Ingition problems with forced air heating


Old 02-07-04, 09:25 AM
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Question Ingition problems with forced air heating


I have a fairly old forced air heater that has developed a new problem. At first, when the heater was activated the furnace make a quick clicking sound before the burners ignited. The clicking sound would last for about 20 to 30 seconds.

This went on for about a day. Now what happens is the furnace makes a whirring sound, maybe a fan belt or pulley that is loose. This goes on for a little while and it builds up speed as it does. (about a minute). The burners ignite and the furnace will stay on for a few minutes, goes off then the whole process starts again.
Warm air is coming out of the bonnet. I don't hear the clicking noise, but I'm *assuming* it's still there... I just can't hear it over the other mechanical noises.

As an aside, the distribution fan seems to work fine. I can feel the air being forced through the vents.

I'm wondering if the input valve to the burner is clogged, but the whirring sound? I'm going down to have a look at it in a few minutes. Does anyone have a suggestion as to what to look out for?

Thanks for the help!
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Old 02-07-04, 12:22 PM
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Follow up

Hi again,

I watched the furnace in action and it appears that my assumption about a clogged gas valve is incorrect.

The pilot light and the burners are igniting just fine. This is what appears to be happening (I'm not familiar with furnaces in general so take this with a grain of salt):

A motor connected to a fan (blower, etc.) is trying to turn on, but fails for about 20 to 30 seconds. During that time the pilot is ignited and goes out after a few seconds because nothing else is happening. Eventually the pilot light ignites and whatever condition needed to open the gas valve to the burners is there and the burners ignite. Once that happens the fan that distributes the air to the vents turns on.

The burners stay on between 20 to 40 seconds and then turn off. I don't know if that is correct operation or not. I have no idea if that is enough time to heat the air in the bonnet up enough to trip the limiter solenoid. The burners igniting and turning off repeats itself a few more times.

After a few minutes the furnace turns off again. During this whole time that motor I mentioned is wheezing.

It looks to me like the motor is for a fan or something for the flue (the flue is clogged?).

I've attached a picture of the unit that shows the part I'm talking about. Any suggestions would be greatly appreciated.

Old 02-08-04, 05:43 AM
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Need a bit more info.

What Brand and model of furnace do you have?

Do you have the old basic model, 80% or 90%?

As for whirling sound, is it the inducer or the main fan?

The pictures don't work in this form.. So if you have them post them on a link and we can see it from there.. otherwise. e-mail it.
Old 02-08-04, 06:07 AM
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Like Jay says we need some more info.

Be sure your furnace and filter are clean.

The start-up should go like this,

1. Draft inducer should come on. "small blower hooked to the flue pipe"

2. The pilot should light.

3. The main burner should ignite.

4. The main blower should start.

If it's the draft inducer giving you this problem check for any obstructions in the wheel. "Does it turn by hand?" If its not wanting to start it will probably need replaced.

When it does a succesful start the burners should not shut down until the space temp has been met then they will shut off and the main blower will run for a few minutes and it will shut off.
Old 02-20-04, 09:34 PM
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Hi Guys,

Thanks for the input. I found out what was wrong pretty quickly, but an emergency trip to the hospital (not related to this) made me put the whole thing aside for a few days.

The inducer motor was the problem. The noise/seal gasket that separates the motor chasis from the flue chamber crumbled and fell into the rotor causing it to stick. I disassembled the motor to get the pieces of rubber out. Unfortunately I had to cut off the blower wheel in the process because the screw that held it on the motor shaft was so corroded it was impossible to remove any other way.

To make a long story short, I found another blower wheel that matched the old one but was short in height 1/4".

The furnace pilot ignites now, but the burners will only stay lit for 5 seconds without the main blower on and for about 15 seconds with the main blower on. My uneducated guess is the flue is not pulling enough air to keep the burners on for any longer period of time. Is this 1/4" difference the culprit for this or should I check for something else?

Thanks again!
Old 02-23-04, 09:32 PM
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Hi again,

I've figured out the second problem with this thing and it's working again. The electrical connector from the control board to the draft induction motor was making intermittent contact causing the furnace to shut off.

Sorry I can't give any specifics about this furnace other than it is a Carrier and it's old. It might be twenty or more years old. The installers put the furnace front side down, model number and all, so I can't get under it to see. I'm also not able to find anything legible on the unit other than part numbers for individual parts. The same goes with the air conditioning unit outside. All I know is that it is a Chrysler and that's old.

Anyway, here's my question now. I'm observing that the burners will have yellow tips on the flame occasionally. I've always thought that for natural gas this meant that the methane wasn't combusting completely and the yellowing was due to carbon burning incompletely. I've checked my CO detector in the house and haven't seen any detectable levels of CO, but should I still be concerned about that?

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