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gigascott · Jan 19, 2006, 04:19 PM

I recently experienced a problem with my heat. I have a programmable thermostat and noticed that the heat wasn't on when it was suppose to be. The temperature was set to 75 but the temperature in the room was only 68. I canceled the program and manually set the temperature again to 75. The heat came on and everything seemed fine. Then about 10-15 minutes later cool air started to come through the vents. I went outside and found that the my Trane gas furnace had turn off even though it hadn't reach the set temperature. I played around a little with the thermostat but nothing fixed the problem. Then about an hour later warm air started to come out of the vents. But then about 10-15 minutes later the heat turned off and cool air came out of the vents. It seems like it can start up fine but then something is causing it to shut off like it was overheating or a safety switch is be activated. Any thoughts? Thanks.

labman · Jan 19, 2006, 05:29 PM

When the cool air starts, go to the back of the thermostat or the connections at the furnace. Check to see if you have 24 volts between the white wire, W, and the B or C terminal which may have a blue wire. If you have the power to the white wire, the problem is the furnace, not the thermostat. Another check is to short the W and R terminals together at the furnace. My guess something is wrong on the control board. The next check is to follow the white wire to gas valve. Check for 24 volts between the 2 wires to the coil. If you have power, and gas isn't on, the valve is bad. A new one may require fussy adjustments.

If the control board is getting a problem signal from a heat sensor or limit switch, I would expect the blower to shut off after the timed cycle.

gigascott · Jan 20, 2006, 09:58 AM

Thanks for the info. I am pretty sure that it is the furnace that is the problem. I went online and found the manual for the furnace. In the manual is a list of flashing LED codes that will help figure out what the problem is. I went home during lunch, turned the heat on, and watched the LED. The furnace started as it normally does but then shut off like it has been doing. The LED indicates that there is a "System Lockout (No Flame)" problem. Do you understand what is happening and what needs replacing? Thanks again for your time.

labman · Jan 20, 2006, 10:42 AM

I was thinking of suggesting checking for the error codes. It sounds to me like it is the gas valve. Before replacing it, I would check the terminals on it for power. If it has power, and the gas isn't on, it is bad.

gigascott · Jan 20, 2006, 11:38 AM

Does this really explain the problem? The gas valve seems to be turning on at least at first because I can feel warm air coming from the vents and out of the furnace. Then it seems like it then closes. Would a bad gas valve just close like this? I don't fully understand the internal workings of the furnace but it seems like something is causing the gas valve to close, no? Also, in your first response you mentioned that replacing a gas valve "may require fussy adjustments". Is this something that I can replace myself or should I have a professional do it? Thanks for your help.

labman · Jan 20, 2006, 01:58 PM

Many electrical devices use coils. I am not sure exactly why, but as coils reach the end of their life, they continue to work cold, but not after they warm up. They keep quitting sooner and sooner and finally not at all.

As for replacing it yourself, I am not sure. You might go back to the manual and see if it says anything about the adjustment.

gigascott · Jan 22, 2006, 01:18 PM

Thanks for the info. I am not sure if it is the gas valve after all. Unfortunately, I am having a problem measuring the voltage at the gas valve. There isn't much room to get probes onto the contacts. Anyway, as I have stated earlier the Control Board is indicating a "System Lockout (No Flame)". I thought that the flame sensor might be dirty. I took it out and it was pretty clean. I made it more clean and then reinstalled it. I them turned the furnace on again and it did the same thing it has been doing; coming on, heating, and then turning off. Sometimes the blower stays on blowing cold air through the vents and sometimes the whole thing just turns off. Do you think it could just be the control board? If it was any other sensor, the LED inidicator would blink a different pattern. Should I still check the gas valve? Should I disconnect the leads going to the gas valve for 24V? Or do I need to measure the voltage with it still plugged in? Let me know. Thanks.

labman · Jan 22, 2006, 01:34 PM

Control boards are expensive, but easier to replace and often what it takes. If you buy one, and it doesn't fix this problem, it might prevent the next. In any case, you will have a working board to try the next time.

You do need to have the gas valve plugged in to check the voltage. However, you may be able to find easier places to measure it. The common, often a blue wire connected to B or C on the board and to the transformer, can be checked anywhere. You may be able to check the hot wire where it comes off the board.

gigascott · Jan 22, 2006, 01:44 PM

Thanks for the info. Yeah, even if a new control board doesn't fix the problem. Having it around would be nice.

The wires going to the gas valve are blue and brown. They are connected to the control board via a big connector with the other sensors. Getting access to the connections at the connector is also difficult. I will try it though and see if I can get anything. Thanks for your help.

gigascott · Jan 27, 2006, 11:50 AM

Resolution: It turns out that it was the gas valve. Unfortunately I was not able to replace it myself. In my area, NC, you are not allowed to purchase a new gas valve without having the appropriate license. I have to resort to having it replaced professionally. I didn't really mind this since I didn't want to risk haing and leaks in my gas line as a result of replacing the gas valve.

FYI, the service person was able to verify that it was the gas valve by unplugging the wires to the gas valve and checking the voltage between the two wires when the system was trying to ignite.

Thanks for you help.

labman · Jan 27, 2006, 01:01 PM

I am glad you posted back. As I said, gas valves are tricky to adjust and certainly you didn't go wrong letting a professional do it. When I put in a new furnace and A/C about 10 years ago, I saved money by doing most of the work myself, but the contractor insisted on starting up the furnace. I was surprised how long the technician fiddled around with the adjustments. Of course, they had to silver solder the A/C tubing and fill it with Freon. I missed being able to buy the precharged components by 6 months.

trudgeon · Apr 27, 2010, 10:03 AM

Question; Can the following conversion work?

I currently have an older BDP (carrier) down draft combination furnace/a-coil pushing air into ducting in the concrete slab with returns in the ceiling attic in a one story home 1800 sq. ft. Due to settling I have some damage to the in-slab ducting and want to convert to a ceiling forced air system. The existing system is functional, with the condenser outside rated at 3.5 tons. Existing condenser is model number(567C036RCU) Existing furnace model number(396DAC036080) Existing A-Coil is mounted below the fire box and on the floor to the ducting.

I was given a 2001 TRANE system (excellant working order ) as follows;CONDENSOR XB 1000 model # TTB048D100AO, FURNACE model# TUE180A948K3, and A-COIL model number TXC049C4HP CD. This system has the A-Coil mounted above the fire box with the furnace mounted to the floor ducting. My plan is to make a new Plenium Box covering the existing floor ducting and come out the side of the PLenium box and up the wall into the attic and install flexible ducting for ceiling mounted registers, Any help regarding this fesibility plan would be helpful.

botnet · Dec 15, 2010, 10:57 AM

The gas valve is bad if there is gas getting to the valve but no gas on the other side of the valve (no flame on the primary burner).

hadi22 · Mar 2, 2011, 03:27 PM

Hi
My name is hadi kazak
I would like to ask you a question for a fornaise
When I start my fournaise the pilot become red and the flamme go well 5 sec leter the fournaise stops
Could you tell me me please what it could be the probleme
The first time I went there the flamme goese well sec time the fournaise stops after 5 sec
Please let me know if it could be the flamme sancer or the gaz valve
Its an trane xr90