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myqhenry 05-30-2011 05:03 PM

Heat but no A/C with new thermostat
Hi All,
First, thanks to everyone as this is the first time posting and this site has provided quite a bit of valuable information for me in the past. Now it's almost 90 degrees out and I cannot get my A/C to properly function and it's pretty hot in here to say the least.

This past Winter I replaced a White Rogers 1F78 (5/2) thermostat with a Honeywell Prestige thermostat. Heating works perfectly and the new thermostat (with remote) allows me to properly heat my newborn's room. When installing I noticed that there was a separate set of wires (white and red) that I'm assuming are routed up to the A/C unit on the roof -- note that I'm in a large condo building. When installing the thermostat for heating, these were not used and I'm sure this is the reason for my A/C not properly functioning.

When I select "Cool" on the new thermostat the fan comes on, but does not blow cold air. I ran up the roof confirming that the A/C unit was not powered on.

The original thermostat was wired with the following:
Grey cable:
Brown -- Not used
Yellow -- Not used
Green -- Green terminal
Red -- Red terminal (w/ jumper to RC)
White -- White terminal

White cable (one not currently used):
Red -- Yellow terminal
White -- Twisted with blue wire from grey cable

The furnace was wired with the following:
Grey cable:
Yellow -- Yellow terminal
Green -- Green terminal
Red -- Red terminal
Blue -- Common terminal

The current thermostat was wired with the following:
Grey cable:
Brown -- Common terminal
Yellow -- Yellow terminal
Green -- Green terminal
Red -- Red terminal (w/ jumper to RC)
White -- White terminal

White cable -- not used

The current furnace has the following:
Grey cable:
Yellow -- Yellow terminal
Green -- Green terminal
Red -- Red terminal
Brown -- Common terminal

Below are some pics for reference.

Old thermostat terminals:

New thermostat terminals:

Current furnace terminals:

Any suggestions?

Thank you!

yuri 05-30-2011 05:12 PM

The 2 wires from the AC condensor on the roof should go to Y and C on the furnace control board. Not sure why they were removed.

myqhenry 05-30-2011 05:23 PM

Unfortunately, it does not look like I'd be able to move those wires without opening up the walls. Any other suggestions/workarounds?

Note that our unit was new construction and the old thermostat config was how it came.

yuri 05-30-2011 05:27 PM

I would be VERY VERY careful with what you do in a condo. You need to know for SURE those are the wires etc. If you hookup something wrong and things get damaged the property manager will be all over you for liability reasons. I would talk to them and see if they have a maintenance dept or guy or company they use and go thru the proper channels.

myqhenry 05-30-2011 05:29 PM

I'm 100% sure and have verified. Does the original setup make any sense?

JJboy 05-30-2011 05:55 PM

You can take a close up picture of wires in the board?

Look at this thermostat is set to heart (O/B) no colling

yuri 05-30-2011 06:07 PM

Looks like it is wired properly on the tstat. Why were the AC wires removed. There is no workaround. They need to be connected.

myqhenry 05-30-2011 06:08 PM

Close-up on the new thermostat? If so, see

According to the Honeywell install directions, it should be the following for a 1H/1C system (two transformers):
C -- 24VAC common
R -- Power (heating)
Rc -- Power (cooling)
W -- Heat Relay
Y -- Compressor Contacts
G -- Fan Relay

myqhenry 05-30-2011 06:15 PM

When the new tstat was installed, there was no terminal for those A/C wires and they were left not connected to anything. I can't recall why it was left this way.

The original config had the red wire from the A/C going to the yellow terminal on the tstat and had the white wire from the A/C twisted with the blue wire connecting to the common terminal on the furnace. Scratching my head of why this worked with the old tstat. Note that I haven't tried this on the new worried that it might cause more harm than good.

Just an FYI, after we moved into our unit we noticed that there were some oddities on how things were installed. We later learned that not the best contractors were used. Maybe the original setup was an easier way of doing things to make their lives easier...


JJboy 05-30-2011 06:29 PM

Did you test all wires for continuity? It Seems that Yellow was replaced in the old configuration for some reason.

yuri 05-30-2011 06:36 PM

You can hook it up the way it was B4. It is a bush league hacker way but they saved wire by not having to run it the extra distance to the furnace.:wink:

myqhenry 05-30-2011 06:54 PM

Bush league -- sounds about right for my building :)

JJBoy, yes all wires were tested.

What are your thoughts on just running the red wire (from the A/C) directly to the yellow terminal on the tstat without the twisting the white wire (from the A/C) to the blue wire to be run back to the common terminal on the furnace?


yuri 05-30-2011 07:01 PM

AC needs to be connected to Y and C at the tstat or furnace. Won't run w/o a C connection. Don't twist it. Poor connection will overload the transformer. Need to use a #29 or 31 marette or solder and tape the connection. Y and C need to go to the furnace also. Y starts the furnace fan on newer units G does it on very old ones and C is the return/neutral line.

myqhenry 05-30-2011 07:08 PM


Sorry for the amateur questions, but what if I left my current common wire setup the way it is and soldered the A/C white to the blue wire and connected the blue wire along with the current common wire (brown) to the common terminal on the furnace?

Thx again!

yuri 05-30-2011 08:14 PM

All I can do is go with the pic of the tstat subbase which is wired correctly. I cannot see the rest of the wires from here or the control board clearly. It is not all that complicated. Hook the white wire from the AC to the C on the tstat (orange) Other wire from AC/red to the Y terminal on the tstat/yellow wire. Make sure the Y from the tstat/yellow goes to the Y on the board and the C/orange goes to the board on C and that the colors don't get changed mid way from the tstat to the furnace. If they do then you have a HUGE problem and they need to be traced out with an ohmeter.

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