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Forgive my lack of knowledge here. I bought my husband an Amazon smart thermostat awhile back so that it would go with all of our other Alexa smart home stuff. We hooked it up where the previous (battery powered) thermostat was and it wouldn’t power on. He works in hvac so we used the multimeter to get reads at the furnace and at the thermostat and the thermostat read was too low—we needed to run a new wire. I don’t remember all the exact specifics because we tried this maybe a year ago when I first bought it, and then gave up trying once it was too complicated and have just used the old thermostat. Motivation to figure it out again came when we got an email from the power company saying we could be saving big dollars every month by registering a smart thermostat. Plus I just want it to work and don’t like giving up.

We can’t fish the new wire using the old wire because the old one is stapled to the studs. Further, the thermostat is on our main floor at the top of the basement steps. The furnace is down those steps, across the hallway in the laundry room. It’s a finished basement, so we stopped trying when it sounded like punching multiple holes in drywall/ceiling were the only options. The only other option we could think of was moving the thermostat, but putting it in a place right above the boiler requiring less drywall punching would mean putting it in the kitchen—not a good spot.

Am I missing some other option here that doesn’t involve me learning electric and drywall repair?

ETA photos: I left for work before taking pics, but we never paid to take down our Zillow listing so I swear these aren't stock photos. Added arrows and captions to help, but I apologize for having nothing clearer.

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    If it goes along the joists, should only need a hole at each end, across the joists is a PITA. I would read the fine print on saving big dollars, sounds like a gimmick of saving big with just a thermostat. Can save big by turning it down a couple of degrees.
    – crip659
    Commented Dec 6, 2023 at 23:04
  • There's not enough info to really answer, a batter operated smart thermostat should be able to control anything a regular thermostat can control. Also, drywall is not as hard as people think. Don't make holes, instead find studs, and cut out rectangular sections, then put them back after.
    – Ariel
    Commented Dec 6, 2023 at 23:19
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    Since you bought it, register it and worry about the installation later. They may ask to see the receipt.
    – QTX
    Commented Dec 7, 2023 at 0:50
  • Can you post photos of the wiring at both the thermostat and the furnace ends please? Commented Dec 7, 2023 at 3:49
  • Photos added to original post--sorry they aren't better!
    – Lydnort
    Commented Dec 7, 2023 at 16:41

3 Answers 3

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If your problem is the most common one, lack of a C wire, the easiest alternative is to use a C-wire adapter. You install a pair of boxes, one at the furnace and one at the thermostat, which delivers power to the thermostat and also lets the thermostat do its thing, all over the existing wires.

And with the Amazon thermostat in particular there is a variant that has the thermostat-end box built in to the thermostat. Return the one you have and get that one! Then there's no box upstairs, only one at the furnace, and no new wiring at all. The new thermostat will work over the existing wires. You just have to connect the wires in a special way to the special Amazon thermostat.

All this assumes that your only issue is the lack of a C wire. If the old thermostat worked, that is almost certainly the problem.

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  • Thank you! I am pretty sure that back when we first tried to mess with it, I did order the c-wire adapter and it did not end up being the issue. I remember being irritated that I couldn't return it by the time we figured out I didn't need it.
    – Lydnort
    Commented Dec 7, 2023 at 17:26
  • Ok, but I can't think of a reason your old thermostat would continue working but the new one would not, other than a missing C wire unless they are totally incompatible like you have a milliamp-based furnace or a 120V-based furnace. You need to determine the problem, and your requirements, before you focus on fishing cable. Possibly worth a new question to help diagnose the issue if you think it's not lack of a C wire? There are easy ways to prove that.
    – jay613
    Commented Dec 7, 2023 at 18:05
  • Would it have to do with the different multimeter readings at the furnace end versus the thermostat end? I remember that the readings were different, and based on our research the voltage was too low to power the wifi device versus one with battery backup.
    – Lydnort
    Commented Dec 7, 2023 at 19:10
  • I feel like my husband (who does boilers, not electric) said that it could mean the wire was bad, split somewhere, had a staple through it, or something along those lines.
    – Lydnort
    Commented Dec 7, 2023 at 19:11
  • Also, the old one had a battery. Works with lower voltage with the battery as backup
    – Lydnort
    Commented Dec 8, 2023 at 2:54
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There may be other options.

If there are enough existing thermostat wires for all necessary functions and the only issue is that the wires work well enough for a basic thermostat but don't have enough power for the smart thermostat then:

  • You may be able to replace the transformer (in or near the HVAC system) to provide more power.
  • You may be able to add supplemental power for the thermostat using a transformer installed near the thermostat while still handling all HVAC functions through the existing wires.

That last option also applies if the problem is that the thermostat needs an additional wire to get power from the HVAC transformer.

All of this depends on the specific make/model of thermostat and possibly also on what wires are currently used for different functions with the existing thermostat.

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  • Thanks for this. I did read some about the 24v transformer, but saw some posts that said it only works when you have a heat only situation. I don’t know enough to know whether that applies to me. But yes. Enough existing wires for a regular thermostat, just not enough power for the fancy new thermostat.
    – Lydnort
    Commented Dec 7, 2023 at 2:10
  • Photos added for reference
    – Lydnort
    Commented Dec 7, 2023 at 17:02
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Another option is to simply use some wire covers and run the cable down along the baseboards. If you have quarter-round, they even make quarter-round shaped wire covers.

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  • Thank you! We could do this, but it would have to run around the corner of the main floor, down the stairs, up to and across the ceiling, back down to the floor and into the laundry room, and around the perimeter of the laundry room to get to the furnace. Is that pretty typical for this type of work-around, or is that unsafe? I have no background for context here, lol.
    – Lydnort
    Commented Dec 7, 2023 at 15:32
  • Since we don't have a picture of your house, it's hard to make really good route suggestions. Can you sketch a floor plan or something, showing both the main floor and basement? Are the basement walls finished?
    – Huesmann
    Commented Dec 7, 2023 at 15:33
  • Just added photos. Basement walls/ceilings are finished.
    – Lydnort
    Commented Dec 7, 2023 at 16:42

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