Gas Furnace Cycles


Old 11-25-02, 09:06 PM
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Question gas furnace cycles until temp reached

I have a 17 year old Bryant 395CAV which continuously cycles on and off until the temperature is reached. Shouldn't it stay on all the time until the temp is reached? What could be wrong?

Here is what happens:
1) some motor starts in the furnace, not the blower, maybe the inducer motor
2) after about 25 second after the motor starts, you can hear the gas turn on
3) after about 45 seconds of the gas being on, the blower motor starts. the motor in 1 still sounds as if it is going.
4) after about 3-4 minutes, the gas turns off, and the other motor turns off, the blower is still going
5) after about 3-4 minutes, the blower motor turns off
6) after about 5 minutes, the whole thing starts again.

This cycle continues until the temperature is reached, not just once it's reached. I played around, set the tstat to 85, and it cycles over and over again until the temp is reached.
It has a brand new filter.

Is there anything I can do or do I need to contact a professional?

I read an article in Popular Mechanics that pointed to three possible problems:
-a) high-limit temp sensor not working, thinks it is overheating
-b) low flame signal from flame sensor not operating or it is sensing a low flame or burner not lit anymore
-c) flame roll out switch sensing insufficient combustion air and shuts of the gas

Any ideas?

Last edited by zim123; 11-26-02 at 02:18 PM.
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Old 11-26-02, 05:28 PM
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Hello zim123 and Welcome to the Heating and Cooling forum and our Do-It-Yourself Web Site.

Chances are the furnace is cycling because of an intake air restriction. Check the filter. Intake restriction could also be caused by dirty A/C coils.

If the unit is equiped with a flame sensor element, it may be dirty. Try cleaning it with steel wool or a fine wire brush. Reinstall exactly as found.

If this does not resolve the problem, the sensor element should be replaced.

Other Suggestions:
Read several postings within this forum on the topic of "Gas Furnace Problems." The questions already asked describe like or similarly related problem descriptions as your question.

These prior postings and any or all the replies offered within them contain almost all the required information needed pertaining to the many potential and or possible problems with the heating unit like your describing.

Additional Suggestions:
Read the manufacturers online web site for product information, problem possibilities, solving methods, part locations, disassembly, reassembly methods, repair procedures, instructions, pictorials, schematics, all or part of which may be available online.

An excellent source for orginal replacement parts is your local retail appliance parts store. Appliance parts dealers test and carry replacement parts for all appliances. Parts dealers are listed in the phone book under appliances.

Retail parts dealers can also help determine what the possible problem may be. Bring the make, model and serial numbers if you stop in at the store. The info will help to determine the ignition system and possible problem causes.

If you need further assistance, use the REPLY button to add any additional information or questions. Using this method also moves the topic back up to the top of the list automatically.

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Old 11-27-02, 08:35 AM
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After reading the postings for a good hour or two yesterday (wow, lot's of good stuff!), I feel much more informed and I tried a couple of things, however with no luck:
1) unhooked the thermostat and wired together red/white to turn furnace on, it continues to cycle, which rules out a thermostat problem.
2) tried to look at air restriction: my filter is brand new, the duct work on the intake side of the filter is very clean, the inside of the box where the blower motor is is very clean, the blower motor blades are very clean. Everything for that matter looks very clean inside the furnace, no dirt/dust, no carbon or other visible buildup or anything.
3) one of the postings mentioned cleaning the flame sensor with a fine sand paper. The sensor wasn't very dirty, didn't have much of any build up or anything, I tried to clean it anyway, although without really taking it off.

Couple remaining questions/things to try:
- I'll check the AC coils which are in the outbound air shaft to see if they are gunked up and restricting the outbound air. It doesn't look easy to get to, there is a 9x9 panel on the side which is below the aprilaire humidifier and above where I assume the coils to be.
- I couldn't find the limit switch, could my scenario be caused because the high limit is set to low which is when the gas turns off?
- Does the flame sensor shut the gas off if it determines there is no flame? So that's why it may be my problem? The amount of time the gas is on is pretty consistent. Are you saying that maybe the flame sensor goes bad when it reaches a certain temp and sends the wrong signal back saying the flame is off so shut off the gas?

Thanks so far, keep the ideas coming!
Old 11-27-02, 09:12 AM
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Is your furnace a HSI or spark?

If it's HSI (Hot Surface Ignitor "Glows red/orange" to light the burner".. Make sure they are connected on the control board or modular that controls the gas valve, and ignitor..

Make sure your ground wire is grouned to a ground.

if they all pass, then chances are you may need a new igntion cotrol modular.

If you have spark, make sure the pilot flame is burning on the rod. You've already mention cleaning it. so I take it you have spark..

Again, check your ground wire are grounded, and chances are your modular might be bad on it as well..

Do you have a pressure swich?? Chances are your pressure swich maybe tripping from your flue pipe might be plugged or maybe the heat exchanger as well.. see where the hose from the pressure swich leads to..

Old 11-27-02, 11:32 AM
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It's a hot surface igniter. It turns off after the flame has started and gone for a few seconds.

I'm not sure if it has a pressure switch, the mtce papers I have and the Bryant site don't get into those details, any idea where I can get those details online?

I'll check for the ground, thanks.

I also opened up and looked at the AC coils, they look pretty clean, although I can only see them from the top, the blower motor is underneath. But it looks like it's clean.

It was the flame sensor that I cleaned, sorry if I confused you.

How much is a new flame sensor? Maybe I should just pop for one of those to see if it solves it?

Thanks for the ideas!

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