I have just recently installed a new thermostat (Honeywell RTH7500) for my old Coleman Evcon (AH16P20BA) system. Everything works perfectly except for the heat (and maybe the cold... but we'd have to turn it to 60 to check that lol). It kicks on when it is supposed to, but it just blows air - neither hot nor cold. The old thermostat is a Honeywell T874R1152.

On the old thermostat:
O is orange
L is pink
Y is yellow
G is green
R is red
X is black
W2 is white
E is bridged to W2

The two other contacts, W3 and B, had nothing connected to them.

On the new thermostat, I connected everything as it was on the old thermostat except for the X (black) cable. There is no "X" input on the new thermostat, but there is a "C" (I'm guessing common) - which there was not on the old thermostat. The installation manual reads that X may be plugged into C on the grounds that "something" needs to be plugged into C if L is in use. So, I did that.

We've tried some other wiring options, but this one works best (at least the fan comes on). We've also tried tinkering with the settings, but - again - the "all electric" setup works best (which is odd since we believe it is a heat pump system).

The old thermostat reads "Heat Pump Thermostat" on the front and all the excess wires connect to the "Heat Pump" connections on the new thermostat. Also, sometimes the "EM Heat" and/or "Aux Heat" light comes on with the old thermostat. So, we're guessing we have a heat pump system.

I can upload scans of the installation guide, pictures of the new and old wiring (we currently have reinstalled the old thermostat), and provide any further details. Quite frankly, we're flummoxed.

New development: Now our heater does nothing; even with the old thermostat hooked back up. It's going to be a cold night...

--EDIT--

Here is a link to a photo album containing a scan of the wiring guide from the installation manual plus pictures containing possibly important information.

http://s3.photobucket.com/albums/y94/vvhispers/Answers/

Okay, we fixed the problem of it not working at all. Somehow we blew a fuse in the furnace. Since we now know we are on a heat pump system we reconnected the new thermostat and set it up for a heat pump.

Now the problem is with the emergency heat. The normal heat works well enough, but the EM Heat only throws out cold air. I checked switching over the changeover valve, but to no avail.

It's snowing outside... I'd love to get this fixed. Lol

Ever figure this out, same issue here

Nope. Emergency heat still doesn't work. Heat pump works moderately well, but not exactly well enough for the colder nights.