Gas Furnace short cycling

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Old 02-14-06, 02:03 PM
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Gas Furnace short cycling

Need help with gas furnace, I have had certified technician come to service the furnace last October. He changed the filter and did whatever tech's usually do to check the furnace. I mentioned that the furnace the prior year appeared to shut down prematurely between heating cycles. He said it was probably the thermostat. So I went out to buy another one, but the problem continued. The service tech came back and said it was still the thermostat. So I bought another one! It still behaves the same way. I do not want to call them back and fork over another $150.00 to have them tell me the same thing again. Can you give me any pointers on other things that it could be? The last thermostat I bought is an old model that you can adjust the anticipator setting. I have tried different settings and there is no change. This leads me to believe it was never the thermostat! This is a gas forced air furnace that was installed in 1987. Help!
 
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Old 02-14-06, 07:50 PM
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Hello trying. Welcome to my Gas Appliances topic & the Do-It-Yourself Web Site.

Gas furnace questions are answered in this forum. Therefore I moved your question here. The other pros will also help.

Most common cause is an air intake flow restriction. Some possible causes:

Dirty air filter.
Restricted intake air vent(s). (Where the filter is likely located.)
Restricted registers in the rooms or too many closed registers.
Restriction (Excessive lint & dust) in the A/C coils, if equipped with A/C.
Fan blower motor speed to slow or motor defective. (Running to slowly)

Check back on your question several more times. Heating professionals and other members posting replies in this forum topic may offer you additional advice, possible causes, solutions and or repair methods.

Kindly use the reply button to add updated additional information. Using the reply button moves the topic back up to the top of the list of current questions automatically.

Be sure the electrical power to the appliance is turned off, the thermostat is turned down and the gas control valve is in the off postion before attempting any repairs.

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Old 02-15-06, 05:53 PM
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Short Cycling

When you said the furnace shut down prematurely, did you mean it shut down before the room reached the set point on the thermostat?
 
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Old 02-15-06, 08:31 PM
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The furnace turns on, stays on briefly (maybe 10 seconds), shuts off, almost immediately turns on again (fan blows and heat comes out), shuts off, again turns on again. this can go on for up to 6 times and the time the heat is on and the fan is blowing for longer periods each time. The temperature eventually gets to the desired temperator.
Also, I checked the intake duct and it was a little dirty, nothing totally obstructing it though, so I cleaned it out. The air filter was replaced by the tech that was initially called in.
 
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Old 02-15-06, 09:05 PM
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Trying

Here is something you can do to help us diagnose the problem:
When the furnace first comes on, watch it to see if the flame is staying on & just the fan is cycling or if flame & all is shutting down. Please post back with the results of your observations.
 
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Old 02-18-06, 02:59 PM
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Thumbs up solution found

Okay, I did make sure the flame and the fan were both shutting down at the same time. My husband also removed the front panel and noticed a wire that was not connected to anything. I believe it was from the fan switch and it was obvious where it was supposed to go because the screw was not tight. So, we turned off the power and put the connector back on and guess what! No more short cycling. Now, the question I have is why didn't the gas tech see this and instead send me shopping for a thermostat! In any event, it was not something I expected but I am grateful we decided to take a look ourselves. Thank again for all your suggestions, it has been a frustrating winter! I am also grateful there doesn't appear to be a damage done, at least as of now. Still not sure what to do about that gas tech though......
 
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Old 02-18-06, 06:51 PM
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Service tech

I suggest you write to his service manager documenting what you found & asking for this tech to spoken to regarding his troubleshooting skills or lack there of. Thermostats are usually very easy to check by bypassing them. Had the tech done this, he would have known right away the problem was not in the stat. Short of a letter, there isn't a lot you can do.
 
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