I'm working (again) on a Heil. It's about 80K, upflow model. Older one, a 70 or 80 percenter.
Customer's complaint, keeps on running. House gets hot, and they have to turn it off at the swtich on the side of the furnace.
He's changed the you-know-what a couple times. I pulled a wire for the YKW.
So, the fellow called me, and it's running right along. Wants to know if he should change the YKW again. No, don't do that.
Last time it happened, I got him to call me right over. I pulled the main cover, and tapped ont he box with the electric. Went right out. Aha! Relay isn't releasing. Replace the relay.
And a couple weeks later, it's happening again. What's the odds of a bad relay a couple months later? Not all that much.
what's the next couple things to check? Sticky gas valve would cause that problem. Any other common problems?