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Seems that I have a gas furnace that is having some sort of heat/air sensing issue.

Basically it shuts off after running for a minute or two. But if I remove the top plate of the furnace itself it runs just fine.

It has an automatic ignitor which is doing just fine. Also no errors on the motherboard. But as soon as I put the top plate (area of ignition and flames) back on it will short cycle on me.

Too many registers closed? I just cleaned the filters out so that isn't it...

More Info: Model - will get the exact when I get home but its a Rheem high efficiency model bought about 5 years ago.

Any recent changes - Finished basement 3-4 months ago and added to duct lines - the duct lines have been up 5 months.

Can I see pilot light? There is no pilot light since it is automatic ignition. The ignition is working fine. Flames come on strong with cover on or off - so don't think it is a gas issue and I have several other appliances with no gas issues. With cover on it does seem like there is a grasp for air eventually (after a couple minutes). I can't see the flames with the cover on but it seems to go through a normal shut-down and cycles back on in a few minutes.

Filter - Filters are brand new.

Does the blower run when unit is off? No. As mentioned before I can't find anything odd except for the short cycling and the what it seems like lack of air.

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The two PVC pipes meet about 4 feet above and go out the back of the house. The one on the left is always a little warm and the one on the right cool. There doesn't seem to be anything obstructing the pipes but I also did the "paper" test while the furnace was on and it didn't suck the paper in. Today was about 20 degrees here and I could notice warm air coming from the pipe outside (not blowing but you could tell).

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This is with furnace door off. Everything seems to be working fine. Has electronic ignition so no pilot.

Other note: We have an electronic air filter that has been turned off for a while because of some construction we are doing. Found out today that it is dead. Shouldn't matter to the furnace but who knows.

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Is this a high efficiency furnace? Have you checked if the air intake pipe is clogged? –  Edwin Jan 21 at 1:23
    
The air intake is just about 10 feet of PVC out of my house. It is only about 4 years old and has a screen so not sure how it would be clogged. Any good way to check the air flow is right? –  DMoore Jan 21 at 1:43
    
I don't have a good way. Maybe a sheet of paper outside when the unit is on? The reason I ask is because when the door to the combustion chamber is open, there is typically unrestricted combustion airflow. When it's closed and properly sealed, the combustion air comes from the intake pipe. The other thing to check is the draft inducer motor. –  Edwin Jan 21 at 3:31
    
@Edwin - will try the paper out tomorrow. Thanks for the help. Spent 3 hours troubleshooting it today because an iffy thermostat that batteries died happened at the same time. –  DMoore Jan 21 at 5:03
    
Supplying the make and model of the unit is usually helpful. What have you checked during your 3 hours of troubleshooting? Did you make any changes, or do anything just before this started happening? Posting a video of the furnace operating abnormally would be helpful. Can you see the burners and/or pilot with the cover on? If so, does the flame get smaller before the furnace shuts down (or act as if it's starved for air), or does it go from full blast to nothing? Have you checked to make sure your gas pressure is good and consistent? How often do you change the filter? –  Tester101 Jan 21 at 11:45

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