2

My apartment has a central heating/cooling HVAC system consisting of a gas-fired hot water heater which feeds hot water to a First Co air handler for heating, and a rooftop compressor which feeds coolant for air conditioning. I'm wondering if I can hook up the Common wire to a Nest given the following detailed configuration.

There is already a similar question here but with less detail.

The details of my setup is as follows:

  • First Co TAQ1824 Air Handler from 1996
  • Rooftop AC compressor - uknown brand/model
  • Johnson Controls cold weather shutoff for AC Compressor
  • Honeywell L6006C1034 "Aquastat" Circulator on hot water output of hot water tank. I think this is a just a temperature-based shutoff for heat control so we're not trying to heat with cold water.
  • Honeywell V804F1135 "Zone Valve" on hot water inlet to air handler
  • Honeywell TH1110D1000 digital dumb Thermostat with batter power.

It originally had a mechanical thermostat. The bundled furnace wire to the thermostat includes a black common wire which is not connected on either end.

The following installation instructions and wiring diagram are taped to the side of the air handler and clearly indicate a Brown Common wire exiting the handler.

First Co TAQ1824 Installation Instructions

I took the time to trace out all the existing connections in my system and I've included them here:

Furnace Wiring Schematic

I recently tried to connect a Nest E thermostat in the place of the existing Honeywell unit but it does not detect any power. It indicates about 2.5V at its inputs, and when I measure them with a multi-meter I see the same. This should be more like 25-37V. I'm not sure why the Nest is unable to do its normal power stealing in this system, but in any case, I want to hook up the common wire now.

I should be able to just connect the black wire in the bundled furnace wire between the thermostat's 'C' terminal and the air handler's Brown output wire, i.e. add the following dotted purple connection:

Common Wire Connection

Is that safe to do in this scenario? The one point I'm unsure about is the note in the air handler instructions that says "Should any add-on equipment also have a Class 2 transformer furnished, care must be taken to prevent interconnecting outputs of the two transformers by using a thermostat with isolating contacts."

The only item in my system I can't easily inspect is the AC Compressor, so I don't know if this warning applies to my scenario. I also don't know if the Nest or my existing thermostat have isolating contacts. Google searches were not revealing.

Any suggestions on making this connection are appreciated.

PS: On a safety note, although the hot water heater is gas-fired, it appears to be independent of the air handler/ AC/ thermostat system I outlined above. None of the electrical devices I'm discussing here directly interact with, control, or ignite gas. The hot water heater does its own thing, and keeps itself heated as hot water is pulled out either by the air handler or other household usage.

  • 1
    For systems with two transformers, you would normally have a red wire for heat and a separate for cooling (commonly labeled Rh and Rc on thermostats). So, it seems like you just have one transformer. – JPhi1618 Mar 11 at 18:46
  • Do you measure 24v between the red and brown wire exiting the air handler? Also, common is commonly attached to the metal body of the air handler, so you can also check for continuity between brown and a good metal "ground". – JPhi1618 Mar 11 at 18:50
  • @JPhi1618 Re: your first comment: Thanks for the tip! I have noticed discussion of Rh and Rc online, I didn't realize that was what they referred to. In my system there is only the one R wire coming from the air handler to the thermostat. As outlined in my sketch, when the thermostat wants to run AC, the Cool signal goes through the cold weather shutoff which, if it's warm enough outdoors, passes it on to the AC unit, who's only connection back to the rest of the system is to the air handler's Common wire. – SSilk Mar 11 at 18:50
  • @JPhi1618 Regarding your second comment, I will check those measurements tonight and see what I find. – SSilk Mar 11 at 18:53
  • 1
    @JPhi1618 I checked all my voltages and confirmed that the voltage between Red and Brown at the air handler was 24V AC RMS. I connected the black wire of the bundled furnace wire to the air handler's brown Common wire and and the Nest's Common terminal and the Nest is now getting powered properly now and is able to charge its battery. If you want to re-post your first comment as an answer I will accept it. Thanks. – SSilk Mar 17 at 23:51
1

For systems with two transformers, you would normally have a red wire for heat and a separate for cooling (commonly labeled Rh and Rc on thermostats). So, it seems like you just have one transformer.

Did you measure 24v between the red and brown wire exiting the air handler? Also, common is commonly attached to the metal body of the air handler, so you can also check for continuity between brown and a good metal "ground".

Great diagrams! They make answering a question like this much easier.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.