How to Upgrade to a Smart Thermostat with Four Wires | This Old House: Live

Ask This Old House home technology expert Ross Trethewey demonstrates how to install a new smart thermostat without an existing common wire.

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Cost: $150

Time: 1 hour

Difficulty: Easy

Shopping List:

Tools:

Steps for Installing a Smart Thermostat:
1. Shut the power off and disconnect the old thermostat using the small flathead screwdriver. Ross suggests taking a picture of the wiring configuration to keep track of which wire goes where.
Remove the wall screws holding up the thermostat before disconnecting the wiring to prevent the wires from falling into the wall.
2. Mount the baseplate of the smart thermostat to the wall using the provided screws.
Wire the smart thermostat to the correct terminals.
a. In general, the heating wire (often white), goes into the “W” port, the cooling wire (often yellow), goes into the “Y” port, the wire that sends power from the HVAC system to the thermostat (often red), goes into the “R” port, the fan wire (often green), goes into the “G” port, and the common wire (often blue), which should go into the “C” port.
b. If you don’t have a common wire, Ross recommends wiring the G wire into the common port instead. Most people don’t use the “fan only” mode, so while you’d give up control over the fan in the system, you’d gain constant power to the smart thermostat. You’ll also have to change the wire at the furnace from G to C.
5. Connect the thermostat to the baseplate.
6. Configure the thermostat following the manufacturer’s settings.

Resources:

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This Old House, Ask This Old House, DIY, Home Improvement, DIY Ideas, Renovation, Renovation Ideas, How To Fix, How To Install, How To Build, Ross Trethewey, smart thermostat, heating, electrical

How to Upgrade to a Smart Thermostat with Four Wires | This Old House: Live

[robertsteich]

I would’ve put a piece of tape over the hole and poke the wires through it. This minimizes air leakage from the inside of the wall. Which can give inaccurate temperatures to the thermostat.

[ptg]

Seems simple enough ! At the furnace side, I've seen other instructions that says in addition to moving green wire to "C" terminal, you need to install a short jumper wire between "G" terminal and "Y" terminal inside the furnace's control board ?

[the-bulb]

Many homes have the old circle turn with 2 or 3 wires I would like to see
How you go from that to a new updated style

[pliontis]

I don't know about this cheap E model you used, but a couple of the normal 2nd gen ones that i am using for years now, are wired normally (all the cables at the correct place - including the fan) and work flawlessly. The battery was only used a couple of times we had a power failure.

[mansharker]

I would have ran a new line with more wire pairs inside the cable, for control of the fan as well.

[finallyamommy]

Will the fan be stuck on auto or constantly on?

[Hookset]

I would have fixed the holes and painted the wall before mounting the new control.

[mattb]

Google just needs to make the Nest compensate and adjust the real room air temperature measurement against the cold-soak wall temperature. The cold wall still throws off the temperature measurement and causes unnecessary call-for-heat cycles. All they need is a thermal couple dongle that hangs in the wall cavity connected from their base-plate.

[ZachCullenDrums]

I’ve thought about doing mine this way but there’s already something attached to the C Wire terminal down at the furnace but no C wire behind my thermostat. Can I connect it anyway?

[beefmelt]

My question is this. If I set the temperature to something like 75 degrees, is the thermostat able to operate the heating furnace or the AC unit, whichever needs to be running to maintain my temperature setting without me tending the thermostat?

[seankoons]

Wait if you get rid of green which is fab control. What happens. Don’t you need that?

[peewee]

Question. What happens to the G wire spot since isnt connected to the correct spot since it’s replaced the yellow wire in this sense. Since there is nothing on the G terminals, what’s the consequence?

[-old-Forthischet]

A trick I learned as a telecom tech was to cinch a LARGE cable tie onto the wire BEFORE it falls back into the wall because all it takes is that one/first time.
Another tip is that many of these thermostats do not charge the batteries so beware that AAA size batteries don't last very long with consistent use. I change out the batteries every summer season in my condo and remove the batteries in my RV thermostat when not in use.

[reaperbot]

could have ran a newer line of cables with to support the added functions. depending on how the original wiring was setup that is. sadly it's not always a straight shot from the thermostat to the heating/cooling unit. but when fishing new wiring you can tape the newer one to the older one. so you can fish the new line in place of the old line when you pull out the older line. however it can be tricky for someone with little/no experience.

[yuniorlazo]

You are brilliant٫ you safe me a trip to the store thanks brother👍

[dertythegrower]

I am amazed you still found something that was made to last more than 10 years, let alone over 20. The thing almost looks like you just installed it to do this video

[zacheryaltamirano]

Are all terminals in the artic or on the unit itself that’s in a closet like mine?

[carlosfiriarte]

Do you need to set the gas as Auto or on?

[EMBer]

How hard would it be to pull a new wire? In case it breaks or you want to upgrade.

[soniamaldonado]

If I’m not switching the yellow to the Common slot would there be any reason I’d have to even touch the furnace? In other words can I change the thermostat without having to do anything to it? Thx.