Author Topic: Replacing a line voltage thermostat with a standard one (Update: Solved)  (Read 2146 times)

ShoulderThingThatGoesUp

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I have electric baseboard heaters in my "third floor" (really a finished walkup attic). There's a Honeywell thermostat on it currently which does a terrible job keeping a temperature and is super easy to leave on all night accidentally. (We mostly use it as a playroom for our daughter, but she doesn't sleep up there. Someday people will sleep up there when we have more kids.) I got a programmable thermostat for Christmas (a Nest, facepunch away) and went to install it...turns out the thing is wired like a switch. After some research I've figured out that this means it's a line voltage thermostat.

Further research shows that one can buy a relay which takes line-voltage thermostat wires on one end and puts out regular thermostat wires on the other end, allowing me to use a regular thermostat in the room.

Questions:
1. Is this a bad project to DIY? Should I pay a real electrician to do it right the first time? I have replaced plenty of switches, light fixtures, outlets, and two dishwashers, but I have no experience with "new" wiring.
2. How exactly do I find the part that I need? I have tried searching for instructions but this doesn't appear to be a very common problem. I expect there are some diagnostic steps to take but I don't know what they are.
3. Will I be able to shove the relay in an "old work" box in the wall so I can conveniently mount the thermostat?
« Last Edit: January 18, 2016, 08:24:50 AM by ShoulderThingThatGoesUp »

lthenderson

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You might want to Google the subject. I've heard of quite a few people who have done what you propose and have problems getting the Nest to play nice with their furnace. All of them end up just getting a different thermostat and as a Nest owner (though on a low voltage system), I think you would be better off anyway. Mine finally connected to the internet after two weeks of being unable to do so and yes, this is the third year I've owned it and it does this quite regularly, among other quirky things.

ShoulderThingThatGoesUp

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I have another one controlling my steam radiators that does great, so I was hoping to integrate everything in that respect.

ShoulderThingThatGoesUp

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I am getting electricians' quotes for this as I haven't been able to find a suitable DIY guide.

Update: the first guy who called back said "just buy a line voltage programmable thermostat." Well, fair enough.
« Last Edit: December 29, 2015, 07:57:28 AM by ShoulderThingThatGoesUp »

soupcxan

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ShoulderThingThatGoesUp

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My wife is very set on the nest because of the auto-away so we don't forget to turn off the heat when we leave the floor, and because we already have one downstairs.

ShoulderThingThatGoesUp

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Re: Replacing a line voltage thermostat with a standard one (Update: Solved)
« Reply #6 on: January 18, 2016, 08:33:08 AM »
Got it done. It required a transformer to go from 240 volts to 24 volts; I used this one.

The Nest will turn on with two wires or three, but to actually get the heat to turn on you need to use C, W, and R. Paid a friend who does HVAC for a living to solve that problem for me.

The steps were:

0. Turn off power to the unit. I have two breakers, one for the white wire and one for the black.
1. Get old switch box holding old thermostat out of the wall. It was a new work box, so it was nailed to the stud before installation. I got out a little drywall around it and then used a hammer and chisel to take off the tabs holding the box to the nail so I wouldn't have to make a giant hole in the drywall.
2. Attach transformer to the new junction box using the supplied nut that fits into a 1/2-inch knockout. I then threaded the thermostat wire from the transformer back into the box
3. Bring in all the wires - three sets of white and black, one coming from the breaker box, the other two going to the baseboards - into the new metal box.
4. Put the metal box in the wall. It had some small holes in the side that I put nails through to attach it to the stud. The old nails that held up the old box I just hammered out of the way. The transformer fit behind the drywall so with a little persuasion I got everything in there.
5. Attach wires: all the whites connected to the transformer's blue wire, the black from the breaker connected to the black from the transformer, and the blacks to the heaters connected to the red on the transformer.
6. Wire up the thermostat, put it on the wall, and turn the breakers back on. It works!

Note that when you have two Nests in the same house the app wants to link them for auto-away purposes, which is the exact opposite of what I wanted. So, I had to set them up as separate houses in the app.

paddedhat

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Re: Replacing a line voltage thermostat with a standard one (Update: Solved)
« Reply #7 on: January 18, 2016, 06:52:32 PM »
Good job. If you ever need to remove an plastic existing nail on box again, try this. Take a hacksaw blade and tape 1/2 of the blade up with electrical tape to function as a handle. Remove the cover plate, slide the blade between the box and the stud and cut the nails off. Clean, easy, no damage to the sheetrock.

ShoulderThingThatGoesUp

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Re: Replacing a line voltage thermostat with a standard one (Update: Solved)
« Reply #8 on: January 19, 2016, 03:54:44 AM »
Good job. If you ever need to remove an plastic existing nail on box again, try this. Take a hacksaw blade and tape 1/2 of the blade up with electrical tape to function as a handle. Remove the cover plate, slide the blade between the box and the stud and cut the nails off. Clean, easy, no damage to the sheetrock.

That sounds like a much more pleasant way to do it. The plastic cover that came with the thermostat covered the drywall bruising that I did do, so it worked out, but I'm sure I would have been done sooner this way - getting the thing out was a huge portion of the time.

ShoulderThingThatGoesUp

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Necro-ing this with some questions. The electrical box this is attached to projects into a sealed attic space. There's no A/C attached to the thermostat, but we have a window unit in the room, and I've noticed that the temperatures on the thermostat are totally bonkers hot, except when you realize it's telling you the temperature from the attic.

Question 1. What's the best way to insulate behind the thermostat, since it's apparently letting hot air (or cold, in the winter) into the room? A thin layer of foam that I poke the wires through?

Question 2. Can I connect the Nest to an attic fan that I might someday put in that space?