind that mostly cater to mechanical contractors) and ask if they will dispose of the old thermostat for you. In my area there are several heating supply companies that will gladly recycle old thermostats with mercury switches. Also, many cities and states have a hazardous waste disposal place for consumers with small amounts of toxic stuff like mercury.



Tools Used:

  • Cordless Drill/Driver
  • Screwdrivers
  • Pliers

Materials Used:

  • Setback Thermostat, Ace Hardware ATX500 Series
  • AA Batteries (2)

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Written February 25, 2005