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Discussion Starter #1
Hey everyone,

I recently left the house for a few days, I turned the thermostat down to 58 degrees on a Carrier forced hot air gas furnace. When I returned four days later I bumped to 64 degrees. The furnace came on and the hot air started to blow, after a few minutes the force of the air doubled and became very loud. The furnace is around 8 years old and I've been in the house for 5 years. It has never performed like this and it's waking me up in the middle of the night with it's ferocious blowing. Can anyone explain this?

thanks,

a new guy
 

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I'm Your Huckleberry
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Is it still blowing hot air when it ramps up? If a limit switch trips (due to excessive heat/overheating) this is exactly how a furnace acts, it blows on high speed BUT the gas is shut off so it won't heat.
 

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Discussion Starter #3
Yes, the air is still hot during the ramp up. There also seems to be a loud "thud" noise when the hard blowing stops.

thanks
 

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I'm Your Huckleberry
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I would double check that. What may be happening is a limit switch is tripping, the blower ramps up, and after the furnace cools off a bit the switch resets and resumes heating. It could be the residual heat that it's blowing in between.

Next time it happens go and look at the led light on the furnace. You should have a window on the furnace door making that light visible. See if it's throwing a code.
 

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Ok, I pulled the cover and witnessed a few things. It starts up and runs for a few minutes, the pilot light goes out and seems to stop blowing completely. A blinking code starts, 3 quick blinks, a pause and then 2 long blinks. Then the ramp up starts, the pilot light fires and the blinking code stays lit. I checked the code on the back of the panel, Low-heat pressure switch did not close or reopened, does that sound correct?

Thanks for your help, I guess it's time to call a service person.
 

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I'm Your Huckleberry
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Not so fast, you can probably take care of this on your own if you are willing. The pressure switch is sucked closed by the exhaust fan motor, the little one attached to the metal exhaust pipe. There is a tube that attaches from the draft inducer motor to the pressure switch. That tube or the ports where the tube connects to can become clogged. You can use a good sized straightened out paper clip, remove the tube at the motor and at the pressure switch, and make sure both ports as well as the tube is unrestricted.

Mostly it'll happen at the motor port. Be careful when removing the tube, it'll break easily.

On the motor side you can ram the paper clip in there pretty good, on the switch side you need to be very gentle so as to not damage the switch.

It could be that, it could be your exhaust pipe is blocked. You can remove that pipe frmo the furnace and bang on it a bit, see if any dirt falls out.

 

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Discussion Starter #9
Yes it's a 58 mvp. The photo you attached does not quite match up with what I'm looking at. I might give it a go when I have some free time tomorrow night.

Thanks again.
 

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You have a 2 stage furnace, and when you set your thermostat up those 4 degrees it went to second stage, which requires a higher air flow.

Apparently, your furnace is oversized if its never gone to second stage before.
 
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Discussion Starter #12
2nd stage

It has gone to second stage before. Now it goes there every time the furnace fires, it can't be normal.
 

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It has gone to second stage before. Now it goes there every time the furnace fires, it can't be normal.
Depends how old it is outside, it could be working normal for those outdoor temps.
 
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