How to Replace a Thermostat

Co-authored by Mark Spelman

Last Updated: July 19, 2023

A thermostat is a tool that regulates heating and cooling, whether it be in your home or in your car. Replacing inefficient thermostats can help you save money on utility bills or, in your vehicle, keep you safe on the roads. Whatever your situation, replacing it yourself is a task that is surprisingly simple. See Step 1 of your preferred method below.

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Method 1

Method 1 of 2:

Replacing a Thermostat in Your Home

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1
Purchase a replacement thermostat that will work with your system. Review the compatibilities listed on the packaging of the replacement thermostat. Most replacement thermostats are compatible with all common systems.
- However, if your system is unique, finding a replacement thermostat may be difficult. Here are your basic options (information that should be easily found on the packaging):^{[1]
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  - "Works with 1 stage heating or cooling": Used when you have separate heating and air conditioning units
  - "Works with 2 stage or multi-stage heating or cooling": Used for heating or cooling units that have designated high and low speeds
  - "Works with Direct Line Voltage": Used with 110 or 240 direct current power sources to power the thermostat (generally seen in older homes)
  - "Works with 24mV": Used with fireplaces, floors, or wall furnaces
  - "Zoned HVAC": Used when both heating and cooling are individually controlled in different areas from the same system
2
Review the manufacturer instructions for wiring your replacement thermostat. Most thermostats utilize similar installation methods; however, it's beneficial to read all materials and preview all pictures provided for how to install your new thermostat. Or you risk literally being stuck in the cold!
- Reading instructions is a total drag, yes. But this is not something you want to mess up! Read them and study the pictures. You want yours to match detail for detail.
- It’s also a smart idea to always take pictures of the existing wiring before you start.
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Turn the switches off at the breaker box that pertain to your thermostat, furnace and air conditioner. Turning off the power to the thermostat reduces the chances of electrical injuries as you remove the old thermostat and install the new one.
4
Remove the old thermostat from the wall. Most thermostats slide upward from where they are attached to the wall. Loosen the screws that attach the wall plate to the wall, if there is one.
- Some thermostats have a base and sub-base. You need to remove the entire thermostat -- all you should be left with is exposed wires and a bare wall, nothing else.
- If the wires you expose are corroded and there is enough wire length, re-strip the wires. Otherwise, scrape the ends with a utility knife until they are shiny.
5
Pay attention to how the old thermostat was wired as you disconnect it. This is the most important step. Most thermostats wires are coded, but some (if done by an amateur previously) can be coded incorrectly. To make sure you're doing it right:
- With a piece of tape, write a letter on each wire, matching the letter of the connection on the thermostat base. If the blue wire was in connection B, write "B" on the piece of tape, and put the tape on the wire. Label or designate any wires that were loose and unconnected to your thermostat as well.
- Ignore the colors of wires, except for your own identification purposes. Thermostats wired by non-professionals do not usually adhere to codes, so the colors may not match what they should.
6
Keep the disconnected wires hanging out of the wall. Tie the wires together or tape them to the wall to keep them from falling back into the wall. A lost wire will turn this rather easy process into quite the debacle.
- Pro tip? Wrap all the wires around a pencil. The weight of the pencil is just enough to keep the wires from going anywhere.
7
Install the replacement wall plate. Use the new wall plate as a template to mark where the holes you need to drill for the screws will be. Use a level if necessary. Then, drill the holes, and screw the replacement wall plate into its new position on the wall.
- If your new thermostat has a mercury tube (that is to say, if your new thermostat is rather old school), your device needs to be completely level or it won't render accurate readings. Using a level is very important in this scenario and is not just for aesthetic reasons.
- Make sure you're drilling holes that match the size of your screws. A 3/16" drill bit is fairly standard.
- Your thermostat definitely comes with screws, and probably comes with anchors. Make sure to use the anchors, too. They support the system on the wall.
8
Hook up the thermostat to the wires. Use your notes or labels to re-connect the wires to the thermostat – or, follow the pictures you took of the pre-existing wiring. You can twist the wires onto the thermostats connectors, or follow the directions manual provided by the manufacturer.
- Your new thermostat should have the same corresponding code on the back, unless otherwise noted in the instructions. If you're at all in doubt, contact a heating and cooling company.
- Some thermostats are as simple as a two-wire system. Some have 5. If you have empty ports or connections, don't worry. Your thermostat is most likely fine.
9
Place the thermostat on the wall. Replace all wires back into the wall, if extra length is exposed. Place the thermostat flush against the wall, slightly above the wall plate. Slide it down so it catches the grooves (or screws) on the wall plate to sit in place.
- If your thermostat isn't in a good location (it's exposed to drafts or heat, which can mess up the readings, or it's too high or low for you), you'll need to contact a professional to have wires moved.
10
Activate the power back to the thermostat, furnace and air conditioner. Switch on the proper switches in the breaker box to restore power. Give it a minute to kick on.
- And don't forget to install the batteries! Most systems require 2 AA batteries to work. Make sure the batteries aren't old, they're in place, and the polarities are correct
11
Check to make sure your replacement thermostat is working. Set the thermostat so the furnace and the air conditioner will come on at different times. Give your furnace and air conditioner at least 5 minutes each time to activate. If the thermostat does not work properly, retrace your steps to see where you made a mistake.
- You may need to hit the reset button on your new thermostat. Some will not start until this button is pushed.
Each type of thermostat is different, so read your manual if you have questions. Just remember that a programmable thermostat can save you loads of money -- have it be cooler when you're gone and warmer when you're there. It'll turn off without you, saving you money and saving energy to boot!

Method 2

Method 2 of 2:

Replacing a Thermostat in Your Car

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1
Make sure your car has cooled down. It won't be a good day if you singe off your eyebrows and get third-degree burns on your hands, so turn off your car and let it cool down before you open up the hood and get to work. Let it sit for at least an hour before you start dissecting its parts.
- It's not a dumb idea to protect yourself with goggles or gloves, either. If you don't want anything getting in your eyes or your hands getting covered in gunk, get out the protective gear. And, of course, a shirt you don't mind smudging with grease or oil.
2
Drain the antifreeze out of your car. The thermostat and radiator hose are linked to your car's cooling system; if you don't drain out the coolant, you're gonna get water everywhere when you start disassembling. Here's how:
- Place a bucket (or some sort of container) underneath your radiator. You'll have somewhere between 4 to 8 cups of fluid pouring out, so don't skimp on the size of your container.
- At the bottom of the radiator, there should be a draining screw or cap (technically, it's a radiator drain cock). Twist this to the left, opening it up.
- Let all the water and coolant flow out. Keep the cap somewhere you won't lose it.
3
Locate your thermostat. Every model of car is different. Some thermostats you could spot from a mile away, others will be an obstacle to even the most trained of eyes. If looking at your car's engine is like reading gibberish, find the radiator hose and follow it to the end -- that'll be where your thermostat is located.
- The body of the thermostat is probably metal with a bit of gold in the center and possible a rubber ring around the edges. It resembles a top or dreidel in shape and size, or, a small plunger.
- If you're not quite sure what you're doing, consult your manual or look up the location online. It's better to know what you're looking for instead of poking around and possibly injuring yourself.
4
Remove the radiator hose and take apart the thermostat casing. The hose is most likely clamped to the thermostat casing. Unscrew this and set it aside. Move onto the thermostat casing, exposing the thermostat itself. You will definitely need a screwdriver, and you may need pliers, for the record.
- Most vehicles have a two-bolt or three-bolt system for the thermostat casing.
- If corrosion and gunk is building up prominently, clean up the area before you go about adding your new thermostat.
- A bit of water will probably come out with removal of the hose. This is normal.
5
If desired, test your thermostat. Is it possible that your thermostat works, it's just stuck closed or a different part of your vehicle is starting to kick the bucket, affecting your thermostat's ability to make accurate readings? If so, it's very easy to test your thermostat. Here's how:
- Get a pot of water boiling.
- Insert your thermostat. The thermostat should open at around 190 ºF (88 ºC). Since water boils at 212ºF (100ºC), this is more than enough.
- If the thermostat doesn't open in the water (and then close when it cools), you need a new one.
6
Swap out your old thermostat for the new one. From here on out, it's mainly a matter of reassembly -- the easy stuff. Replace your thermostat just like it the old one was positioned. If applicable, replace the rubber ring, too, sealing the edges.
- If the area is building up dirt and grime, wipe it down with some cleaner first. You want to maximize the life of your thermostat and not have to deal with this again anytime soon.
7
Reassemble the system. You remember what everything looked like, right? Here's a brief checklist:
- Make sure the thermostat is snug and in place.
- Bolt down the thermostat casing over top of the thermostat. Finger start your bolt and then bust out your pliers or socket wrench and get to tightening. Be careful not to strip the bolts.
- Replace the radiator hose and clamp. The radiator should be snug on the outside of the thermostat casing and the clamp must be well-tightened.
8
Replace the coolant and check for leaks. If the coolant you just drained out was fairly new, just use the same stuff in the bucket and add it back in. If it was old, you may be better off throwing the contents of the bucket out and just using new coolant. Either way, replace the coolant and make sure the radiator drain cock is on tight.
- Once replaced, check for leaks. Your car needs coolant to operate safely. If yours is leaking, you literally won't get very far.
You're done! Now all you have to do is keep an eye on your temperature gauge. If it's acting up, do a double check to make sure you secured everything appropriately. If so, you may need to consult a mechanic -- the problem probably lies elsewhere.