On Monday, April 17, 2017 at 9:25:41 AM UTC-4, Percival P. Cassidy wrote:
On 04/16/2017 05:23 PM, I wrote:

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.

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

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.