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Old 01-18-2012, 09:46 PM   #1
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Furnace shutting down after 30 seconds


I have an older Tempstar gas forced air furnace. It starts up, runs for 30 seconds, shuts down for about 10 seconds then starts up again. Same thing over and over. Have replaced the limit switch, cleaned the flame sensor. Problem still exists. Any ideas?

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Old 01-18-2012, 10:19 PM   #2
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Furnace shutting down after 30 seconds


Age/ model?

the likely sequence of operation is as follows:
1. Tstat call for heat
2. inducer motor starts and closes pressure switch
3. after a short pre purge igntor will be energized
4. gas valve pilot solinoid is energized and delivers gas to the pilot.
5. pilot flame ignites.
6. sensor confirms the pilot flame.
7. main gas burners comes on and ignite.
8. main blower is energized
8. tstat is satisfied and inducer is de-energized
9. main blower de-energized

How far does it go through this process?
Do you have a diagnostic led light blinking through a door grill or inspection port.

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Old 01-19-2012, 04:05 PM   #3
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Furnace shutting down after 30 seconds


Limit goes out for a reason. I bet anything you have a plugged up filter or indoor coil. To even rule out bad t-stat with power off put white and red together and turn back on. If it keeps running then bad stat but most likely plugged up coil.
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Old 01-21-2012, 10:22 AM   #4
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Furnace shutting down after 30 seconds


Thanks for suggestions. Update: what actually happens is it heats up, main fan starts, flames shut off, fan shuts off, flames starts up again.
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Old 01-21-2012, 10:25 AM   #5
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Furnace shutting down after 30 seconds


Filter is brand new. Have even tried it without filter. Heat exchanger is all clean.
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Old 01-21-2012, 10:27 AM   #6
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Furnace shutting down after 30 seconds


Old Temstar. Too old for led indicator.
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Old 01-21-2012, 10:33 AM   #7
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Furnace shutting down after 30 seconds


post pics of it with both doors off. It is an ICP product and lots of them have cracked heat exchangers or popped rivets which hold the heat ex together. air may be disturbing the flame due to a cracked heat ex. may need to get an experienced Pro to check for that.
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Old 01-21-2012, 07:26 PM   #8
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Furnace shutting down after 30 seconds


Quote:
Originally Posted by yuri
post pics of it with both doors off. It is an ICP product and lots of them have cracked heat exchangers or popped rivets which hold the heat ex together. air may be disturbing the flame due to a cracked heat ex. may need to get an experienced Pro to check for that.
Can't figure out how to post photos. Will check the heat exchanger. Thank you.
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Old 01-21-2012, 07:34 PM   #9
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Furnace shutting down after 30 seconds


Quote:
Originally Posted by photvidgal View Post
Can't figure out how to post photos. Will check the heat exchanger. Thank you.
an easy check is pull blower assy look for rings in bottom of blower...most of the times if exchanger has problems rings will drop there...
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Old 01-22-2012, 09:38 AM   #10
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Furnace shutting down after 30 seconds


post them on photobucket/image shack/flickr or (go advanced) in this box and then (manage attachments) and upload them. put links here from pbucket or others
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Old 01-22-2012, 09:40 AM   #11
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Furnace shutting down after 30 seconds


that works some of the time but not for cracks, still need to get a Pro with mirrors and a scope.
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Last edited by yuri; 01-22-2012 at 10:50 AM.
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Old 01-22-2012, 10:47 AM   #12
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Furnace shutting down after 30 seconds


Checked blower. No problems seen.
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Old 01-22-2012, 10:52 AM   #13
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Furnace shutting down after 30 seconds


I have worked on hundreds of them and know where they crack. They only way to see the cracks is with a mirror/experience of where they crack/inspection scope or to pull out the heat ex from the furnace (gut it like a deer). I am more concerned with your safety as well as solving the problem.
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Old 01-23-2012, 03:15 PM   #14
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Furnace shutting down after 30 seconds


We have 2 furnaces so have switched over to the other one. We so appreciate all the help. We will be revisiting this shortly. Want to get them both functioning soon.
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Old 01-24-2012, 12:24 AM   #15
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Furnace shutting down after 30 seconds


Quote:
Originally Posted by Tator1076 View Post
Limit goes out for a reason. I bet anything you have a plugged up filter or indoor coil. To even rule out bad t-stat with power off put white and red together and turn back on. If it keeps running then bad stat but most likely plugged up coil.
Is it safe to do this?

Or should you just try another thermostat?

Thanks

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