I live in Arizona where my two-story house has a separate HVAC system for upstairs and downstairs. Both furnaces are in the attic and A/C condensors are outside. The house was new in July 2004 and we are the original owners. The orignal tstats were Totaline P374-1100 - one on each floor.

In Dec '07 I replaced the upstairs unit with a Honeywell RTH6300B found at Home Depot and all was well... until the A/C was needed this weekend for the first time since its installation. Now, the A/C, fan, and heat does not work on the upstairs system.

I installed the Totaline unit from downtairs to the upstairs wiring and everything worked perfectly. So off to Home Depot I went and exchanged the Honeywell unit. Got back and installed the new tstat.

During the install, two of the wires touched and blew the 5 amp fuse in the furnace circuit board (I know, I know I should have disconnected power :o ). Anyway, the Honeywell unit had a jumper installed connecting the R (red) & Rc (brown) terminals. Hooked everything up, flicked the switch at the furnace and proceeded to blow another fuse. Took out the jumper, replaced the fuse, and that part seems to be working properly. I should also mention that the transformer has 120V going in and 24V coming out (28.4V actual) so it appears to be OK.

The problem is that the new tsat does not work at all, indicating that the one I took back was OK probably. I swapped back the two again. With the Totaline unit upstairs or downstairs, all is well. The Honeywell unit on either upstairs or downstairs system will not operate anything.

How is this possible? I would think that these are interchangeable.

Start by checking the wiring from the thermostat area to the unit. Disconnect all the wires (identify them before removal) Take a ohm reading to make sure each wire is sound. Once that is done at least you know it is not a wiring fault. Also check to make sure none of the wires are shorting to another near the back of the thermostat.

If you had a jumper on RH RC and connected separate wires to both terminals that could be a problem that lead to damage if the jumper was left in during that time. (you said you blew a fuse)

At the furnacre you can also jumper the wires for the heat or cool and see if the furnace will run. This will eliminate a problem with the furnace and put it on wiring of thermostat.

Read this carefully in regards to your install.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-1717.pdf

Thanks for the info. Last night I hooked up the Totaline tstat (from downstairs where the temp control is working properly) upstairs so that we could have A/C for the evening and we did. The furnace blew and the condenser condensed :) . I'm on hold with a Honeywell tech at the moment, but some light seemed to be shined on the situation:

R+W = 28V
Rc+Y = 0
Rc+G = 0.3V
R+Rc = 28V

The guy stated that because of the Rc+G = 0.3 (not 28V), that it indicated my cooling transformer was bad or erattic. I checked the transformer on the furnace attached to the circuit board and it had 128V coming in and 28V going out.

Two questions:
1. Where is the cooling transformer typically located?
2. Is it possible that the cooling transformer is bad AND have the A/C work with the Totaline thermostat?

In most systems there is no cooling transformer. The transformer in the air handler or furnace in the attic usually supply voltage for both heat and cool. R(((heating)) and RC (cooling) should usually have no voltage reading between them. Now if you do have another transformer it will be in the outside unit.

How many wires did your original thermostat actually use and what were there colors/attachment point labels.

The Totaline has the following terminals and (wire color attached):

W1/O/B (white wire)
Y1 (yellow wire)
G (green wire)
W3/ECO (no pin)
CK1 (no pin)
CK2 (no pin)
R (red wire)
C (brown wire)

NOTE: it is worth mentioning that there is a second bank of terminals that are not being used at all (W2, Y2, RS2, RS+5, RS1, RSGND)

The Honeywell has the following terminals and (wire color attached):

G (green wire)
W (white wire)
"Not USED"
Y (yellow wire)
R (red wire)
Rc (brown wire)

My thought all along is that is has either been a setting that I have missed in the new unit (but I have been through the manual a couple times and reviewed the settings with the tech) or some change in the wiring between the two thermostats, but this is my first crack at a HVAC problem.

EDIT - To answer your actual question, existing Totaline used five wires (G, W, Y, R, Rc/Brown). I am using those same wires on the Honeywell. There is a blue and orange wire wrapped at each end.

Woops you have no C on the new honeywell. I have to leave here but you might want to read what you do in that case in the honeywell directions. I do not think brown goes to C.

Well, I get to wear the dunce cap for a day. I was obviously too smart to read the instructions CAREFULLY. They clearly state that the "C" wire is not used, but I assumed that it was >> 5 wires on the old unit must mean 5 wires on the new unit + the assumption that Rc is probably C.

Basically, the "C" wire gets taped up and I use the jumper between the R & Rc. A/C is humming now - go figure.

hvac1000 - thank you very much. Sometimes, the simple stuff is the hardest to detect by yourself.

Glad we could help.