Background: I have an older Trane furnace (manufactured 8/1994) made by American Furnace Inc. (The Trane Company, Trenton, NJ) model TUJ080A936A0. The module controller is by White-Rodgers Emerson Electric Co. Model 50A52-101. There is no gas pilot light.

Problem: Recently, my gas valve will turn itself off prior to the desired temp. Based on the controller, it will attempt to restart (I see the ignitor turns on) for 2 attempts, however, after it turns itself off, any reattempt is to no avail. Even with the ignitor on (bright orange color), the gas valve will not open (i.e., no gas).

Temporary solution: I have to turn the thermostat OFF, then turn it on again. Since the temperature setting is still above current room temperature, the furnace will turn on as normal. Unfortunately, after several minutes it will turn itself off again. This process will repeat continuously, i.e., I have to turn the thermostat OFF and ON again to get it going. Also note that if I did it too soon after the gas valve turns itself off, by cycling the thermostat will not make the gas furnace working.

Need help: Please help me to understand what could be the problem

patrick5150

heating anticipater on thermostat is set wrong, some you can go into the programming of the stat and calibrate. But something else you said about it locking out and will not restart is a safety built into system. respond if you need more help.

kliem

Thanks for the response. One thing I didn't mention, I switched back the thermostat to the old fashion Honeywell rotary dial (based on mercury switch), the furnace still exhibits the same symptom. I even went as far as using my good thermostat from the other 2 furnaces (I have 3 in the house -- basement, main and upper floor has independent and separata furnace) but there is no different. That's when I am totally at a lost!

patrick5150

Now I'm not at the location but I will try to help you. Now on your Honywell T87 take the round donut cover off the first donut you see and behind that you will see the anticipater it should be 4.5 so the gas will shut off 1.5 degrees before set point. Now if you look at it it will say longer and shorter adjust to 4.5. there is the correct way of set it but you need a amp meter, and some wire. Let me know.

kliem

Thank you for getting back to me. I look at the T87F, the maximum anticipator reading is 4.2 -- my default setting since we bought the house, until I changed it to the digital thermometer so that I can set the desired temperature at desired time. I have amp meter and wire, what are these for?
FYI, we I reinstalled my T87F back (as I mentioned the problem still persisted), I notice when I went up to the attic to look at the furnace, the airconditioning pipe was frozen (has condensation). I wonder if I put the wire incorrectly. I put the blue wire to B; the red to R; the white to W; and the green to G -- this is for the T87F. However, my digital thermometer, everything is the same as T87F except the green to YELLOW terminal on the thermometer. It's been like that for the last 10 years.
I understand I deviate from my real problem, but I removed the T87F because I was afraid that I put the green to the G is wrong causing the airconditioning pipe to be frozen. As you probably know that there is YELLOW terminal on T87F too.
Your input and further suggestion will be very much appreciated.

patrick5150

Here is the the wiring as follows: red to R, white to W, green to G, yellow to Y that is it. no blue, anywhere just the first 4 wires. Now if you want to set the antis here is how its done, get about 2' feet of wire about the gage of the wire that is hooked up at thermostat, now strip about a 1/8" off each end, now rape it around a baseball, now shut the power off to the furnace and go the thermostat an get to the wiring base connections the wire that you raped around the baseball unscrew the red and attach one side to R and the other side to W so you will have 2 wires on R and W now turn power back on and get amp meter set it to on low amp reading, now you will need a jumper wire with clips on each side because now you must jumper out R to W and your furnace will start a call for heat, now once your system is up and running complete where you have the first wire where you raped around with baseball just to get circles in wire hook your meter up and read amps what your amps are is the where you should set your heating anticipater. The amps read is the same as the setting. IF your ac is running it is because of the blue and yellow take that blue off completely that is a common wire just follow the first directions at the first of this email but alway remember before you work with the wiring shut power off or you will damage
other things.

acetc

Your problem is not in your thermostat, I believe the problem is either the flame sensor or the ignition control module, try sanding the flame sensor lightly to clean it . The flame sensor is in the burner area and will have one wire from it to the module.
The heat anticipator will cause short cycling if not adjusted correctly but will not cause it to go into the lockout of the module. By the way, the correct way to set the anticipator is to wrap a piece of wire ten times thru the amp snap around , then disconnect the "W"wire at the furnace and connect it to one end of this wire that you wrapped around the amp meter leg with the other end of the wire connected where the "W" wire was connected on the furnace. This will multiply the reading on your amp meter by ten, this gives you the ability to measure the low amp draw thru the thermostat that most meters could not measure, the reading is then divided by ten (example : reading 4.5 = .45) The important thing is that it is wired in series (in line) to the thermostat and not parallel. Good luck, Mike

kliem

Acetc, you seem to know your stuff. However, I'm not familiar with the name of each component in the furnace. I agree with you that it is not my thermostat. Is the flame sensor the one to ignite the gas? If there is the case, my igniter is always 'on' to get the flame to start, unfortunately, the gas valve does not open to allow the gas to burn. If the igniter works all the time, is it possible that the ignition control module is the one at fault?
Regarding the heat anticipator, I really don't know what that is. I do have all the tool to test but don't know where to start.
My current problem is either the gas valve does not open when the thermostat setting is higher than the room temperature. Sometimes, this will work, however, the gas valve will open to allow the flame to start but it is getting shorter and shorter cycling.
Some one told me that the type of furnace I have is known to have a design problem, therefore, Trane has come up with a new kit to replace this unrealiable design. I was just wondering whether I just get the HVAC tech just to replace it with a new kit. I made some inquiry, the cost is around $750 including labor. The new furnace will cause $1400 with 5 year warranty on all part with exception heat exchanger for 20 years. I hate to spend $750 let alone $1400 if I can fix it myself. What do you think? Please help!

acetc

First off the flame sensor is in front of the burner in the flame area, it will look like a wire or 1/8" rod with a single wire attached to one end, clean with fine sand paper or steel wool, your ignition control modual may be causing the problem or the gas control valve, if you have a volt meter check the voltage to the wires on the gas valve when the igniter is glowing wait 45 seconds, you should see 24 volts from the modual to the gas valve, if you don't see voltage it is probably the modual, the gas valve will only stay energised for about 7 sec. if the flame sensor does not sense a flame the modual will de-energise the gas valve and this sequence will repeat itself and may go into a lockout after three tries, when you turn it off and back on you are resetting the lockout, Let me know what you find out, and i may be of more help. The repair prices are high, if it comes down to repairing the furnace call someone else, ask them before hand how much to replace these parts, best, Mike