I purchased a Honeywell RTH6500 Wifi Thermostat, which requires a C wire. My old thermostat showed a connected blue C wire. However, it was battery-powered. If I removed the battery, the thermostat screen would go dark. After I connected the RTH6500, I measured with a multimeter, which showed 0V on the C wire. All the other wires showed positive voltage. I went up to the air handler and saw there was a unconnected blue wire. Would I be ok stripping the blue wire and connecting it to the other two in the wire nut?

In the picture that shows the wires near the air handler, I'm guessing the three sets of wires are going to the (1) air handler, (2) heat pump, and (3) thermostat. You can see the green wire is also unconnected on a different set. The green wire on the thermostat is connected to G, which I think is the fan. I think that set of wires is probably going to the heat pump, since it doesn't have a fan?

I appreciate any help. Thank you.

old thermostat's back panel wiring

wiring near air handler

I just want to double I'm doing the right thing before I open up the air handler. It looks like the wires are coming out of the panel on the right, so I'm going to open that one up. Also, I've turned the breakers for the both air handlers and both heat pumps off. That should cover it, right?

air handler pic 1 air handler pic 2 electric panel

I opened up my air handler, and I'm seeing the blue wire connected to B terminal on the air handler. I found a wiring guide for Trane, but it doesn't exactly say B is common. The Honeywell wiring guide shows that B can go into the C terminal. I'm not sure what to make of this.

air handler terminal enter image description here

  • Where does the white wire connect on the furnace, if anywhere? – ThreePhaseEel Mar 7 '19 at 4:01
  • I don't have a furnace, I don't think. On my old thermostat, the white wire was connected to O. On the wiring diagram for my old thermostat, it shows O is connected to the heat pump's reversing valve. pro.luxproducts.com/wp-content/uploads/2017/09/… – avy Mar 7 '19 at 4:05
  • Well, air-handler/indoor-unit then -- really all the same from our standpoint here – ThreePhaseEel Mar 7 '19 at 4:11
  • I'll have to get back to you on this. I need to find a hex screwdriver to open up the air handler's panel. – avy Mar 7 '19 at 4:52

You need to open the air handler and confirm the wiring. Never trust the color of the wires. If I had to guess in a hurry in a panic situation I would try the blue connection. But you are right in front of it. You are two screws away from confirmation.

Shut the power off at the switch first.

How to tell if your B wire is really a common wire in disguise:

Read through the manual for your old thermostat. If you have the manual for your thermostat (or can find it online), it may describe the function associated with the B connector and help you determine if it’s meant for a heat pump wire or a common wire.

Does your thermostat have a connector labeled C? Whether or not there is a wire attached to it, if your thermostat has a separate connector labeled C in addition to a connector labeled B, then the wire labeled B should be connected to the O/B connector when installing the Nest Learning Thermostat or to the /OB connector when installing the Nest Thermostat E.

Do you have a heat pump? It helps to know what kind of system you have. If you have a heat pump and you don’t have a separate wire labeled O, then the wire labeled B should be connected to the Nest Learning Thermostat's O/B connector or to the Nest Thermostat E’s /OB connector.

What color is the wire labeled B? While the color of the wires connected to a thermostat often don’t adhere to industry standards, they can provide a clue in situations where you’re still uncertain about the function of a wire labeled B. If your thermostat doesn’t have a separate connector labeled C, and you’re not sure what type of system you have, make a note of the wire color connected to B. A brown wire is mostly likely a heat pump wire and should be connected to the O/B connector on the Nest Learning Thermostat or the /OB connector on the Nest Thermostat E. But if the B wire is blue, it should most likely be connected to the C connector.

That's from Nests website

I don't work with heat pumps enough to say "yes connect to B"

I would look at how the system is wired up and determine if it's suitable as a common.

There is one terminal in the image that is not visible, is it a C terminal?


To conclude,

This is as far as I know, unique to Trane heat pumps. Trane only employs the O terminal for the reverseing valve and do not mark or identify a C terminal and substitute it with the B terminal. Seems odd to me but Trane does do some weird things.

Anyway you should be good to go on the air handler B terminal.

  • The one next to the G terminal (green wire) is T. – avy Mar 9 '19 at 20:16

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