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How to Replace an Old Thermostat

It’s remarkable how many ways you can lower utility bills.

Add more insulation, install better windows, get a more efficient HVAC system, etc.

Last week ran over to help my tenant because his thermostat was on the fritz. After 15 minutes he had a new programmable thermostat at his figure tips.

I wanted to show you this tutorial so you could see how easy this project is to do.

The programmable thermostat I installed cost all of $25.

You can get the same one and instantly start saving money. Let’s dive in!

Supplies for a Thermostat Project

These are the supplies that made my project easier

Here’s the funny thing about today’s tutorial: I shot the video while Rich watched Germany play Brazil in the World Cup.

Amazing game, if you’re a Germany fan.

Some how I got the video done without a hitch, haha. There are several bonus tips you’ll want to see.

Here’s what you’ll learn,

How to replace an old thermostat

Let’s get started and bring your thermostat into the 21st century.

Thermostat Project Preparedness

First things first, turn off the power to your heating & cooling system.

Usually there’s an on/off switch on the unit. But if you can’t find one turn off the power at your electric panel or fuse box.

Turn off power

You wouldn’t want your brain operated on without anesthesia, right.

The thermostat is the brain of your HVAC system. By the way, HVAC is short for heating, ventilation, and air conditioning (just in case you’re wondering).

Thermostats usually have a base and a body.

The base is attached to the wall via screws and the body clips to the base. My old digital thermostat that went belly up had 4 tabs holding it to the base.

Your thermostat may be different. Try removing your thermostat by first tilting the bottom up toward the ceiling. You may have to be forceful.

Once the thermostat becomes loose you can slide it up off the base.

Remove thermostat from base

It’s safe to say that most of the time you’ll feel like you’re breaking the thermostat when removing the body from the base.

If your old thermostat has mercury in it you’ll have to dispose of it in a safe way. Check with your local waste company on this.

The base of your thermostat will look like a bomb with all the different colored wires. Seriously, it brings back memories from those cheesy 1980s action films. I feel like Sylvester Stallone when replacing thermostats.

Here’s one tool Sly didn’t have back in 1985: a camera phone.

This is perhaps the best tool ever for any kind of electrical work. Take a picture of your thermostat’s wiring and save it for reference.

Take picture of wiring

Unscrew the wires from the terminals using your screwdriver and you’re ready to the installation of the new thermostat.

Wire and Mount the New Thermostat

Stickers come with the new thermostat. Add the appropriate labeled stickers to your wires based on the terminals they’re attached to.

For example, if the green wire is attached to the G terminal add the G sticker to it. But if you see the Blue wire is attached to the W1 terminal add the W1 sticker to that wire.

Add stickers to wires

Carefully unscrew the base from the wall while holding the wires.

Here’s my first HUGE tip: prevent the wires from falling into the wall by attaching vise grip pliers to them.

Vise grips holding wires

One of my nightmares is to have the wires fall down into the wall. Fishing them out would be a pain!!! So I’d hate to see this happen to you.

Here’s my second HUGE tip: add blue painter’s tape (about 12 inches worth) right below the thermostat location. Make it into an L shape.

The tape will catch the drywall or plaster dust when you drill the holes for the new thermostat base.

Add blue tape to wall

Slide the new base over the wires, level it (which I didn’t do in the video), and mark the two holes.

Drill the holes according to the directions and place the plastic inserts into your wall.

I used my combination stripper to trim the wires. The bare copper was too long for the new terminal slots.

You’ll need a super small precision screwdriver to loosen the terminal screws.

Loosen terminal screws

You can find these at any hardware store for about $6.

They come in handy for bathroom accessory set screws, too!!

When I installed the Nest Thermostat it came with a screwdriver, but for $249 there’d better be one in the package 🙂

Watch that tutorial right here.

There are so many different wiring configurations for thermostats. You’ll have to read the directions and determine what works for your HVAC system.

If you get confused go ahead and call the technical service line of the thermostat manufacturer because they’re usually fantastic.

Check out my video to see how I ended up wiring my thermostat. Even though the temp was about 85 degrees in the house I tested the thermostat to see if it would turn on the furnace. Man it was hot!!! (for Pittsburgh, haha).

I’s easy to replace an old thermostat with a new one.

Thermostat Project

What’s Next

We also have a great tutorial on how to install a Nest Learning thermostat – which I LOVE!

If you’re doing a bathroom remodel and need help, join one of our online courses – they’ll make your bathroom renovation much easier!

Let me know if you have any questions and I’ll do my best to help.

Thanks as always for reading, watching, and being part of our awesome community.



Replace Old Thermostats

  1. easy project that really does pay! I like the vise grips tip because of the weight … last time I did a thermostat – I actually think I grabbed a binder clip – which worked well but you know isn’t necessarily so ironclad. … loosing a wire in a wall (and trust me it’s happened to me) can take the easiest project to pot in a millisecond. good share Jeff. Cheers. ~jb

    1. I like your binder clip idea jb, that’s a great solution.

      That feeling of doom when a project that was suppose to be easy then turns sour – man that stinks!!!

      All these little tips can help with preventing that. Hope you’re having a great day buddy.

      1. Ahhhhhh,… the old “feeling of doom”. How well I know it. Often, it precedes the feeling of another trip to the hardware store.

        This is a project I did a few years ago. It was the first of many little (and not so little) energy-savings projects we’ve done that have added up to huge savings on our electric bill. Great post Jeff.

        1. You hit the nail on the head my friend, I know that feeling, too. There’s nothing like having to jump in the car for another trip to buy something you forgot.

          I’d love to know the other energy-saving projects!!

  2. Ryan says:

    I have been looking to install a new thermostat for our A/C but we also have a swamp (evaporated) cooler attached to the same thermostat. We have never used the swamp cooler since moving into our house and don’t plan to. Do we have to buy any special thermostat or how do we wire it safely?

    1. You and Mike have interesting HVAC systems. I’d get all the particulars about your setup Ryan and give the manufacturer a call. Find a thermostat you like and ask the technical service rep if it’s compatible with your system.

      Wish I had a better solution but alas, HVAC isn’t my topic of expertise. Keep me posted on what you find out since other fans likely have a similar setup.

    2. Hey Ryan – Just browsing comments and thought I’d share what I did in the same situation. We hated the nasty old swamp cooler too. I made sure the unit’s electrical disconnect was turned off on the roof. I capped all the control wires at the theromostat with tiny, tiny wire nuts. I taped the evap cooler wiring around my normal HVAC cable (so it wouldn’t get completely lost) and tucked it back in the wall, behind the thermostat. I suppose I should disconnect the control wiring at the unit too. Someday, I’m going to rip the massive swamp monster off the roof and roof over the hole. For now, it’s just an eyesore. Good luck!

      1. Thanks John for helping Ryan. I have to clue what a swamp cooler is, it sounds like something that needs beer in it!!

        But I think that’s not it’s purpose, haha.

        1. Ryan says:

          Thanks John and Jeff for your advice, i will contact the manufacture and i like the idea of capping off the wures and attaching them to the a/c connection. However, recent monsoon storm damage to the a/c unit may have solved my issue, or at least delayed this particular DIY project for a little while.

  3. Mike Ruf says:

    I have a question for you on a variable in installing a new programmable thermostat. The one I have has a heat pump on it. I have 2 issues? Do I need to buy a special thermostat that handles the heat pump portion? How much do those run compared to the $29 models? Do they have wi-fi linking android adjustable models and cost? Final question – not on this topic. I have a house built in 2002 in Kansas City and when I bought it new in 2003, none of the sinks have shut off valves so I have to shut off the water for the whole house(which seems like it would have broken code in 2003???). Does that endanger the heat pump since it seems to run year round and the air conditioner is running 16/7 as opposed to 24/7. If I turn the water off the either install shutoffs or a new faucet do I need to shut the AC off also?

    Any help/opinions would be awesome – no liability held in your answers. Love your website!

    1. Hey Mike, great questions.

      Definitely make sure your new thermostat can handle your HVAC system, specifically the heat pump portion. If you’re in doubt give the manufacture a call and speak to a technical service representative.

      I get a lot of help just doing this.

      The newer wi-fi enabled thermostats can be very expensive. I actually installed a Nest thermostat in our house and like it a lot. Primarily because I can control the temperature when we’re away from home.

      It cost about $249 but I’m sure you can find thermostats that are a bit more cost effective.

      I’m not too familiar with heat pumps and how they work. But I’m wondering if your heat pump is on a separate system from your house water line. If so, you should be able to add shutoffs to the sink.

      If you get your heat pump serviced annually this would a fantastic question for your HVAC person. If they say it’s okay to add the shutoffs you could easily do it with SharkBites.

      It’s as simple as cutting the copper or pvc or PEX and sliding on the shutoff. Of course there are a few extra things to do like making the cut square but you can totally do this.

  4. Jimmy says:

    I am told I have a dual pump system (or something to that effect). Your video said to choose either gas or electric. What if we have a dual system.

    1. Hi Jimmy, you’ll have to get a thermostat specifically for a dual pump system and set it up according to the instructions.

      Sorry I can’t be more helpful. But a good starting point is to pick the right thermostat for your system 🙂

  5. Steve says:

    I replaced my thermostat last year. After i got it all wired, the furnace/AC would not run. I went online to one of those sites where you can ask questions to professionals for $$$$$.
    The guy told me a fuse in the furnace probably blew when i changed the thermostat. It is a small fuse like the ones in cars. I went to an auto store and bought some. He told me where it was in the furnace and when i replaced it…it worked. Just an FYI is this happens to any of your viewers. The AC guy says this is common when you replace a thermostat.
    Steve in Missouri

    1. Steve, this is an AWESOME tip!!!

      Thanks so much for sharing because I had no idea this happens. You totally made my day.

      Keep the great ideas coming my friend 🙂

  6. Amir says:

    Hi Jeff,
    i tried to replace my old Honeywell 921 thermostat with new one, but the old connection between the Relay Box BDR91 is not correct. A-B-C connection points are left not connected.
    would you please help to show me how to connect the Relay Box to the boiler. my boiler type is Ideal logic 24.

    Thank you in advance for the help.


  7. Andrew says:

    I have an old digital thermostat which showing the wrong temperature but is attached a hard wire sensor. What kind of thermostat I have to buy?. I do not like to change on the smart thermostat. Can you give me any advice, please?
    Thanks for any help.

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