Honeywell digital room thermostat (C) Daniel FriedmanHow to Install or Replace a Room Thermostat
12 steps & 15 minutes to install a new thermostat

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How to install a heating or cooling room thermostat quickly and successfully:

This article describes 12 easy steps in removing a room thermostat and installing, then wiring-up its replacement. We show how simple it is to replace a wall thermostat. We add a few warnings that can help avoid trouble along the way to an easy 15-minute procedure to install the new thermostat.

All you need is your new wall thermostat, a medium or small flat head ro phillips screwdriver, and perhaps the patience to read the instructions from the thermostat manufacturer. We also include toll-free numbers that allow you to call the thermostat manufacturer's hot line in case you have a question or a difficulty.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Step by Step Photo Guide to Removing the Old and Installing the New Room Thermostat

Traditional wall thermostatBefore Buying Your New Thermostat be sure you actually need one

First are you sure you actually need a new thermostat? If you are buying a new unit because the old thermostat is "not working" be sure you have checked a few things:

Ok so forget all that, we're ready to plow ahead.

How to Choose & Buy your New Thermostat

Honeywell room thermostat

You may have already purchased the replacement thermostat you want. If not, the number of choices is both small (in some big box stores) or huge (more than 250 choices at an online store like

Honeywell digital room thermostat (C) Daniel Friedman

Watch out: Most new low voltage thermostats can operate almost any kind of heating and cooling system, but here are some things to watch out for:

Honeywell digital room thermostat (C) Daniel Friedman

At THERMOSTATS, HEATING / COOLING we describe different types of room thermostats, including both very simple models and more sophisticated programmable room thermostats.

Technical Details about Thermostat Features

At HEAT ANTICIPATOR ADJUSTMENT we describe choices among thermostats that have a heat anticipator and those that don't, and we explain the situations for which you might want a thermostat that has this feature (such as if your heating system tends to overshoot or make the room hotter than the thermostat setting). Usually you can skip worrying about this.

But if you know you want this feature, see WHERE TO BUY a Room Thermostat with a HEAT ANTICIPATOR. That same article explains why you would or would not care about heat anticipators.

At THERMOSTAT CYCLE RATE SWITCH you'll see a summary of some hidden or internal switches that are found in most replacement thermostats and that adapt the thermostat to a variety of uses including the control of fans and heat pumps or backup heat for heat pumps.

OK so you've already got your new thermostat. We've already spent about as much time reading the above as it should take to remove the old and install tine new room thermostat.

Remove the Old Room Thermostat

Old thermostat to be removed (C) Daniel Friedman

Below we've simply popped off the thermostat face.

Old thermostat to be removed (C) Daniel Friedman

Here are the steps to follow when replacing a thermostat:

Low Voltage Transformer (C) Daniel Friedman

1. Turn off power: At above left, here's the old thermostat that we're going to replace.

You can see by the fact that the display contains numbers that I have not yet turned off electrical power. If I short wires together maybe nothing will happen, but there's a chance I'll damage other heating system components or possibly blow the transformer (blue arrow in our photo).

So we turn off power before going any further.

Watch out: Turn off electrical power to the heating or cooling system before messing with thermostat wires. If you need help see ELECTRICAL POWER SWITCH FOR HEAT.

2. Pull off the thermostat assembly. You can see the neat job I did in wiring the old unit to be removed - at above right.,

3. Take note of the wires that are connected in the old thermostat: note the wire colors and the identify of the terminals to which each is connected.

Watch out: because heaven knows what fools may have messed with thermostat wiring before, do not assume anything. Indeed in this case we can see that just two wires are being used: a red wire and a white wire. But also check to confirm that on the old thermostat the red wire was connected to the R terminal and the white ware was connected to the W terminal. But you might find something else.

Removing old thermostat - wires extrended (C) Daniel Friedman

4. Disconnect the old wires and label them if necessary

Removing old thermostat - wires extrended (C) Daniel Friedman

Honeywell and just about any other thermostat manufacturer will give you a set of stick-on labels that you should use to indicate not the actual color of the wire but the ID of the thermostat terminal to which the wire was connected on the old thermostat.

Since I had just two wires, red and white, and because the red was connected to R and the white to W on the old thermostat, I didn't use these labels. But if wire colors didn't match the terminal IDs or if I had multiple wires I would use the labels.

Notice that I pulled the wires out and un-kinked them. Power was off so I didn't worry about taping bare wire ends.

5. Position the new room thermostat mounting base in place & mark for drilling pilot holes for screws. You've already unpacked it - and read the installation instructions, right?

I'm holding the thermostat base on the wall as close to level as I can get it. Notice those two orange lines I added to the photo?

Those mark the screw openings for mounting the thermostat base. Notice that the left screw hole is oblong and horizontal in shape while the one at right is vertical - these openings will allow slight adjustment of the backing plate to perfectly level when I install the mounting screws.

Positioning new wall thermostat in place (C) Daniel Friedman

With older mercury bulb thermostats the unit would be out of calibration if not mounted level.With newer thermistor-type thermostats the unit will work properly if not perfectly level, but if it looks crooked on the wall it'll be annoying.

Use a pencil to mark where you will need to drill pilot holes for the mounting screws.

Positioning new wall thermostat in place (C) Daniel Friedman

6. Drill pilot holes for the mounting screws. In drywall you might need to use the plastic anchors and sheet metal screws provided by the manufacturer in which case you'll select an appropriately sized drill bit.

Watch out: Don't make the hole too big or the anchor will turn or push right through the wall. In my case I'm mounting the thermostat on a wall covered with wood paneling so I skip the plastic anchors and screw right to the paneling.

Positioning new wall thermostat in place (C) Daniel Friedman

At left is a little trick for selecting the right sized drill bit without having to be very smart. I hold the drill bit behind the screw. If I can see screw threads sticking out past the edges of the drill bit my bit is probably OK.

In hard materials like wood or metal a perfect size drill bit would be about the size of the solid core of the screw but smaller than the outer diameter of the screw threads. Just sayin ...

Positioning new wall thermostat in place (C) Daniel Friedman

Above you can see I've begun to drill with care right in the center of the hole I marked on the wall. And I've pushed the thermostat wires well out of the way for obvious reasons.

7. Mount the thermostat base to the wall using the new screw pilot holes.

Positioning new wall thermostat in place (C) Daniel Friedman

Above you can see the result of my precision drilling. It's a little off, yes? Now we understand why the mounting plate holes allow for about 3/16" of adjustment as we mount the thermostat plate to the wall.

Below in my photo you'll see that I've hand-started the two screws in place before using the screwdriver - this helps keep the thermostat mounting plate more or less level and in place while I drive in the screws (below).

Positioning new wall thermostat in place (C) Daniel Friedman

Notice that we centered the plate so that the thermostat wires are not pinched nor damaged as the mounting plate is secured to the wall. Also notice that even though I know the power is off, I keep those red and white wires carefully separated.

Watch out: if screwing into drywall make sure that you don't over-tighten screws and end up loosening the mount of the thermostat to the wall or you'll be starting over (or using larger screws) - the thermostat is going to be poked and prodded with years of use so it needs to be securely mounted on the wall.

8. Level the thermostat and prepare the thermostat wires for attachment

I get the thermostat plate level by eye or by actually using a level, and then snug up the mounting screws (below left).

Positioning new wall thermostat in place (C) Daniel Friedman

Watch out: Before attaching my red and white thermostat wires I notice that the stripped-back ends are nicked and a little too long for the new connectors.

A nicked wire might break off later, and a wire too long leaves uninsulated wire exposed where I don't want it. So I trim off extra wire end and material (below).

Positioning new wall thermostat in place (C) Daniel Friedman

9. Attach the thermostat wires to the proper terminals on the thermostat mounting plate.

Watch out: I know we prefer to use them to kneel on or hold a sandwich but really, it's a good idea to read the instructions from the manufacturer before connecting up wires.

For example in this thermostat the manufacturer has included a jumper between the rightmost terminal and the second from right.

My red wire is to go in the second from right terminal (marked R) and I'm to leave the jumper in place (photo at below left).

Positioning new wall thermostat in place (C) Daniel Friedman

I attach the white wire next, to the terminal marked W (photo above right). Notice that I tried to make a neat coil of the un-used wires and that they are cut with no stripped-back insulation.

I want these to be safely out of the way of touching any thermostat parts when I button up the installation.

Positioning new wall thermostat in place (C) Daniel Friedman

If you need help figuring out the wiring for your thermostat see the instructions that came with your unit, or if you've already thrown those away, see our detailed thermostat wiring guide at THERMOSTAT WIRE CONNECTIONS.

10. Check the settings of any internal switches in the thermostat.

A small screwdriver is the right tool for snugging up the wire screws. Tight but not cut or damaged. (below left).

Now we should check to see if there are any internal switches in the thermostat and if so, we need to be sure they are set correctly. (below right).

Examples of how these switches should be set for this and other thermostat models are given at THERMOSTAT SWITCHES, INTERNAL but also you should check the instructions for your own thermostat model.

Positioning new wall thermostat in place (C) Daniel Friedman

Watch out: This is another good time to read the instructions as many thermostats have one or two little switches that you might need to change from their factory settings. In this case factory settings were fine.

Positioning new wall thermostat in place (C) Daniel Friedman

11. Install batteries in the thermostat and mount it to the backing plate. Use fresh batteries so you don't have to re-visit the whole operation any time soon.

This thermostat uses two AA batteries to remember its program should there be a power failure.

Positioning new wall thermostat in place (C) Daniel Friedman

As soon as you insert batteries you will see the thermostat come alive.

Most but not all current room thermostats consist of a backing plate that is screwed to the wall and the thermostat body that mounts to the backing plate by pushing a connector (red arrow in photo at left) onto receiving pins on the backer plate. Plastic clips secure the thermostat to the backing plate.

Positioning new wall thermostat in place (C) Daniel Friedman

Watch out: you should not have to use great force to mount the thermostat and its connector to the backing plate. Watch out to avoid bending pins or pinching wires.

A few thermostat models such as 3M's Filtrete instead mount the entire thermostat body to the wall and then cover various connections and switches with smaller plastic covers.

Watch out: the Filtrete™ 3M-22 is a battery-operated thermostat. If the batteries fail the thermostat may fail to provide heating or cooling and the building may suffer accordingly.

12. Set the thermostat clock, day, time, and then set up the set-back program

Positioning new wall thermostat in place (C) Daniel Friedman

As soon as I touched the SET button (under my thumb in the photo) I saw the display light up - which told me that the thermostat was properly connected to its thermostat wires and was receiving low voltage power from the heating system.

A couple of minutes to set the clock and choose my program and we were in business.

Verify that the thermostat actually turns on your heating or cooling system. If the set temperature is higher than the room temperature and we are in heating mode, the heating system should come on.

If the set temperature is lower than room temperature and we are in cooling mode the A/C should come on (though in this example we were hooking up this thermostat for heating only).

Remember to check the HEAT OFF COOL and FAN AUTO ON switches (located on the bottom of this unit). If you need help those switches are explained at

Where to Get More Help with Thermostat Wiring or Installation?


Continue reading at THERMOSTAT WIRE FUNCTIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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