Hi Experts,

Here's the problem:

The furnace works normally except.
It starts and runs for about 7 minutes. Then, the burner shuts off and the blower continues to run. Then the blower stops, 40 seconds later, the burner relights and the cycle continues until the desired tempurature is achieved.
I don't think this is normal behavoir, but I could be wrong.

I think the blower plenum is sensing an overheat condition and is shutting down the burner but I'll let you experts tell me the real story.

Thanks!

Randy

 

It sounds like a temp sensor is taking action based on a temp rise to a high setpoint. A good control design would turn off the burner and keep the blower running until the temp sensor reverses on a temp fall below the setpoint. Check the air plenum for an obstruction. It could also be the temp sensor is starting to fail. Get it fixed before cold weather comes.

 

A furnace has safeties built in to prevent overheating and other problems. You're experiencing "short cycling", which as Beelzebob mentions can be caused by an obstruction in the plenum: furnaces rely on air flow to operate properly. Other causes / easy fixes to look for are..
~ clogged air filter
~ blocked air return: cold air has to flow back to the furnace for re-heating, and these can be blocked by furniture or bedding. Returns are vents around the house where air flows *in* to the wall.

Less common things you can check for:
~ blocked vent to the roof
~ squirrel cage blower veins clogged with dust and / or pet hair

Good luck with it!
Dave O

 

There appears to be an assumed diagnosis that the furnace is over-heating causing short-cycling. This is very possible, but rather than looking to correct this issue I would first confirm it. Find the recommended temperature rise on the data plate and check to see if your furnace is within, or out of it's range. That should only take a few minutes. If it truly is an over heat situation consider meter-checking which limit switch it is. You may have a tripping vent, blower, or main limit switch, each, if you have one would give you different directions to check. If it isn't over-heating there are other directions to check for short-cycling. Is your thermostat good, is it on a wall next to a supply duct or vent giving an inaccurately high reading only while hot air is blowing, are you getting any error code(s), could you have a bad fan limit switch? Sometimes more focus on the problem will help you narrow down the solution. Good luck.

 

Based on the OP's description.... it's overheating.

Most furnaces have a control board with diagnostic LED.
That board will be in the blower compartment. You can take the blower door off to familiarize yourself with its location and to determine if it does have an LED. The fault will clear and the LED will stop flashing when the door is removed. Many doors have a little round sight glass where you can look thru and see the LED.

 

I strongly stand by my comment that if you assume that your furnace is over heating a temperature rise check should be made to confirm the problem. Failure to do so would be negligent. How often have service techs found localized hot spots (small cracks/dropped rivet rings) behind a high limit switch that presents as a LED coded overheat, I have a few times a year. I'm I saying this is the issue here?, no. -Highest odds would favor a dirty filter-. Could it be something else, maybe. I've often found that diagnosing without actually checking can often be less than accurate. It would be foolish not to spend the few extra minutes to check and confirm.

 

In my opinion, I doubt the temperature would be stabilized in 7 minutes where a temp rise reading would be significant.

 

If you don't feel a temperature rise takes place in that amount of time, then how does the limit switch sense that it is significant enough to open. If it doesn't get hot enough to read, then you might want to start checking elsewhere. Have you ever checked locations other than supply/return vents and/or know the return number even before the burn? I could go on with other ways to confirm a possible unnatural rise, but I feel it sad to have to justify such a quick, normal, and confirming check anyone feeling a system is overheating should be making. But that's only my opinion, and apparently the opinion of the people who put the temperature rise heat range on data decals.