Hi, I have a Central A/C & Heat Unit on my roof, it is about 18 years old. The A/C works fine. I have a prof replace the ignitor last year and it gets a little better. However, I noticed that the heat still have problem whenever it started continually running for more than 20 or 30 minutes, then it won't cycle on & off. However, it works fine if it run less than 15 minutes. Or I can adjust the temp, so that it will reach the desired temperature for less than 15 minutes and it shuts off and comes back on perfectly. The point is that it won't cycle back automatically afterwards as long as it ran non-stop for more than 25 minutes! I have a prof come check it out, he said the unit need to be replaced. Please Help or some idea what need to be repaira and how much it will cost. Thank You!!

Check your filters. Then check and maybe replace the high limit switch. What I am thinking is that if it runs long enough, the high limit switch kicks out and shuts it off. Either the limit switch could be bad, or the thing is there to shut the furnace off if it isn't getting enough air flow. The blower could be on its last legs too.

If so, and it is an older, less efficient unit, the prof might be right abut replacing it. If my ideas don't fix the problem, you might see if somebody else says the same as the first. Any trade has its crooks and fine, honorable men doing a difficult job.