I have an old Carrier/Bryant natural gas central heating and A/C system circa 1980. I recently installed a new thermostat and when I traced down the existing wiring I found something interesting: there is no A/C transformer in the furnace, only the furnace transformer. The two wires from the A/C condenser unit are wired directly to the Control board. The ground wire to the "C" and the contactor wire to "Y".

The thermostat wire has the standard colors: red, white, green, and blue. The red connects the "RH" on the stat to the "R" on the control board. The others are "G" to "GC", "W" to "W", and the blue connects "Y" to "Y" respectively. A jumper connects "RH" to "RC" at the stat. What's interesting here is that the "G" from the stat is supposed to connect to "GH" on the board. If I move the green wire from "GC" to "GH", the furnace will function, but the fan will not operate in the "ON" position.

The system has been operating "OK" over the years except for the fact that I have to replace the control board about every five years, which seems odd. My questions are: 1. Should a transformer have been installed for the A/C? 2. Is the wiring as I described it "normal" for a "4-wire" system? 3. Could it be that this wiring is responsible for the control board going bad so often?

I see no reason to make things more complicated with a second transformer. For heating the red wire is switched to the gas valve and blower relay. For cooling it is the A/C and blower relays. If the transformer is too marginal to supply either one, may as well replace it with one that will serve both.

The wiring is normal, except, I am not familiar with the GC and GH terminals. Usually the furnace just has a G for fan on. If fan on won't work with the green connected to GH, and the furnace with it connected to GC, you could try connecting the green to GC, and jumper the W to GH. However, that might not supply power to the blower for the cool down cycle after the gas goes off.

A new board every 3 years sounds expensive. Do you have frequent thunder storms in your area? After I had to replace the board in my furnace, I added a surge protector.

Thanks for the reply Labman, but wouldn't you know it, just as the cold weather is arriving here in the Chicago area, my furnace has begun to cycle in an odd manner! The heating cycle starts normally, but then goes into a cycle where the burners and the blower only alternate instead of operating at the same time. This continues until there is no longer a call for heat.

I'm guessing that this might be caused by a bad or dirty limit switch (plenum thermostat) or maybe another new control board is needed. I have a replacement limit switch (HH12za189) on hand as well as another control board.

My questions are: 1. what are the symptoms of a bad or dirty limit switch? 2. Should the protective "sleeves" (on the arms of the switch that go in to the chamber) be removed prior to installation? What are they there for?

If the limit switch is dirty, it will be slow to respond to changes in temperature. That could cause just the problem you are having. Burner comes on heats stuff up. The other limit switch shuts the burner off because the blower didn't come on, the limit switch finally get hot enough to start the blower which cools the furnace down shutting itself off, and finally the other limit switch cools off restarting the burner. I am not sure what you mean by protective sleeves on the limit switch arms. If there is a covering over a probe, yes, it come off when installed.

Labman:
The sleeves on the limit switch can be seen at this site:

http://bestbuyheatingandaircondition...tegory_Code=FL

In this picture they are green. I'm thinking they should remain on.

Not sure myself, but it does look like leave them on.

Labman - I replaced the limit switch this morning and the old one still had the sleeves on it, so I in stalled the new one as is and the furnace is cycling more normally.

I do have one remaining concern. When the furnace has to make up a large temperature disparity, like when it must go from a night setting of 62 to a day setting of 70, it cycles numerous times. I would think that as long as the stat was calling for more heat, the blower would stay on and the burners would turn on and off according to the limits of the limit switch. My furnace just has a single stage blower, so the blower must switch on and off numerous times putting more wear on the relays and blower motor. What do you think?

Glad you are making some progress. I hate to say replace parts you aren't sure are bad, so happy to hear replacing the switch did help. How long is long? It is possible the upper limit switch may trip when the furnace has been on for a long time shutting it off while the thermostat still calls for heat. Might try checking it when the furnace cycles off.