Trane XE78 blows cold air, burner does not ignite

[station079]

I've done some research on the internet and think I know what the problem is. The blinking red light blinks four times indicating the "thermal protection device open".

The unit had been working fine this whole season. We stopped using it a few weeks ago when the weather warmed up a bit. The past few days it got a bit cold here and we tried running it. It is now just blowing cold air. I found the red light blinking as stated above. Normally when it is working properly, the burner kicks on and then the blower. Now the blower starts up right away and no heating up of the igniter at all.

This unit is a little over 18 years old and has worked flawlessly since it was new.

I'm fairly handy and feel that I could probably replace what needs to be replaced, but I'm wondering if it would be prudent to have someone come and look at it since there is a possibility that the thermal protection tripped for a reason, not just because it was old or worn out.

I would appreciate any tips, suggestions or help you all may have. Thanks in advance for your help.

[paul52446m]

I've done some research on the internet and think I know what the problem is. The blinking red light blinks four times indicating the "thermal protection device open".

The unit had been working fine this whole season. We stopped using it a few weeks ago when the weather warmed up a bit. The past few days it got a bit cold here and we tried running it. It is now just blowing cold air. I found the red light blinking as stated above. Normally when it is working properly, the burner kicks on and then the blower. Now the blower starts up right away and no heating up of the igniter at all.

This unit is a little over 18 years old and has worked flawlessly since it was new.

I'm fairly handy and feel that I could probably replace what needs to be replaced, but I'm wondering if it would be prudent to have someone come and look at it since there is a possibility that the thermal protection tripped for a reason, not just because it was old or worn out.

I would appreciate any tips, suggestions or help you all may have. Thanks in advance for your help.

A thermal protection switch cab be those little roll out switches that are around the burners and on the inducer housing. These type switch have a little red button that have to be manually reset, so check for that.
But the most probable thing would be a limit switch that is open, because when this happens it makes the blower run all the time. You might have a limit switch back in beside you blower. I you find the limit , try jumping the wires and see if it fires. You will have to have the door back on to have the door switch made. Paul

[station079]

A thermal protection switch cab be those little roll out switches that are around the burners and on the inducer housing. These type switch have a little red button that have to be manually reset, so check for that.
But the most probable thing would be a limit switch that is open, because when this happens it makes the blower run all the time. You might have a limit switch back in beside you blower. I you find the limit , try jumping the wires and see if it fires. You will have to have the door back on to have the door switch made. Paul

Paul, thanks for the reply. I checked all around the burner and the inducer housing and found nothing with a little red button on it. I found two what appear to be switches mounted on the framework above the gas line supplying the burners on both ends of the gas line. These are what appear to be ceramic with two wires on each switch. On the opposite side of where the wires connect are what look like wires or thin rods in the shape of a "U". Sorry for not being able to describe these better. Could these be the thermal protection switches? They appear to be intact.

I can take a picture of what I'm talking about and post it if that would help.

Thanks again for your time.

[paul52446m]

Paul, thanks for the reply. I checked all around the burner and the inducer housing and found nothing with a little red button on it. I found two what appear to be switches mounted on the framework above the gas line supplying the burners on both ends of the gas line. These are what appear to be ceramic with two wires on each switch. On the opposite side of where the wires connect are what look like wires or thin rods in the shape of a "U". Sorry for not being able to describe these better. Could these be the thermal protection switches? They appear to be intact.

I can take a picture of what I'm talking about and post it if that would help.

Thanks again for your time.

yes those are thermal protection switches. You can jump across those one at a time to see if it will fire up. But like i said in my last post it could be a limit switch not made and that would cause the blower to go on. paul

[station079]

yes those are thermal protection switches. You can jump across those one at a time to see if it will fire up. But like i said in my last post it could be a limit switch not made and that would cause the blower to go on. paul

I jumped the two switches and it made no difference.

When you say that the limit switch is not made it will run all the time, I don't have that problem, it only runs when the thermostat calls for heat. If I turn off the thermostat or it gets to the end of the program, the blower shuts off on it's own.

[paul52446m]

I jumped the two switches and it made no difference.

When you say that the limit switch is not made it will run all the time, I don't have that problem, it only runs when the thermostat calls for heat. If I turn off the thermostat or it gets to the end of the program, the blower shuts off on it's own.

What i am saying is when a limit switch is open then any time the furnace is calling for heat the blower will run , and that what you are getting. Paul

[station079]

What i am saying is when a limit switch is open then any time the furnace is calling for heat the blower will run , and that what you are getting. Paul

Got it, I will try and find the limit switch.

Thanks.

[station079]

Paul, it took me a while to get back about this, but I got the problem solved. I took a look at the wiring schematic and found where the actual Hi Temp Limit Switch was located. I jumped the wires and the furnace lit up.

I ordered a new switch and installed it today and it worked like it should.

Thanks for your help on this, I really appreciate it.

[station079]

Thought I'd update this thread seeing that what turned out to be the problem wasn't the Hi Temp Limit switch.

Technically, the reason the Hi Temp limit switch failed was because there was a crack in the combustion chamber.

When I installed the new switch earlier this year, the time for using the furnace was over for the season.

This year when we fired up the furnace, everything went well, the burners kicked on and then went out after burning for about 3 minutes, the trouble light started blinking out the Hi Temp Limit code again.

I called a Tech out and he found that the combustion chamber had a crack. He showed me how the flame was blown around when the circulation fan kicked in.

Anyway, I have a new furnace installed and it works much better than the old one.