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I am just starting out with wiring and some other home improvements. Decided to install a Nest 3rd generation thermostat. I had a TH5110D Honeywell thermostat, so I pulled it out and found 5 wires running into the wall. The R, Y, G, W were connected to Thermostat, and C was not connected. This is fine for Honeywell since it was running on 3v AAA batteries. However, I did not check the voltage on the R line.

After checking that the wire config was ok with Nest, I set out to installing it. After wiring up, the Nest was not turning on. So charged it with microUSB for some time, and then tried to install it but it said the Rh had too low or no voltage.

When I checked the voltage between R and W (after turning on the breaker for my HVAC) I realized it only had 750mv. I read online that 750mv is being used in old thermopile configs. I have a 2010 American Standard AUE1B080A9361A, with gas heater, external cooling AC and a combined fan on furnace. I am surprised this system is not using 24v. Or maybe it is using it internally, just to thermostat it was sending 750mv.

In any case, I did not want to change anything on HVAC, so I want to keep two system (NEST and HVAC) connected through set of relays. This is what I am thinking of wiring up, enter image description here

I will use a 24volt AC external adapter and connect it to Nest, as well as the terminals on the relays for W, Y and G. The old wires from furnace controller board will go into relays and will never receive 24v, only was the original R wire was sending, i.e. 750mv.

Can anyone review and see if I am doing anything wrong?

Thank you.

  • Can you post photos of the wiring at the furnace end and/or the wiring diagram of the furnace? From the literature I found, your furnace definitely should be a standard 24VAC unit... – ThreePhaseEel Nov 8 '18 at 3:35
  • You are right, I forgot to check the actual Furnace Control Unit before asking the question (should have done my homework), and the system is indeed 24VAC. The C wire on furnace control board was not connected, because it was not needed before. I just connected that one on the board, then checked the AC voltage between R and C near my thermostat outlet and it was 27 something. Apparently the 750mv was some grounding feedback (it was 713mv to be exact) that I measured before. And also the old thermostat I had can handle 750mv system, which led me to believe that it's 750mv system. Thanks. – ad1988 Nov 8 '18 at 16:08
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It was my fault. I did not check the Furnace Control board before posting the question. The system is 24VAC. The C wire was not connected on my control board by the previous owner/installer because it was not needed. Connected the C wire, and now I read 27v AC between R and C. Tried connecting Nest, and that works too. Tested Fan, heating and cooling, all good.

(P.S. I feel stupid for not realizing the control board was on the bottom panel, I thought the board was hidden somewhere inside, and I was reluctant to unscrew anything major on the furnace)

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