Short cycling

Butch_2

I have a gas fired hot air system that is short cycling. The gow bar gets hot, the gas turns on. the fan starts and then the gas turns off. The filter and safety button are fine. Any ideas? Thanks in advance!

Mark Eatherton1

Sounds like

a bad circuit board. Sometimes, flip flopping polarity on the power to the unit helps, or confirming a good ground.

But generally speaking, if polarity was reversed, thre is usualy a diagnostic light on the board that would tell you that.

ME

don_9

Wow!

The teacher is giving advice on force air..So people can
change.Smiley face inserted here.

Hey teach, that was some good stuff in the contractor mag
this month.

I agree with the teacher,I find a couple bad grounds that can cause this type of problem.

Other reason,could be..Anticapator on tstat not set properly.
Poor flame signal back to the board.

As mention above,there should be diagnostic light to point you in the right direction.

Duncan_9

Hmmm

You're saying that AS SOON AS THE BLOWER TURNS ON the gas valve goes off? Repeatedly? Each and EVERY time? Definitely cause and effect?

Is there an induced draft blower (and pressure switch) on the unit?

Have you removed the filter and tried cycling it that way?

What model gas valve?

Where does the 24 volts to the gas valve get interrupted when this happens?

Mark Eatherton1

Thanks for the kudos...

I hope you and everyone gets some good from the articles.

Contrary to popular belief, I HAVE worked on a forced air furnace or two, and am always willing to help regardless of race, creed, type, air or water:-)

ME

jim lockard

Flame sensor

Try cleaning the flame sensor opposite the Glow bar on the burner rack just a little metal rod connected to a wire held in with a 1/4" hex head screw. remove it clean with sandcloth or even scrape with a fingernail file, reinstall and see if things do not change. If not check your grounds, tap the gas valve gently on the side. Let us know . Best wishes J. Lockard

Duncan_9

Hmmm

You're saying that AS SOON AS the blower turns on the gas valve goes off? Instantly? Each and EVERY time? Definitely cause and effect?

Is there an induced draft blower (and pressure switch) on the unit?

Have you removed the filter and tried cycling it that way?

What make and model gas valve?

At what point in the control circuit does the 24 volts to the gas valve get interrupted when the burners go out?

pperkins

fluke

There's nothing you can't fix when you use your electrical meter..you need to know operation of furnace then check it electrically....I had a carrier hot air furnace with a cracked heat exchanger.kept popping 24v fuse when fan came on,i removed burner from that chamber just to give them heat,went through many 3amp fuses that night.come to find out, the fan was blowing a small chunk of foil around inside of heat exch.the foil would short the limit to ground.ill never forget that one.....one thing i know ,after 15 years in this line of work...you never stop learning!!!.

pperkins

fluke

There's nothing you can't fix when you use your electrical meter..you need to know operation of furnace then check it electrically....I had a carrier hot air furnace with a cracked heat exchanger.kept popping 24v fuse when fan came on,i removed burner from that chamber just to give them heat,went through many 3amp fuses that night.come to find out, the fan was blowing a small chunk of foil around inside of heat exch.the foil would short the limit to ground.ill never forget that one.....one thing i know ,after 15 years in this line of work...you never stop learning!!!.

Glenn Harrison_2

If the gas turns off every time when the blower comes on

then you are tripping the main limit, almost guaranteed. How long does the gas stay on before going out and the blower coming on? Does this unit have any diagnostic lights on it? What brand, model, and serial number is your unit? Have you had anything done to your system recently, such as a new thermostat, maybe?