My furnace short cycling

The glow plug comes on, the burners start and about 15 seconds later the burners quit. It just keeps repeating this way. There is a button size limit switch by the 4th burner at the opposite end of the glow plug and I bought a new one but it is still short cycling…

There is a rod about an 1/8" by 4" long that hangs down into the 4th burner. What is it? Could that be the problem? Then above all the burners in the middle I see another button size limit switch, what is that for?

Mark, most systems have some type of flame proving device. If the flame is not "sensed within the set time the gas valve will shut down to a safe condition.

What you describe sounds like there is no signal from this device. The rod you describe could be a cappillary tube that funtions as this device. Does that rod have a tube going all the wy back to the gas valve?

A photo or two might help to troublehoot you rproblem. Good luck.

Same thing happened to me this winter. I called my HVAC guy…$75. He took steel wool to some components??? and it fixed the problem. I get fliers from him every spring and fall reminding me of his preventative maintenance services he offers to avoid these problems.:stuck_out_tongue:

Same here last year.

We tell everyone else to have regular maintenance, why don’t we follow our own advice? :sarcasm::???::-k

This can be associated with gas pressure regulation, as well as component failure.

There are so many sensors to insure things are safe, they must systematically diagnosed. Your HVAC Tech is likely the only one that is qualified for this and it is well worth the money spent. Let them do the parts changing if they can’t figure it out the first time around.

That limit switch you are describing has to do with flame roll out. You can see this happen if it is the case.

There are limit devices, proving devices, sequential devices, and safety devices. You must understand each one and how to test them (without replacing them).

It is about as probable to diagnose the issue from here as it would be to inspect the house from the Internet. You are an expert, let the experts do their job.

Is the unit have a board with a fault indicator on it? It sounds like the Flame sensor is not picking up the ground. Ive seen equipment that operated properly when 1st fired up only to fail later on due to a poor ground. Also check the polarity on the cord most units wont operate on heat if this is wrong.

Mark, I’ve had several instances like yours. My heating tech guy did the same thing…he took a plumbers fitting cleaner(little wire brush) or like another suggested, emery sandpaper, and cleaned that sensor rod. Worked perfectly. He said it was usually due to washer/dryer cleaners and additives building up on the sensor. He also said if they are in the same general area its likely to occur more often?

Might as well give it shot…cheap fix…not much to lose.

Home inspectors should take their own advise. We would recommend an item like this to a professional tradesman but when it comes to doing it ourselves we become reluctant to follow that same sage advise. As Dave A. stated there are numerous devices installed to perform safety functions and someone may correct one thing by fixing it themselves, allowing the furnace to function and completely miss another problem by not doing the full system analysis the tech would do to insure the system is operating correctly. Recommend you call the HVAC man.

Thanks guys. Darrell, I will give it a try . Thanks…

Thanks guys. Darrell it worked!!! Saved me some cash!!

That’s great Mark…it’s always nice to save a few bucks especially when its something simple like that and they have to charge you 1 hrs. labor just to come and clean off a little sensor.:smiley: </IMG>