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gas furnace keeps cycling?when thermostat calls for heat exhaust fan starts and runs.burner ignites. main fan starts and runs. it all runs for a while than everything shuts down except for the main fan before it reaches set temp. then when main fan does shut down exhaust fan starts again , burner ignites and main fan starts. it keeps on cycling like this. anyone have any ideas? it runs for about 5 to 10 minutes per cycle.

Is it a new high efficient furnace? If you have a manual it should give you a code book to read the flashing codes.  
 
Next I will say is , from my expierience these new high efficient furnaces fault out if you set the thermostat to rise more than 1 deg at a time.

I had a similar problem last year and found that the expensive filters I was using were restricting air flow thru my Furnace enough to trip it out on a high temperature alarm then it would shut down and cool off enough to start up again and keep going till it tripped out again switched to cheap filters and haven't had a problem since

I had that problem in one place I lived, and my grandmother routinely has that. In that case, it's the high-limit sensor either being dirty or just worn out.  
 
It's about a $200 job to get that changed out.

I had a similar thing happen last year.  
Furnace kicks in, vent starts up and flame ignites shortly after but only holds for about 3-5 seconds. Then a short while after flame ignites again and so forth.  
Reason was, my sensor was dirty. It couldn't register the flame and therefore shut of the gas flow.  
The DIY fix...look up YouTube for step-by-step help. It only costs 20 minutes of your time, a screwdriver and a sheet of fine sand paper.  
Worked like a charm for me.

I had a similar thing happen last year.  
Furnace kicks in, fan starts up and flames ignite shortly after but only hold for about 3-5 seconds. Then, a short while after, flame ignites again and so forth.  
Reason was, my sensor was dirty. It could not register the flame and therefore shut of the gas flow.  
The DIY fix...look up YouTube for step-by-step help and where to find the sensor in your furnace. It only costs 20 minutes of your time, a screwdriver and a sheet of fine sand paper.  
Worked like a charm for me.  
 
Edited by , 2016-10-13 10:33:57

It sounds like it could be your flame sensor. Easy clean up or replace usually.

Defiantly. If it was the flame sensor it shuts down right away to prevent a build up of gas.  
The high limit shuts off when there is a build up of excessive heat. This is caused by a lack of air flow caused by a dirty filter or if you have a/ c the under side of the a coil in your furnace is dirty and needs to be cleaned.  
The main fan stays running for a while to cool down the combustion chamber and the high limit opens and the fan shuts down.  
Sequence of operation then starts again.

ok figured out a couple of things. the exhaust fan doesn't seem to be getting enough air. when I close the door where the exhaust fan is the ok light starts flashing and it shuts down. if I leave the door open a crack so that the exhaust fan gets enough air it keeps running and no codes. the other thing is that yes I have a good filter in it and I also have to keep it out a little bit to keep the furnace running. we had the furnace cleaned .but it would be nice to take a/c coils out and wash them. don't think there was much maintenance done in this building before we bought it.

replaced the flame sensor which didn't make any difference.  
 
Where is the high limit switch located?

where does the exhaust fan get its air from?  
 
Edited by ride, 2016-10-16 17:12:56

 
 
Edited by ride, 2016-10-16 17:10:48