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Fireman1557
Jan 6, 2009, 06:11 AM
The Pilot light on my Evcon furnace keeps blowing out. I have installed a roof jack extension, but that didn't seem to help. I just had the lower limit switch replace also. Any Ideas?

hvac1000
Jan 6, 2009, 07:39 AM
Please post a EXACT model number. I cannot look over all the Coleman problems for all there furnaces without a exact number. Coleman had these kind of problems with different units.

Fireman1557
Jan 6, 2009, 08:13 AM
Model 7970-856/E

hvac1000
Jan 6, 2009, 08:59 AM
Check roof jack for inner separation if it has become separated that will cause your problem but since you extended it you should have noticed that while you looked up or down through it while doing the work.

Check pilot orifice to be sure it is clean and putting out a strong pilot

Check manifold gas pressure to high or to low a pressure can cause this problem.

I hate to mention this but a hole in the heat exchanger could also cause this and your furnace is not a spring chicken if you know what I mean. It might be a real good idea to have that checked out.



I also hope that the roof jack extension you used was listed and labeled to be used as a extension for that model furnace. There are many different roof extensions for Coleman furnaces especially in mobile/manufactured homes.

Have you replaced the thermocouple? It could be weak allowing the gas valve to drop the pilot circuit.

Roof/roof jack cap. This HAS to be the original design or it will definitely cause problems. The reason for this is that the roof jack assembly takes in fresh air and also removes the exhaust/flue gasses from the furnace. If the roof jack assembly including the cap is not absolutely correct problems will happen.

I listed a lot of items that could be causing your problem. In your case I do not have a short list of possibilitys so you have your work cut out for you.

Fireman1557
Jan 6, 2009, 11:31 AM
Thank You very much. I just replaced the Thermocouple about an hour ago and it seems to be working fine. I looked all over inside and aside from some rust there does not appear to be any holes in it. The extender I put on was for this furnace and I didn't see any separation. The furnace is a 1993, at what point does it need to be replaced? Also this furnace has a damper control assembly. If that is malfunctioning could that be the cause? As for now it is working, but tonight will be the test.

hvac1000
Jan 6, 2009, 12:45 PM
The furnace is a 1993, at what point does it need to be replaced

As long as it is safe to use it does not need to be replaced.

Explain damper control assembly.

Fireman1557
Jan 6, 2009, 01:19 PM
It is on the right side of the furnace and the duct runs out of the combustion chamber and up past this. I have included a pic of the part from mhparts.com.

http://i41.tinypic.com/2airqkm.jpg

hvac1000
Jan 6, 2009, 01:58 PM
That is a combustion air damper assemble. It shuts off the combustion air when the furnace is not running, It is equipped with a Cherry brand micro switch as a safety. If the switch does not make the unit will not fire. You might also have something called Blend Air on that unit. If so there will be a separate air intake on the roof with a motor in it.
If the motor failed in the open position on the combustion air damper it could allow a back draft also which may contribute to your problem.

hvac1000
Jan 6, 2009, 02:02 PM
It is on the right side of the furnace and the duct runs out of the combustion chamber and up past this. I have included a pic of the part from mhparts.com.

http://i41.tinypic.com/2airqkm.jpg


Blend Air

Fireman1557
Jan 7, 2009, 12:39 PM
Could the -Combustion Exhaust Inlet Motor with Damper Control-be causing this problem I think this is the Blendair thing you posted!

hvac1000
Jan 7, 2009, 01:54 PM
Anything is possible. Now that you mentioned you have a blend air system I suggest you contact a HVAC mobile home repair person since many if not most of the local regular home HVAC companys might not know how to properly service a Blend Air equipped furnace with all its associated extra parts and pieces that must work in harmony with each other.

Fireman1557
Jan 8, 2009, 07:52 AM
I pulled the furnace back apart today and this time I took the gas orfice out of the regulator and low and be hold there was a large spiders nest in there. I cleaned it out the best I could and now I am waiting. There is a push rod on the mounting plate below the regulator that moves a sleeve on the burner. What does that do?

hvac1000
Jan 8, 2009, 11:34 AM
Remember this post to you

Now that you mentioned you have a blend air system I suggest you contact a HVAC mobile home repair person since many if not most of the local regular home HVAC companys might not know how to properly service a Blend Air equipped furnace with all its associated extra parts and pieces that must work in harmony with each other.

I pulled the furnace back apart today and this time I took the gas orfice out of the regulator and low and be hold there was a large spiders nest in there. I cleaned it out the best I could and now I am waiting. There is a push rod on the mounting plate below the regulator that moves a sleeve on the burner. What does that do?

I told you to check the pilot orifice in my first post. Yes spiders do like to get into them.

What are you waiting for? Did you try to run the furnace after the spider web removal?