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· Registered
2 Posts
Discussion Starter · #1 ·
Hi all,

New homeowner here, with a very old HVAC system-- grateful for any help!

I have tried-- and thus far failed-- to replace my old Totaline P710U-21PHP thermostat with a new Honeywell T9 Smart Thermostat (advised Honeywell works better with older HVACs).

The old tstat had fewer terminals: Y, C, R, G, W, O/B-- wired as Y-yellow, C-blue, R-red, G-green, W-white and black, O/B- orange.

The Honeywell has more terminals-- but the wiring matched up. First I tried wiring both the white and black wires into the W terminal on the Honeywell. It powered up, but then I kept getting an "Invalid Wiring" message on the Honeywell. "You connected the aux/backup heat to the W1 terminal on the thermostat. Please use the W2/aux terminal for the aux back-up heat instead." So I tried it both ways, with W-white/W2-black and W-black/W2-white-- got the same error message either way. So I tried both both black and white wire into W2 terminal and, success, error message gone!

The tstat then popped up a new screen/prompt asking to select the "Reversing Valve"-- "Is your reversing valve energized when cooling or heating?" Choose O or B. "Cooling (O Wire) - Select if O wire is connected to the O/B terminal. When cooling mode is active, then the O/B terminal will be energized." Or "Heating (B Wire) - Select if B wire is connected to the O/B terminal. When heating mode is active, then the O/B terminal will be energized." My O wire was wired to O/B on the old tstat, so I selected that. The Honeywell proceeded to finish set up and went to the home screen-- job done, I thought!

As soon as I tried to actually turn on the system (cooling), it immediately blew a 3-amp fuse on the electrical panel in the HVAC. I replaced the fuse, everything proceeded same as before. Got to the homescreen following set up, and I selected the fan--the fan turned on. I tried to turn up the heat, nothing happened. I tried to turn on the cooling, and it blew the fuse again. Seemed the logical thing to try next, to me as a novice, would be to switch selections on the tsat from O to B going into O/B terminal (as reversing valve).

Problem is now, even after replacing the fuse yet again, I can no longer get power to the tstat at all-- it blows the fuse immediately, as soon as I turn the breaker back on. So I'm at a bit of a loss.

I've attached a bunch of photos that may be helpful. Only other perhaps helpful information, is that looking at the actual wiring on the furnace, there is one set of wires heading out of the system to the tstat, and one set of wires heading up onto the roof. Nothing is wired to W2 on the furnace. Yet, there are 2 white wires and one black wire wired to W1. One of the whites heading up to the roof. Not sure if that's helpful.

Any suggestions on how to correct the wiring? Is the problem with the W/W1 white/black issue? Is O the correct selection for reversing valve (from everything I read online, most systems use O)? How to get power back to the tstat without the fuse blowing?

Appreciate any suggestions. Thanks in advance!!



· In Loving Memory
42,671 Posts
Check that you are actually getting 24 volts between R and C at the thermostat.

· Registered
3,257 Posts
Could be a short in the wall box since the wires have been disturbed. If it has a metal box and mud ring for the mount, maybe served by conduit and grounded, one of the stat mounting screws may have pierced the wiring causing a short. (the "C" terminal is grounded at the furnace so any control load side wire getting nicked and grounded to a grounded box will represent a control line short and take out the 3.2 amp fuse.

That's just clutching at straws so far. Review all your work for some flaw, just in case.

If still no joy, seeking out a short is often done by measuring the resistance between the "C" screw and all the other screws at the stat, with all wires connected. Any wire with a very low reading or a reading well below all others would be suspect.

· Registered
1,346 Posts
Not sure if this helps but...

In my case, old thermostat had a C terminal but it was empty, not used. So, old thermostat has battery backup, to keep settings.

New thermostat is pending delivery. Requires a C wire, no batteries.

To get ready for new thermostat, checked air handler board to see color coding of what C terminal should be. It is brown. But it later connected to a blue wire that went to thermostat area. Blue wire was curled around wire bundle but not connected to thermostat.

Advice is to check wire colors at board, follow that they did not change.


I would take out the orange wire and put in the black on the O/B terminal. Sorry if it costs another fuse.
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