Furnace cycling on and off

[jb1]

I have an approx 25 yr old coleman natural gas furnace. During the recent cold snap, we noticed that our furnace hasn't been able to raise the house temp above about 67 degrees and I decided to check our furnace. I noticed that although the furnace fan is running continually, the burners turn on for approx 60-90 seconds, then turn off for approx. the same amount of time. It cycles like this continually. I checked the filter (relatively clean) and even removed it to test with no observed effect. I'm not sure how long this has been occurring, but assume this is not normal behavior. I put an ohmeter on a sensor near the top of the furnace (I assume this is the high temp. cut out) and found that it was closed when the furnace was cool but opened just prior to the burners turning off. As far as I know, all our heat vents are open. I'm wondering what things I should check out prior to calling someone to check it out.

[Hube]

Welcome to the forum; This would seem like the furnace is cutting out on "high limit". The "open" contacts will cut off the burners when the heat chamber gets to too hot a temperature. (usually approx 180) Have you checked to see if the fan is operating properly.If belt driven, it could be slipping, too slow rpm, or the fan motor is not operating up to par. Check for any obstruction in and around the blower's housing, and the blades, Also make sure the Return air to the furnace is not blocked. Filter should be clean/ renewed. Let us know how you make out, we are here to help. Hube