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Old 10-24-2011, 05:21 PM   #1
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Draft fan, igniter, and gas lit but no main blower


Hello everyone,

I have an old gas furnace which didn't fire up correctly this season The outwardly noticeable behaviors:

1) Fan on auto, arrange for a call for heat, forced draft motor starts, the igniter glows, the gas valve activates, and presto the burners are lit and remain lit. It runs like that for a spell but the main circulation blower nevers starts and the furnace shuts down fairly soon.

2) If I change the fan setting to fan always on and then arrange for a call for heat, the furnace fires up and remains burning well beyond the time it would have shutdown in scenario 1. I only performed one brief test of this and then I set the heat/cool setting to off while keeping the fan on always on so as to mimic what it would normally do and cool things down. I'm not sure how it would behave if I let it do its thing while the fan was set to always on.

Given the age of this thing I'm strongly leaning towards getting quotes and having it replaced rather than having it repaired. However, I can't help but be curious about a) possible or likely causes of the behavior I describe, and b) if it would be safe to, in the event of an unexpected cold snap, briefly run it with the fan set to always on.

At present and for the near term at least, it should remain warm enough that I wouldn't want any heat to begin with. I don't intend to repair anything myself at this point. I'd just welcome comments if someone cares to offer them. Last I checked, the system was capable of turning the fan on by itself when the fan is set to auto and it is in cooling mode. Which makes me think it might not be a main board problem but rather something else which might not be a huge deal to fix. A quick/cheap repair would definitely buy me some time and give a company more time to install a new system.

Thanks.

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Old 10-24-2011, 07:15 PM   #2
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Draft fan, igniter, and gas lit but no main blower


Could be a bad winding in the motor, or the control board isn't energizing the motor for heat.

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Old 10-24-2011, 07:41 PM   #3
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Draft fan, igniter, and gas lit but no main blower


Or it could be a fan switch .
If it had a bad winding it wouldn't run.
The fan switch could be located on the circut board.
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Old 10-24-2011, 07:57 PM   #4
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Draft fan, igniter, and gas lit but no main blower


Thanks for the replies in light of how vague my first post was It has made me realize that there is at least one difference between energizing the motor for cooling (which it could do *last I checked*) and energizing the motor for heat (which it apparently can't do). The motor is at least 2-speed, possibly three. When in heat mode the furnace would if it were working run the fan at low speed. When in cooling mode or when the fan is set to always on the fan runs on a higher speed. At least that is how it was jumpered or hard wired. There for a second I was imagining a microcontroller selecting speeds and driving the blower motor via the exact same wiring in both heat and cool modes, thus wondering what the microcontroller might want to see but isn't seeing. I take it running the fan at higher than normal CFM would drive temperature rise below nominal and thus not be good for the heat exchanger? Does it seem reasonable to think that since it cuts itself off when the blower motor doesn't start that it is do to a high temperature limit feature kicking in?
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Old 10-24-2011, 08:06 PM   #5
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Draft fan, igniter, and gas lit but no main blower


Quote:
Originally Posted by REP View Post
Or it could be a fan switch .
If it had a bad winding it wouldn't run.
The fan switch could be located on the circut board.

Seen a lot of motors that would run on one winding but not the other.
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Old 10-24-2011, 08:54 PM   #6
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Draft fan, igniter, and gas lit but no main blower


yes if burnner is running and after a few monents if fan does not start furnace has a limit switch that opens at high temps.....what kind of furnace is this......it may have blower control board,,,it may have combo fan and limit....
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Old 10-24-2011, 09:09 PM   #7
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Draft fan, igniter, and gas lit but no main blower


It is a Snyder General GUA040A010AIN

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