How To Install A Smart Thermostat | Ask This Old House

In this video, This Old House home technology expert Ross Trethewey helps a homeowner upgrade his existing thermostat to a smart model by finding the power source the new device needs.

With everything smart thermostats can do (scheduling, voice control, remote access, and remote temperature sensing), it’s obvious they’re a major upgrade over standard models. But, with the upgrade comes the need for constant 24-volt power with a common wire. When a homeowner couldn’t figure out how to wire his new smart thermostat, he called home technology expert Ross Trethewey for help.

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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 Install A Smart Thermostat | Ask This Old House

[max-zlvm]

I cannot convey how helpful this video was. I always wondered why my smart thermostat had no fan control, now it makes sense: the fan wire has been repurposed for the common wire.

[peterjanis]

I was pretty impressed with the way this was handled. I wouldn't have thought Ross would think on his toes in the way he did by eliminating the fan wire. Not sure I would have thought of that either. Really good easy fix, eliminating cutting drywall, drilling holes, rewiring, or even doing the dedicated transformer for the common (another PITA) well done

[bingo]

This has nothing to do with this episode but I have learned so much from Tom Richard Rodgers’s and norm I pray that you keep this going for a long time because even when we are all gone at least our children will still learn you guys have put a lot knowledge out there best schooling I’ve ever ever had

[JetLagRecords]

This Old House, This is perfect! I subscribed right away!

[byronwatkins]

Actually, "OL" is over limit. The meter's applied voltage reached its limit trying to cause current to flow through the circuit.
I would have tried to use the old wire to pull new wire down the wall. If there were no snags, that would have been the best repair. If there were snags, you could always pull the wire back and use your Plan B.

[wadup]

Keyboard warrior here, but.. I would at least look for that splice AT THE FACEPLATE. Wire could have been just short when they replaced the previous thermostat. It looked like there was plenty of plaster covering that wire, and my bet is opening that hole another 2-4” (new faceplate would cover) you would find your splice.

[durangodave]

i would have atleast attempted to find the splice. Put a tone on the wire and follow the sound until it stops, that is your splice. It even could be in an access panel or behind a connection plate.

[andyallen]

In this scenario, use honeywell thermostat which provide a c-wire adapter. You can control all fuctions only with 4-wires.

[midwestengimaker]

FYI, the “OL” on the meter is short for “Overload”, not “Open Loop”. It is considered overload because the resistance it is measuring is higher than its max limit.

[jstrick]

I installed smart thermostats and I’m so glad I repainted instead of using those huge wall plates. Looks so much cleaner.

[HarmonsHarbor]

He’s a candidate for spray foam. The 140 degree air handler and utility bill will thank him.

[agoldstein]

I was surprised the heating system was not checked for the splice before sacrificing the fan connection.

[dtemp]

Would it have been possible to see if there was a way to make a common wire with the heat unit? That cable had extra wires too, and if there's a 24V transformer, there's a hot and neutral tap.

[gtarider]

lol I got into trouble on a job for putting that big ugly back plate on. They do sell converters from a 3wire to a 4 wire or just use nest it does not need the common wire

[jasonziegler]

After owning 2 nests and an ecobee I'd like to say an upgrade would be to install a reliable and inexpensive thermostat not connected to the internet

[kenmore]

What about heater control? They have a fancy new thermostat and no control over the heat

[kchiem]

Nest thermostats don't need a C. They basically have a battery and charge up when they run the fan, which happens something like 10-15 mins every hour.

[jamesklaatu]

Does smart mean some else can restrict my power use remotely?

[generallyhelpfulsoftware]

That’s an ecobee. It has remote temperature sensors. Why not mount it on the wall of the floor above and run a new wire. You use your phone to control it anyway, it could be anywhere in the house. I have 5 of them in my house and I physically touch one, maybe twice a month. The thermostat itself doesn’t have to provide temperature data.

[John_Locke_]

My Nest ran for years without a C wire. And then all of a sudden it stopped working unless I charge it for twelve hours every day. But only with the Ac. Runs fine all winter no problem. No idea what the issue is but it's annoying.