Gas forced heat furnace starts blowing chilly air after awhile


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Old 12-09-10, 01:20 PM
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Question Gas forced heat furnace starts blowing chilly air after awhile

Hello all! Have a furnace that seems to work fine. But if it continues running for long periods it begins to blow out cooler air. I checked the pilot lights and no problem there. If I turn down the thermostat and give it a break warm air will begin again. Husbands thinks might be blower motor but other than that clueless, as am I , which is why I depend on this site! If it is blower motor anyone know average price to fix, both service call and diy way. Thanks in advance for the help!
 
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Old 12-09-10, 02:02 PM
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I dont know IL but NJ is 89 to come out reg hours 180 after hours. 220 diognostic and 200-800 for possible parts replace.(depends on what is bad)4000-12000 for new furnace with line set and a/c. Do the math.

Mike NJ
 
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Old 12-09-10, 02:10 PM
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First...check to make sure all the exhaust ducting for the burners/gases is clear. Sounds like it's overheating and shutting down the burners til the blowing air cools it off.

Please explain in detail what its doing. You can listen for most of the issues. Does the blower ever shut off after it starts blowing cold? Or does it run continuously.
 
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Old 12-09-10, 03:48 PM
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Well it really doesn't sound any different. Sounding "normal". No the blower doesn't shut off, it keeps running. A couple of times I just turned the heat down because it was making the house cooler, so it wouldn't have shut off. I just turned it up a couple degrees after giving it a break most of the day and it was nice and toasty. I will keep it going again this evening and see if it starts cool air again.
 
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Old 12-09-10, 04:01 PM
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The gas burners in your furnace are probably shutting down because of high temperature coming out of the heat exchanger. This is most often caused by insufficient air flow and the most common problem affecting air flow is a dirty filter. If it is anything other than a dirty filter then it is probably not a DIY job.

After changing the filter try this, when the furnace starts turn the fan switch on the thermostat from the Auto position to the On position and see if the furnace burners run for a longer time period.
 
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Old 12-29-10, 10:47 AM
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I'm back! okay haven't done anything w the furnace yet. It seemed to be doing better there for awhile, but its back to its old tricks again. The other night I got a clue however! When the furnace was beginning to start again (you can hear the flames starting from upstairs) I heard the flames start but they shut off like 20 sec. later, again they started, and again turned off after maybe 10 sec. this happened about 4 or 5 times. once the blower kicked in (flames were off) it was cool air. Now when I turn the furnace on and the flames stay on and the blower starts, oula! heat. So I've found that when it starts & stops like that it defaults to cool air and I just turn it down before the blower starts. Any insight on that piece of info? Thanks to all!
 
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Old 12-29-10, 05:23 PM
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One thing we know is the fan is being brought on by a timer rather than temperature. Now we need to know a bit more about the furnace itself. Let's start with a make & model. From that we might be able to determine the ignition & flame proving systems.
 
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Old 01-05-11, 06:02 AM
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Haven't been back because furnace was working perfectly for maybe a week.....woke up to 56 degrees this am..... The model is a Trane XE 78. We have been in the house for about 6 years, and it may have been a good 10 yrs old when we moved in. My thermostat does go off the temp in the house and will kick on when it gets below the set degree. Had it on 63 when I went to bed...again 56 this am. I can only assume that it was running all night to get back up to 63. A friend came over and said that flames look good. They are a good color blue, igniter seems to be fine starts up the flames well, but sometimes the flames just shut off when its trying to start up, thats when I get cool air. Have a Bday party this weekend at my house & am praying that it works that day!!!!!
 
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Old 01-05-11, 04:22 PM
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One of the easiest things you can do is to clean the flame sensing rod.
This rod is usually on the opposite end of the burner rack from the ignitor & is usually held in place by one 1/4" hex head screw. Most have one wire going to them. The rod has a porcelain insulator & the metal rod extends into the flame. Remove & clean the metal rod with a scotch brite pad or steel wool, reinstall, & try it. Post back with results.
 
 

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