Honeywell thermostat says "Wait" then reboots

[Percival P. Cassidy]

We have a Honeywell RTH8580WF WiFi Thermostat that was installed by repurposing the G-wire to substitute for the missing C-wire as described in a Honeywell online video: move the G-wire from the G terminal to the C terminal at the furnace end, and jumper the G and Y terminals, and connect the G wire to the C terminal at the thermostat.

Everything worked fine as long as we were trying to heat the premises, but now that we are trying to cool down, the thermostat indicates "Wait" for several minutes, then clicks, and the screen goes blank and the thing "reboots."

The furnace is an ancient Singer, and I can't see a brand name on the A/C compressor.

The same thing happens if I turn off the breaker for the 240V to the compressor unit.

Perce

[Percival P. Cassidy]

Honeywell Tech Support says that although that training video about repurposing the G wire is out there, it is intended only as a temporary solution and may not work on all systems. They suggest either running a separate C wire from the furnace to the thermostat or using one of their Wire Saver modules.

Perce

[trader_4]

I don;t think Honeywell knows what they are talking about. If it works temporarily, then why would it not work permanently? All you're doing is eliminating the wire that you had that optionally turns on the fan manually and re-purposing that wire to be used as the common back to the transformer so that the thermostat has power.

I can't explain what you're seeing though. The "wait" is normal if power has been interrupted or if you're trying to start the AC after the thermostat thinks it's been running very recently. It wants to wait ~ 5 mins for the pressure to reduce. Are you sure you have the wire going to the common side of the transformer and not somewhere else? It looks like it's losing power somehow when it tries to switch on the AC, ie, it waits 5 mins, then when it closes the relay to activate the compressor contactor, it loses power. I would be very surprised if you ran a new wire to the same place you have the re-purposed G wire connected now, reconnected the G wire as the fan wire, and it worked.

[Percival P. Cassidy]

Is it possible that it's unduly stressing the system somehow?

The re-purposed G wire is going to the C terminal both at the thermostat and at the HVAC unit.

There is a skinny 2-conductor cable connected to R and C that looks as though it goes off to the outdoor A/C unit.

Everything worked fine with an RTH7400 and the original connections; of course that thermostat did not need the C wire, since it was powered by its two internal AA cells.

Perce

[Tekkie?]

It needs the fourth wire for cooling. Either add the wire or the module, which is a substitute for the fourth wire,

[trader_4]

How can it possibly be over stressing the system? All you did was remove the G wire that's used to optionally turn on the fan from the thermostat and then use that wire as the common to supply power to the thermostat. It's no different than if you ran a new wire for the common and disconnected the fan control wire, which, AFAIK, is optional.

That seems right. Have you put a volt meter on the R and C coming to the thermostat and monitored what happens when it calls for AC? Particularly after the delay, when it craps out?

That doesn't seem right. The wires going to the outdoor unit should be coming from the control board. One side is common, the other side should be the control signal from the furnace control board that goes to the contactor to turn it on. If R and C were going to the contactor, then teh compressor would be on all the time, no?

[Percival P. Cassidy]

But could jumpering G and Y be doing anything undesirable?

No, I haven't. There's no place to do that easily. There isn't enough slack in the wiring at the thermostat end to make any connections, and at the furnace end, removing the cover will kill the power to it.

Here's the link to a picture of the wiring at the furnace before I re-purposed the G wire and installed the G-to-Y jumper:

Ah, but now I think I see that that two-conductor cable goes to C and Y, not C and R.

Perce

[trader_4]

So splice some test wire on.

Link doesn't work, takes me to dropbox and wants me to sign in.

That sounds better. It's also possible that you might have a bad contactor in the AC that's on it's way out? If it's partially shorted it could draw so much current that the voltage drops, causing the thermostat to lose power. You could try disconnecting the contactor wire and see what happens. If the thermostat then behaves normally, you'll have another clue. The outside AC unit won't turn on of course.

In the discussion you haven't said what gauge wire is used, I'm assuming it's typical 20g, if it's some telephone wire, that would be bad.

[Percival P. Cassidy]

I'll try that later.

Try this:

Look like 18 or 20ga.

The furnace is a Singer (not made since the 80s??) GC232000 series, according to the disintegrating schematic.

Perce

[trader_4]

I wouldn't think so, but who knows? Why don't you try removing the jumper between G and Y at the furnace and then try it and see what happens. The jumper may not even be necessary. On many furnaces the controller board turns on the blower automatically when AC is called for by the thermostat and the G terminal is only used to optionally activate the blower when you're neither heating nor cooling, but just want to move air. That's how mine works.

If you do that test, just don't let the compressor run for more than a minute unless the blower spins up, otherwise you'll over cool the evaporator because it won't have air flow.