Central Air Unit not Sending Enough Power to Thermostat

Question

Right now I don't have specifics on my central air unit (make, model, etc) but hopefully the info I provide will help.

I have a Second Generation (and Third Generation) Nest Thermostat hooked up to the Central Air Unit. I receive intermittent messages of "not enough power" being sent to the thermostat. (The reason I mentioned both Nests is that I've tried both units).

The wire contains one hot wire (Rh), one for the fan (G), and one for the cool/compressor (Y1).

I used a multimeter to test the voltage, and when I touch the positive probe (red wire) to the Rh wire and the negative probe to the G1 wire, I get a strong enough reading (between 27-28 volts). But when I do the same to the Y1 wire I get next to nothing.

After some research, I found people recommending changing the fuse in the unit itself... but when I lifted the panel, I didn't see any fuses that could be replaced. Moreover, the power drain doesn't seem to be consistent (i.e. today it is showing a poor voltage reading and tomorrow it will be fine).

I don't believe its the Nest because I've tried different ones and get the same results. Sorry for the long question, but I wanted to be specific. Any ideas on what is causing the power generation problem?

Answer 1

This is the most common problem with the Nest. You need to hook up a "C" wire.

Here's the thing. Originally, thermostats were passive devices - nothing more than switches. And so, controls were wired in a "switch loop" similar to how switches in houses were wired before current electrical code. Power went in a functional loop: from the 24V transformer, to the thermostat, to the actuator, and back to the transformer.

The R wire is sorta like a "hot" wire, the other wires at the thermostat are sorta like "switched hot". And just like a switch loop doesn't have or need a neutral, neither does the thermostat (the neutral would be the C wire).

Old style dimmers and motion sensors powered themsleves by leaking a bit of power through the incandescent bulb. That fails with efficient CFLs or LEDs. Likewise, the Nest tries to power itself without a C wire by leaking a bit of power through the big clunky old relay that switches on the furnace, fan or A/C. That fails with efficient relays or electronic controls.

In this situation, you need to run a C wire, as the Nest manual discusses. Tester101 is proposing that you may have spare wires in your thermostat cable.

There are other ways to solve the problem, but they're starting to resemble electrical engineering.