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

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.

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

Tools List for Upgrading a Smart Thermostat:
Drill driver []
Small flathead screwdriver []

Shopping List:
Smart thermostat []
Thermostat wire []

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.

Richard installed the InvitaTM Wifi Thermostat 564, which is manufactured by Tekmar Controls (

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


  1. san mac on March 19, 2023 at 6:29 am

    What’s name of that product ?!’

  2. Udder Heaven Ice Cream Retreat on March 19, 2023 at 6:33 am

    What if your burner only has a white and red connection at the burner?

  3. T-tech vids on March 19, 2023 at 6:35 am

    Just buy a Google power connector if you’re using a Google nest, I only had R and W wires for my boiler

  4. priceandpride on March 19, 2023 at 6:37 am

    the wall cover is a monstrosity and the GUI is horrible on that unit

  5. Altyn Dom on March 19, 2023 at 6:39 am

    Poor guy should’ve added as well on top of the oil furnace

  6. MOMO112382 on March 19, 2023 at 6:40 am

    What’s the name of the Hub that is mounted to the peg board. I have a similar Honeywell system like this with no C wire for heat only.

  7. Ru on March 19, 2023 at 6:42 am

    Why does he say – the "Common wire will power the whole rig", Isn’t the C wire the return 24v wire?… The transformer provides 24v to the red wire, and the C wire is the return wire to complete the 24 circuit, so shouldn’t it be the R wire that powers the thermostat?

  8. John on March 19, 2023 at 6:42 am

    I thought that this was a rather poor video. More attention paid to drilling a hole in the wall than on the details of terminal selection for wires to connect to the boiler e.g. there used to be two wires now there are three. Also what facilities were now available to programme the boiler operation. Very shallow piece.

  9. DAVID BRYANT on March 19, 2023 at 6:43 am

    What is the name (brand) of the new device?

  10. Jeff Budzisz on March 19, 2023 at 6:44 am

    Any ideas on how to set up a back up power supply to at least keep the wifi thermostadt telling the boiler to stay at the set temperature when the power grid shuts down?

    Ie: snow storm line damage shuts the grid off for several days. Don’t need the internet, jus keep the place warm.

  11. InDaZone on March 19, 2023 at 6:45 am

    “Without changing the wiring” and then proceeds to change wiring.

  12. Ron Sanders on March 19, 2023 at 6:46 am

    The QR code no longer works. Unsupported garbage.

  13. Superdry14 on March 19, 2023 at 6:47 am

    I thought Richard was a plumber not a sparky??? In the US, is he allowed to do sparky work as well?

  14. Travis Oakes on March 19, 2023 at 6:47 am

    Too much copper is showing on the connections, just asking for problems.

  15. octavius muir on March 19, 2023 at 6:48 am

    Could someone please let me know the name of this thermostat

  16. Ken Combs on March 19, 2023 at 6:51 am

    Hey guys, low voltage wiring. It is not an issue……

  17. James Randolph on March 19, 2023 at 6:51 am

    Nerd question! Anyone know what protocol it speaks across the wire?

  18. Michel Vallee on March 19, 2023 at 6:51 am

    The nest gen 3 works fine with 2 wires …

  19. Dan Lux on March 19, 2023 at 6:52 am

    Get Robertson screws and stop stripping your screws.

  20. Orangutan on March 19, 2023 at 6:53 am

    My baseboard heater (208/230 ) wall thermostat also has only 2 wire. Any suggestion?

  21. Jean-Pierre White on March 19, 2023 at 6:53 am

    Great video. This is a very neat solution to a common problem and the solution is future proof because you can add a more capable furnace later or humidity control and be setup and ready.

    The best part of the video for me however was the discussion at the end and I saw a Clipper Creek EV charger on the wall in the background. Ahhh…..

  22. Tom Pelletier on March 19, 2023 at 6:56 am

    Oh man I hate showing up on a call and seeing a nest, nest belong in trees guys not as a thermostat! Tekmar is a good control though glad he chose that instead. 😊 I would’ve used an EIM module by Honeywell for that and a Honeywell 8000, but nice work anyway. Also probably would’ve cut those wires back so you don’t see any of the conductors.

  23. JetSnow on March 19, 2023 at 6:56 am

    Rf interference is a key.

  24. POLY LIVING on March 19, 2023 at 6:58 am

    What’s the range distance on the tekmar?

  25. IIIllllIIIIlllll on March 19, 2023 at 7:01 am

    Aren’t tekmar trash? Couldn’t you have just added a C wire kit onto the furnace controller, without needing to run new wire?

  26. Csaba Dosa on March 19, 2023 at 7:02 am

    When I try to program my T4 Pro thermostat when I press Menu there is no option for PROG. Help!

  27. George Cunill on March 19, 2023 at 7:03 am

    The control Board of the furnace shows a C terminal but there is no wire connected to the existing Honeywell thermostat which has no C terminal. The R terminal of the furnace has 2 wires connected to the thermostat , one connects to the RC of the thermostat and the other connects to the R of the existing thermostat. The W, Y, and G terminals of the furnace are connected to the W, Y, and G terminals of the thermostat. The C of the furnace connects to the terminal of a Relay contactor.
    I bought a MOES smart wifi thermostat for the furnace and AC that requires de C cable. The Diagram of the thermostat requires a jumper from RC to Rh. The Diagram also shows the terminals W,Y, and G to be connected to the furnace and I should not have any problem with that.
    I am planning to connect the R terminal of the Furnace to the RC terminal of the thermostat with a jumper from RC to Rh. Remove the other connection that goes from R of the furnace to the Rh of the thermostat and use this cable to connect the C from the furnace to the C of the thermostat and would keep the existing connection from the C of the furnace going to the terminal of the Relay also located at the furnace. Do you think that this new wiring could work?
    Note: there is only one transformer at the Furnace

  28. Rosalie DiPietro on March 19, 2023 at 7:03 am

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

  29. ilovefunnyamv2nd on March 19, 2023 at 7:04 am

    there’s really no reason you can’t have sensors that plug into an outlet and transmit back to the base station at say, 700Mhz… when its only grabbing and sending temperature and relative humidity, it doesn’t need to be that complicated…. Give it another decade, and we’ll have better temperature control. until then, this is a great upgrade path when you can’t just pull a new line.

  30. Cortex on March 19, 2023 at 7:05 am

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

  31. MyBikEtube on March 19, 2023 at 7:06 am

    doesnt the nest he shown on the table still work with the 2 wire heating systems? Nest websites says common wire not required

  32. dan b on March 19, 2023 at 7:08 am

    Horrible wire management!!! Why leave so much copper visible, and the stripped leads so long, so as they might short out on each other!

  33. maggot238 on March 19, 2023 at 7:08 am

    Dude, I see a big issue. There is still some wire coating left on those wires. Think ya need to strip em back more

  34. DAVID BRYANT on March 19, 2023 at 7:09 am

    I thought blue was usually associated with heat pumps and that black was the more usual color for the "C" wire….?

  35. James Dill on March 19, 2023 at 7:09 am

    polarity insensitive how dare they

  36. Brian Smart on March 19, 2023 at 7:11 am

    Only one issue. That thermostat is not available to consumers.

  37. Dan Whitman on March 19, 2023 at 7:11 am

    My existing thermostat only has red, blue, and white connected. The yellow and green wires are there, but not connected to the existing thermostat. Should I connect these to my amazon smart thermostat?

  38. Cory R. on March 19, 2023 at 7:13 am

    Is the "hub" also called a "c wire adapter"?

  39. raminator157 on March 19, 2023 at 7:13 am

    I just had this same thermostat installed with my new furnace and heat pump. A word to the wise, this thermostat relays on the internet for the outside temperature. It does not really update the outside temperature correctly. It has been off by 10 degrees for me and yes the zip code is correct. If I go in and refresh the zip code it will pull the proper temperature again. They make a outside sensor, I think I’ll be asking my service person who installed this put it in. As I need it to read properly so my heat pump or furnace kicks on during certain temperatures outside.

  40. SanraS Uzumaki on March 19, 2023 at 7:15 am

    Great video and I think this will help convince my In-Laws to make the upgrade, I think I can do it too, thanks a lot.

  41. Bruce G on March 19, 2023 at 7:15 am

    Why not using a Nest thermostat which not requiring a C wire? It’s under $100 from Google.

  42. Randi Craib on March 19, 2023 at 7:15 am

    That electrician is an excellent salesman. But also can’t most men and woman homeowners figure this out? It’s really extremely understandable. The directions will see you though. Why give up the satisfaction of knowing you did it yourself? Then build on those skills? You’ll be doing really advanced stuff in no time thanks to YouTube directions. The pride that comes with doing this sort of work makes you feel so great!!!

  43. Lucas Smith on March 19, 2023 at 7:17 am

    Dude just run new wires through the wall

  44. Guy Etchells on March 19, 2023 at 7:18 am

    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.

  45. Spikey Dog on March 19, 2023 at 7:18 am

    I think you need more cooper showing on those terminals

  46. Jaze on March 19, 2023 at 7:18 am

    I just looked up this product, it is wireless isn’t it? You’re pretty much providing power by the existing wires and then have a wireless communication for everything else.

  47. Adam Craig on March 19, 2023 at 7:21 am

    So my dude has no AC but he could use a new programmable tstat to “save” money lol

  48. Heywood Jabozoff on March 19, 2023 at 7:23 am

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

  49. Gary Garramone on March 19, 2023 at 7:24 am

    What happens if you have 3 zone heat with 3 thermostats

  50. Zach Osborne on March 19, 2023 at 7:25 am

    As an hvac mechanical journeyman, these videos are always good for a laugh. Can’t tell you how many houses I’ve been to where they “watched a video” but shorted the wires and blew a fuse now “it doesn’t do anything but the lights are on”. Lol These guys help throw me some easy calls from time to time.

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