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.
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?