Author Topic: Furnace Trouble  (Read 5729 times)

Offline alydar66

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Furnace Trouble
« on: December 01, 2007, 10:33:54 AM »
Hi, Im having a problem with my Bryant gas-fired,induced combustion,furnace and cant seem to isolate the problem. When the temp. outside gets down to about 30 degrees I start having problems. Probably because the furnace has to stay on longer to keep the proper temp in the house. What is happening is that the furnace will go and work fine for a while but then it will shut down and have trouble restarting again. I think something is getting to hot because if I shut the  furnace down for a half hour and start it again it works fine. But then after about 15 minutes of being on it shuts down. All my limit switches are manual reset, and I havent had to reset them.
   The inducer fan goes on and stays on for 5-15 seconds, sometimes the pilot clicks on while the fan is running and clicks off. Then the fan shuts off. The fan stays off for about 3 seconds and the process starts over again sometimes up to 20 times before the furnace finally lights. Sometimes when it does light it only stays on for a minute other times it stays on until the thermostat temp is reached. I replaced the Limit Switch that senses the air temp in the furnace but the problem still exists. If I turn the furnace of for 15 minutes the furnace will run fine when I turn it back on. It only seems to have this problem when it has to stay on longer to keep the proper temp. Does anyone have any ideas what the problem may be or can you give me an idea what to check next. I posted this on one of the other boards and someone suggested cleaning the flame sensor, but my schematic only shows a flame sensing switch.  Thank for any help anyone can give.

Offline TECH X

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Re: Furnace Trouble
« Reply #1 on: December 01, 2007, 02:35:07 PM »
Look in front of the burners, usually on the left hand side, you will see a metal rod with a porcelain base, connected to a wire.  That's the flame sensor, but your problem doesn't sound like flame failure.

The furnace goes into a lockout mode when it detects a problem, sometimes the lockout lasts for 3 hours then resets itself.  Re-setting the power will clear the lockout and restart the system.  A dirty filter could cause the furnace to overheat and to lockout.  Make sure the filter is clean. 

There should be diagnostic lights on the furnace board that flash a code.  Get the code next time the furnace locks out and it would help isolate the problem.

Your description sounds like a pressure switch problem.

Offline alydar66

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Re: Furnace Trouble
« Reply #2 on: December 01, 2007, 02:47:16 PM »
This is an older furnace. It doesnt have diagnostic lights.  The only flame sensor it has is the one that senses the pilot.  Its called a standing pilot.  Also, this system doesnt have a lockout and all the limiter switches are manual.  The filter has been changed.  Ill look again at the pressure switch. Thanks

Offline TECH X

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Re: Furnace Trouble
« Reply #3 on: December 01, 2007, 06:28:42 PM »
The pressure switch may be opening because of a venting problem.

Offline alydar66

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Re: Furnace Trouble
« Reply #4 on: December 02, 2007, 12:11:06 AM »
If it was venting problem or a pressure problem why does the furnace work fine when the outside temp. gets above 30-35 degrees.  The longer the furnace stays on the more trouble it has lighting and staying lit. It seems like something is getting to hot when the furnace is on for longer periods of time and something is shutting it down.  But like I said, all the limiter switches are manual reset.

Offline TECH X

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Re: Furnace Trouble
« Reply #5 on: December 03, 2007, 05:36:15 PM »
It's unlikely the primary limit switch is manual reset.  They're usually all automatic reset.

You say you have a standing pilot, but you said the pilot clicks off then on.

It sounds like you have an intermittent pilot.

Sequence,

Thermostat calls for heat.
Inducer motor starts.
Pressure switch closes.
Ignition module or board sends power to electrode, and lights pilot.
Main burner opens and burners fire.

Is there always a pilot burning in the furnace, or just when there's a call for heat?

What does the pilot assembly look like?  How may wires/tubes connect to it?

I no longer think it's a pressure switch problem.  I'm thinking either a problem with the board/module or pilot assembly.  Sometimes there is an electrode that sparks and lights the pilot and senses the flame.  It may need to be cleaned.  Or the module/board may be bad.  Where does the wire from the pilot assembly connect to?  What kind of module or board do you see.  A flame switch usually refers to a mercury switch.  They normally plug directly into the gas valve.

Offline alydar66

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Re: Furnace Trouble
« Reply #6 on: December 04, 2007, 12:11:10 AM »
This is the order that things happen:

1. Furnace is working properly. Its 45 degrees outside and the furnace only has to stay on for short periods of time to keep the house temp. at the thermostat setting.

2. Temperature outside drops to 20 degrees overnight so now the furnace has to stay on for longer periods of time to keep the desired house temperature.

3. Furnace will shut down before temperature  on thermostat is reached.  It shuts down in this order:
         
         a. inducer motor shuts off

         b. furnace flame goes out

         c. main blower shuts off after a couple of minutes

4. Furnace attemps to restart in this order:

         a. inducer motor starts

         b. Im assuming pressure valve closes because I have tested it and it seems to work properly.

         c. pilot sometimes lights and sometimes doesnt.

         d. inducer motor stops. Sometimes the pilot will stay on when the inducer motor stops, other times it goes
             out.

         e. inducer motor starts after a few seconds and the whole process is repeated.

         f.  eventually the furnace will light. Sometimes it takes up to 15 of these cycles for the furnace to light.

         g.  after the furnace lights, sometimes it will stay on for only 15 seconds and other times for 15 minutes, and
              others in between.

5. If I shut the furnace  off for a half hour or so and restart it,  it will start and run fine, until some point it has been on to long and will shut down, and the whole process is repeated.


6.  My schematic shows 1 Limit Switch and 1 Auxilliary Limit Switch, both are manual reset.

7.  I dont know how to add a picture to this forum but this a link to a picture of the Standing Pilot that I have.
     Scroll down the page to the Bryant & Carrier Standing Pilot section. Its the pilot in the middle of the page.

   [url][/urhttp://arnoldservice.com/bryant_&_carrier_flame_sensors.html]

8.  I dont think it would be the dirty pilot assembly,  that wouldnt have anything to do with the furnace
     shutting off, would it?

    THANK YOU so much for taking time to help diagnose my problem.  Hopefully you can narrow down the problem
    after reading this post.  Thanks.

Offline Admin

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Re: Furnace Trouble
« Reply #7 on: December 04, 2007, 10:37:12 PM »
Quote
d. inducer motor stops. Sometimes the pilot will stay on when the inducer motor stops, other times it goes out.

Sounds like a board problem.  When the inducer stops there shouldn't be a pilot.  I would check if you have voltage at the gas valve the next time you catch the pilot staying on when the inducer has stopped.  If there is voltage I would say make sure the pressure switch hasn't stuck closed , if not then change the board.  If there's no voltage at the valve when there is a pilot I would change the valve, it could be sticking open.