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How to Replace Your Thermostat - Step-by-Step Installation Instructions

Old thermostat

There are a number of good reasons to consider replacing your old thermostat.

  • Your old thermostat has stopped working.
  • Your old thermostat is not as accurate as you'd like.
  • You've just redecorated, and the old thermostat is an eyesore.
  • You want the convenience of automatic thermostat adjustments.
  • You can't figure out how to program your thermostat.
  • You want to save money on your utility bills.

Whatever your reasons, thermostat replacement is actually one of the easiest DIY improvements you can do for your home. In fact, you can have a new thermostat installed and operating in as little as 15 minutes. So get your tools ready (you'll need a screwdriver and possibly a drill), read through this guide, and go for it!

Before You Get Started

Choosing the Right Location:
Make sure your thermostat is located in a good spot. It should be on an inside wall, away from the fireplace or windows and out of direct sunlight. Make sure the air flow around it is not blocked by furniture or curtains. If you think that your thermostat needs to be located in a different spot, then you should consult with a heating and cooling professional.

Choosing the Right Thermostat:
When you're buying a new thermostat, make sure you get one that's compatible with your heating and cooling system. Most central heat or central heat and air systems operate with low-volt thermostats that are commonly available. If you have a heat pump, electric baseboard heaters, or floor and wall furnaces, you will have special requirements: read our article on types of thermostats for more specifics.

Before investing in a thermostat with a digital display, find out whether it requires a common wire for operation. A common wire is a low-voltage wire that provides continuous power to the thermostat to run enhanced display screens and functions like Wi-Fi support. To find out if you have a common wire available at your thermostat, just remove the cover and have a look at the wires already connected. If there are three (or five if an AC is also controlled by the thermostat), then you're good to go. If there are only two wires (or four with an AC) then there is no common wire, and you need to have a HVAC professional install one, or select a different thermostat.

Note: If your HVAC system has additional systems connected to it, like a whole house dehumidifier, then your thermostat will have additional wires connected to your thermostat.

Read the Instructions.
While the instructions provided here are generally applicable to all thermostat installations, your new thermostat may have specific requirements. Make sure you are aware of them before you start.

Step-by-Step Thermostat Installation Instructions

1. Shut off the power to the heater and air conditioner at the breaker.

2. Remove old thermostat from wall. Typically the display part of the thermostat will slide off a wall plate.

3. Note how the wires are connected. Each terminal on the thermostat is labeled with a letter (Y, W, B, R, etc.) which might correspond to the color of the wire connected to it. But there is no guarantee code specifications were followed, so it's best to record which wire is connected where. Most thermostats these days come with lettered labels that you attach to the wires, to indicate which terminal they should be reconnected to. If there are no labels, use pieces of masking tape and write the letters yourself, write down which wire connects to which terminal, or just take a picture of the existing configuration.

4. Use a screwdriver to remove the wall plate from the wall.

5. Disconnect the wires from the wall plate. To make sure they don't fall back into the wall, wrap them around a pencil or tape them to the wall, or simply spread them apart to keep them from sliding through the hole.

6. Install the replacement wall plate. It's important that it is absolutely straight, or temperature readings could be inaccurate, so use a level. Use the plate to mark the proper placement, drill your marked holes, and insert drywall anchors if necessary (these may be included with the thermostat). Screw the faceplate into the holes, bringing the wires through the provided opening.

Thermostat installation

7. Connect the wires. Most models will have screw terminals, just connect the wire to the terminal and tighten the screw with a screwdriver. Higher-end thermostats might have terminals that allow you to connect wires by pushing a button. Do not connect any wires that were not connected to the old thermostat.

8. Install batteries. If your thermostat has a digital display, then it probably requires one to three batteries to provide backup power in case of a blackout. Install them now.

9. Slide thermostat display into wall plate.

10. Turn power back on at the main service panel.

Mercury switch thermostat

Now that the power is restored, test the thermostat to make sure it is cycling your heating and cooling systems properly.

Disposing of your old thermostat. Once the new thermostat is installed, you'll have to dispose of the old one. Older mechanical thermostats contain mercury, and must be disposed of properly to prevent mercury from getting into the soil and water of local environments. You can find out if your thermostat has mercury by looking inside; if you see a clear glass cylinder, then the thermostat has mercury. Check to see what your state regulations are, and what recycling programs are available.

Old digital thermostats can be recycled, too. Look for local recycling organizations that accept electronic waste.

Programming Your Thermostat to Save on Heating and Cooling Costs

If you have installed a programmable thermostat, then once it's installed, you need to take a few minutes to program it. In the past, digital thermostats earned the reputation as being notoriously difficult to program. In fact, the EnergyStar program even stopped certifying thermostats because most people just didn't bother to use them properly. But easier interfaces and advancements in technology have improved the ease-of-use, so there's no excuse for not taking advantage of the device you've installed. Just follow these steps and consult your manual for specific instructions.

STEP ONE: Set the Date and Time
Setting the day and time on your newly installed programmable thermostat should be just as easy as setting it on your digital alarm clock. Be sure you have it set properly for AM or PM, or your programming will be off by 12 hours, and it will be cool when you want it warm, and vice versa.

STEP TWO: Setting the Schedule
Many newer programmable thermostats come with out-of-the-box settings that will help you start saving money without having to do anything else. But to get optimal value from your new device, you should take the time to program it according to your personal schedule.

Different programmable thermostats offer different programming options. The simplest have only one programming schedule which will run every day of the week. The most complex devices allow you to select a different schedule for each day of the week. But the most popular options are for 5/2 scheduling (one schedule for the weekdays, one for the weekend), and 5/1/1 schedule (one schedule for weekdays, one for Saturday, one for Sunday). This is a choice you should make when purchasing your thermostat.

Thermostats also vary on the number of temperature adjustments they allow each day. Most people can divide their days into four different operating periods:

Early morning - when you're getting out of bed and getting ready for work

Daytime - when you're gone for the day at work or school

Evening - when you're dining and relaxing at home

Overnight - when you're asleep

Setting the schedule should be very similar to setting the thermostat's clock, with the addition of noting the temperature you would like.

  • Select the day of the week you are setting the program for.
  • Select the time you get out of bed in the morning, along with the desired temperature.
  • Enter the "set back" time - the time you leave home - and the reduced, energy-saving temperature your thermostat should maintain while you are gone.
  • Enter the time you return home from work and tell the thermostat what temperature you'd like.
  • Enter the second set back time and temperature reduction for when you go to bed. (If you like your room cool when you go to sleep, you might want to program this 15 or 20 minutes earlier than your usual bedtime.)

Repeat this process for each day (or group of days) of the week as necessary.

STEP THREE: Choosing the Right Money-Saving Temperatures
There are two things to take into account when you program your thermostat: Your personal comfort level, and the amount of money you want to save.

As far as saving money goes, when you reduce temperatures by 10-15°F in your home for 8 hours a day, you can save as much as 15% on your heating costs. Every 1-degree set back equals 1% in savings. If you have air conditioning, the same rule applies in summer weather, but in reverse. Save money by raising the household a few degrees when it's warm out, especially when you're not at home.

Here are some target, money-saving temperatures for different types of day, depending on the season:

Thermostat settings

Of course, your personal comfort level will affect your own choices, but keep in mind that it is a lot easier to keep one person warm or cool than a whole house. On a cold winter day, 74° might keep you warm and cozy when you're home, but 70° and a sweater will save you more money. Find ways to increase your comfort that don't rely on adjusting the thermostat.

Shop our thermostat collection.