How to Upgrade to a Smart Thermostat without Changing the Wiring | Ask This Old House

Ask This Old House plumbing and heating expert Richard Trethewey replaces an old thermostat with a smart thermostat without replacing the original two wires in the wall.

Time: 2-3 hours
Cost: $300
Skill Level: Difficult

Tools List for Upgrading a Smart Thermostat:

Shopping List:

Steps:
1. Start by turning off the service switch at the furnace.
2. Remove the old thermostat.
a. Pull off the front cover.
b. Unscrew the screws inside.
c. Disconnect the wires.
d. Pull off the back cover.
3. Mount the hub in a convenient location near the furnace using a drill and a few screws.
4. Disconnect the wiring from the thermostat to the furnace and run that wiring to the hub. There’s an obvious place on the hub to screw the wires in properly.
5. Run the thermostat wire from the furnace to the hub. The red wire goes to R, the white wire goes to W, and the blue wire (common wire) goes to C.
6. Mount the new thermostat back cover at the location of the old one. This might require drilling for new screws.
7. Using the old two wires, wire the new thermostat and connect it to the back cover.
8. Turn the service switch back on at the furnace.
9. Download the smart app and program it to the thermostat following the manufacturer’s instructions.

Resources:

About Ask This Old House TV:
From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. Ask This Old House helps you protect and preserve your greatest investment—your home.

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How to Upgrade to a Smart Thermostat without Changing the Wiring | Ask This Old House

[XthisismyusernameX]

Hired a professional plumber to install, after about 2 hours said this was not compatible with my Independence boiler. But hey, I got two free holes in my wall for my trouble.

[beetle]

A good practice is to snip the bare wire shorter so there's non visible once connected to the faceplate. There looked to be at least 5-10mm bare wire shown at 5.58 and these could easily connect and cause a short or even a fire when pushing the housing back into position. This is why the wire has the plastic insulation in the first place.

[heywoodjabozoff]

it might've helped if you showed how the original wires were connected to the furnace relay.

[Russwig]

I bought one of these for a little more than $225. Works great and installed as easily as Richard's did. I too only need it for furnace, no AC.

[LowsJuan]

I learned more in the comments than I did in the video. There are so many variables in this kind of installation, I quess that this shows us a basic install. Probably the biggest issue is whether you and your wife like the same temperature not so much if you can use your phone to set the stat!

[bobvila]

Thank you for this. My friend was actually looking for something like this and no one has recommended anything close... I remember watching this show 25 years ago with my dad.

[Engineer]

Strange location, that thermostat box on a toolboard. Toolboards are meant for tools not for thermostat boxes. And he should have cut off a bit of the copper going into that box at 4:50. It will be ok, but leaving so much bare copper sticking out is a bad habbit.

[casperunnerup]

4:18. I might not matter in this instance. But please don't strip the insulation that far back. Plus the copper of the red wire is nicked, making it easier to break.

[Russwig]

As far as remote sensors go, many houses have wiring for land-line phones to multiple rooms. I have not used my land lines for several years now, since every family member has a cell of their own. The existing lines could be Isolated from the telephone network and repurposed to connect remote sensors to the central interface. Locations may not be ideal but...

[joemoreno]

Cool option for older homes. But seriously the best part of the video was the the baby picture behind them!

[CortexGaming]

Glad the setup was so easy! Turn the power on and the app will install itself and pair to the thermostat!

[agate_jcg]

This is great, but it locks you into this brand of thermostat. Another great option is the Fast-Stat Common Maker. It's a pair of devices, one at the furnace, one that hides in the wall behind your thermostat, that adds a common wire and lets you use any smart thermostat you like (Nest, Ecobee, etc).

[dpielick]

What is the name of the product use in the video?

[rawbacon]

Most old wires are probably stapled on and run thru tiny holes, otherwise you could use the old wire to pull a new one.

[rosaliedipietro]

What is the brand of thermostat you're using? Also, would this work on a steam boiler?

[common_cnts]

Why would you install it on the peg board instead of a more permanent location like one of those studs? You also take up space on the peg board.
Also, too much installation was stripped off. You can easily short the wires.

[wholeNwon]

My T-stats run on 120. I don't know of a "solution" that wouldn't require transformers and relays. But I get along very well with just simple programmable units that don't risk hacking. Batteries last years.

[FightingSioux]

Interesting device. I had a similar issue with my Nest thermostat and old 2 wire boiler. I ran wires to a new location and used an external transformer to power my thermostat.

[GeeTheBuilder]

Richard is a much better plumber than he is an electrician

Way too much copper showing when he fitted the wires to the new unit in the pegboard.

[leroybrown]

This solution is about as costly as having a professional run new wiring, in my area the estimates were $350 and up.

So I went to Home Depot and bought 50 feet of 18/5 thermostat wire for $21 and fished it through the wall myself by tying it to the existing 2 wire (R and W) that the old thermostat used. I've never done it but with a little patience it's not difficult. Every connection is labelled on both the furnace and the thermostat (I need C, R and W and didn't have a C at all). You just plug the wires accordingly. Now I have both a sleek new Honeywell smart-thermostat and future proof wiring to the furnace without all the extra or a giant bill for an HVAC guy.