Hello! I live in an apartment building in Minnesota and have a Comfortmaker heater/AC unit that is less than two years old and also a Honeywell thermostat that is the same age. There is also an A/C unit on the roof and I don't know how old it is. The thermostat has two switches. One is Heat/Off/Cool and the other one is FAN On/Auto. I ALWAYS leave the FAN swich at Auto. I set the other switch depending if I need heating or cooling, since the temp varies here ALOT, and switch it to Off when I leave my apartment for more than 3 hours or so. A couple of days ago I left for work and had the fan switch set for Auto and the other one Off, but when I got home the fan was running non-stop. I checked the switches on the thermostat and they were STILL set at fan Auto and Off. I then experimented with the switches and discovered that neither the heat or A/C was working! The fan stays on non-stop, until I shut off the circuit breaker switch, but I get no heat and no A/C at all! The furnace model number is GUJ050N12A1 NUG3050AFA1. The number on the circuit board is ST9120C3000. All of the wires between the thermostat and circuit board appear to be connected firmly on both ends. HELP!!! Thank you!!! JimBoy By the way, I would just call apartment maintainance, but they are WORTHLESS and I think I know more about this stuff than they do!!!

 

Id call them.It sounds like its the board. You can see first if the transformer is good and has 24 V at it. is about all.

ED

 

does sound like the board. several older units would run the fan when the transformer went out, do not think it is that way on your model.

 

Thanks for the info. I DID check the transformer and it IS putting out 24 volts. I still haven't contacted the apartment maintenence people yet, but I guess I will have to because I am not going to pay for a replacement board myself. I found one on a website for $70.63 + P&H. Jim

 

that is a good price. what website is it? other members may benefit from that info. i am thinking that price is near (within 15 bucks) of what i pay wholesale

 

It is simply "furnaceparts.com". I have not ordered anything from them, so I cannot say anything positive or negative about them. They showed up on a "Google" search that I did. You can search their website by part number, etc. Jim P.S. I STILL have not called maintenence to have my furnace/AC fixed! My apartment is a MESS and I have to clean it up before I allow them to come in here! I have been kind of procrastinating because the temps here have been very nice lately (50's to 70's), but it is supposed to get HOT soon!

 

Hi Jim,

I had to work on a Comfortmaker this winter that had the same situation as yours. I looked up the work order and found that it was a defunked Hi-Limit switch. When this switch is opened, the board tells the Fan to run all the time to cool off the exchanger. This feature over-rides your thermostat. Hope this info helps your maintainence people.

Sincerely,
Aaron B.

 

you are absolutely correct, high limit circuit open will cause your problem, good call gas guy.

check your manual reset rollout switches. also the high limit, ohm them out

 

Is this "high limit switch" on the circuit board or is it something totally separate, and if so, where do I find it/what does it look like! I told the maintanance people today about my problem and they have already ordered a new circuit board!!! They also claim that my furnace is over 5 years old and out of warranty, but I am POSITIVE that they are wrong!!! Jim

 

The "Hi-Limit" is a safety device that simply cuts power to the control circuit when over-heating, power surges, and flame roll-out occures. It is usually an external device with either 2 red wires, 2 orange wires, or 2 brown wires connecting to it, and are usually withing the 24vac circuit of the wiring. The Comfortmaker has up to 3 Hi-Limits depending on your model. There should be some type of wiring schematic on the cover to your unit that will show you where these limits are. Hope this helps...

Sincerely,
The Gas Man

 

In looking at the schematic attached to the removable door on my furnace, I see that there are three switches, "rollout limit", "vent limit", and "main limit", all connected with red wires in series. The ends of the two red wires are then connected to the #1 and #6 pins on the six-pin connector on the circuit board. Exactly how do I go about checking each of these three switches? Do I have to disconnect the two wires from each switch to check for zero or infinite Ohms on the switch terminals, or can I just check the switches while they are still connected to the wires? Thanks! Jim

 

some will have reset buttons, try that first, then ohm out the individual limits. you are on the right track

jump out this circuit briefly to see what happens, if it fires up normally, you are almost there

 

None of these three switches have resets. Apartment maintenence is comming tomorrow to replace the circuit board. I guess if that doesn't solve the problem I will tell them to check these three switches, if they can't figure it out themselves! I can't wait to get this going! It is HOT here! Thanks! Jim

 

You could just turn the power off and put a jumper on one at a time turn the power back on this would tell you if and af the controls there are bad. Id still go for a new board.

ED

 

Yesterday afternoon the apartment maintenance guy stopped by. He installed the new circuit board and the heater/AC didn't do anything different -- it still had the same symptoms! I then asked him to check those three limit switches and sure enough, one was bad! I believe it is the "roll out limit" switch. It is the one mounted about 3 inches above the pilot light. Of course he didn't have a new one to replace the bad one, so for now it is replaced with a jumper wire. The A/C is working perfectly now, the fan does not run all the time and although I have not checked it yet, I assume the heater is also working properly now! My final question for you is : Is it okay to just leave this jumper wire in place of the roll out limit switch until I can get a new one? Thank you for all of your advice and help!!! Two thumbs up!!! Jim

 

Please remember that my last question still has not been answered, so please look at that one first, but I DO have one more question on top of that one! My new circuit board has two DIP switches that need to be set, unlike my old circuit board. The setting is for "heat fan on delay" and the choices are 60, 90, 120, and 150 seconds. From the factory it is set at 120 seconds. I have the papers that came with this new circuit board and they give absolutely no advice or input on how to decide which one of these 4 settings to use! By the way, it was a little bit cool this morning so I decided to see if my furnace is working, and it IS working fine! Thanks again! Jim

 

You're ok with the roll out switch jumped for now but put a note on the calender so it don't get forgotten. YOU DO NOT WANT TO RUN THE FURNACE FOR HEATING WITH A SAFETY BIPASSED. My big question is what caused the roll out switch to fail in the 1st place. When heating season comes around with the new roll out switch in place can you watch the funace fire up? If so the flame should not come out of the excanger at all. If it does it could possibly be cracked or dirty.

You 2nd question. 120 should be fine. It just means the furnace will heat up for 2 minutes before the blower comes on so you don't get a blast of cold air. You can try it at 90 or 60 and if the air coming out of the vent is warm enough leave it there.

 

Thank YOU!!! I have been struggling with this same problem in Atlanta GA for the last 5 days - Bought the replacement Board with no improvement. I have 24 volts going into the board, but nothing going to the Thermostat. The Pilot flame seems to be a bit stronger than Normal..... I believe I can now isolate which switch is bad.
Thanks Again