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Furnace Blows Stronger Just Before Shutting Off

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  • Mar 15th, 2022 7:58 pm
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Furnace Blows Stronger Just Before Shutting Off

I’ve noticed that when I have the heat on and the desired temperature is reached, my furnace will then blow out air a lot stronger for 10-15 seconds before shutting off. Any idea why this occurs?

While it’s running to reach the desired temperature, it runs at a constant air flow but only on the tail end right before shutting off does it get much louder as if it’s blowing out more air.
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Sep 27, 2006
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Not so easy there Ma…
Is that a new condition or has it always existed?
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fergy wrote: Is that a new condition or has it always existed?
We moved into this home back in May 2021. It’s been doing that since we moved in. Also hearing a ton of popping and banging throughout the ducts when the heat is on, didn’t notice these in the summer, could be heat expansion but the banging are these loud pops. If I didn’t know any better, it almost sounds like the duct work banging against the dry wall or the hard wood floor expanding as a result of the duct work being warmer??
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Jun 14, 2008
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To cool down the heating element?
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when was it last cleaned?
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divx wrote: when was it last cleaned?
We had ducts cleaned right after we moved in. Maybe 9 months ago.
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Jagstyles wrote: We had ducts cleaned right after we moved in. Maybe 9 months ago.
Ducts cleaning will be seperate from the furnace cleaning unless the company does both.

For furnace the furnace burners might need to be cleaned especially if it's had life a incomplete combustion. Heat exchanger might need to be vacumed out, The flame sensor is cleaned, the blower and inducer fans are cleaned and lubricated too.
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bubuski wrote: Ducts cleaning will be seperate from the furnace cleaning unless the company does both.

For furnace the furnace burners might need to be cleaned especially if it's had life a incomplete combustion. Heat exchanger might need to be vacumed out, The flame sensor is cleaned, the blower and inducer fans are cleaned and lubricated too.
Any one of those things can cause banging/popping throughout the duct work?
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Jagstyles wrote: We moved into this home back in May 2021. It’s been doing that since we moved in. Also hearing a ton of popping and banging throughout the ducts when the heat is on, didn’t notice these in the summer, could be heat expansion but the banging are these loud pops. If I didn’t know any better, it almost sounds like the duct work banging against the dry wall or the hard wood floor expanding as a result of the duct work being warmer??
It won't be the flooring or the ducts hitting the drywall, they won't be close enough to hit the drywall and it would be more of a thud than a pop or bang if it did hit. It's the ducts expanding due to the heat so that's why you didn’t notice it in the summer.
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Jagstyles wrote: Any one of those things can cause banging/popping throughout the duct work?
Depending on the kind of banging or popping noise but yes they can cause some sort of banging noise resonating in your duct work. On the extreme dirty burners allow NG to build up, when it ignites it causes mini explosions which could a sound like banging at the start of the heat cycle or if the flame goes out mid-cycle. The blower and inducer could also make sounds closer to a loud thump or repeating tapping as they go bad.

Getting the furnace cleaned is usually part of the normal furnace maintenance when most of the parts will be checked. What is the brand and model?
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Jagstyles wrote: We had ducts cleaned right after we moved in. Maybe 9 months ago.
I'm talking about the blower itself.
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Whats the brand/model of furnace.
Some fan speeds can be adjusted if needed. In your case I'm not sure but that sounds like the heat turns off then the fan may speed up to help cool the heat exchanger to prolong life? Just a guess but that would make sense. Maybe @newlyborn will have seen that before while being out in the field?
Jagstyles wrote: I’ve noticed that when I have the heat on and the desired temperature is reached, my furnace will then blow out air a lot stronger for 10-15 seconds before shutting off. Any idea why this occurs?

While it’s running to reach the desired temperature, it runs at a constant air flow but only on the tail end right before shutting off does it get much louder as if it’s blowing out more air.
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Jun 1, 2010
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Jagstyles wrote: I’ve noticed that when I have the heat on and the desired temperature is reached, my furnace will then blow out air a lot stronger for 10-15 seconds before shutting off. Any idea why this occurs?

While it’s running to reach the desired temperature, it runs at a constant air flow but only on the tail end right before shutting off does it get much louder as if it’s blowing out more air.
Is your current furnace a single stage or 2 stage?
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Jun 23, 2019
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What thermostat are you using? If it's an Ecobee make sure fan control is set to HVAC not thermostat. Also Heat Dissipation Time and Cool Dissipation Time set to 0. These settings can cause the fan to run at a different speed after a call for heat/cool.
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you have a variable speed blower fan on your furnace. what is making it run faster at the end of the cycle is unknown. your furnace likely has two stages of heat. if you have a smart thermostat, you may have these two stages wired directly to the thermostat. if you have a traditional thermostat (or a smart thermostat and the installer was lazy and didn't run an extra wire) the HVAV logic board is likely making the decision on what speed to run the blower fan at. my fathers furnace does this based on a combination of previous runtime on heating calls.

also, setting heat dissipation time at zero is a terrible idea. heat dissipation is how long the HVAC blower motor will stay on after the furnace has stopped burning. allowing for a greater amount of heat dissipation time will circulate the heat normally left in the heat exchanger throughout your house, saving you money.
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Jagstyles wrote: We moved into this home back in May 2021. It’s been doing that since we moved in. Also hearing a ton of popping and banging throughout the ducts when the heat is on, didn’t notice these in the summer, could be heat expansion but the banging are these loud pops. If I didn’t know any better, it almost sounds like the duct work banging against the dry wall or the hard wood floor expanding as a result of the duct work being warmer??
This is completely normal. The ducting is metal, and it expands and contracts with heating and subsequent cooling. If the ducts happen to be in contact with the wood framing of your house (which is highly probable) it will rub against it and make noise. The ducting is a hollow tube, and as such makes it like a musical instrument. Learn to live with it, or buy a house with radiant in floor heating.
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Jagstyles wrote: We moved into this home back in May 2021. It’s been doing that since we moved in. Also hearing a ton of popping and banging throughout the ducts when the heat is on, didn’t notice these in the summer, could be heat expansion but the banging are these loud pops. If I didn’t know any better, it almost sounds like the duct work banging against the dry wall or the hard wood floor expanding as a result of the duct work being warmer??
Banging of the ductwork is due to expansion of the metal and is super common. I'm surprised you've never experienced this in your previous homes.
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ZxExN wrote: Banging of the ductwork is due to expansion of the metal and is super common. I'm surprised you've never experienced this in your previous homes.
Oh I have, just not to this extreme. These are loud pops/bangs, much worse than anything in my previous home.
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TrueToy wrote: you have a variable speed blower fan on your furnace. what is making it run faster at the end of the cycle is unknown. your furnace likely has two stages of heat. if you have a smart thermostat, you may have these two stages wired directly to the thermostat. if you have a traditional thermostat (or a smart thermostat and the installer was lazy and didn't run an extra wire) the HVAV logic board is likely making the decision on what speed to run the blower fan at. my fathers furnace does this based on a combination of previous runtime on heating calls.

also, setting heat dissipation time at zero is a terrible idea. heat dissipation is how long the HVAC blower motor will stay on after the furnace has stopped burning. allowing for a greater amount of heat dissipation time will circulate the heat normally left in the heat exchanger throughout your house, saving you money.
How do you know he has a variable speed furnace or two stage furnace? I didn't see him post the model anywhere in the thread. Also The furnace logic board will run the blower for x amount of seconds after a call for heat, it's better for the furnace logic board to control the blower than the thermostat. Lennox installer manual allows you to choose how long to run the blower after a call for heat, if you run the fan too long it will cause a cold draft.

Page 44
https://www.questargas.com/ForEmployees ... 6E_IOM.pdf
The blower-on delay of 30 seconds is not adjustable. The blower-off delay (time that the blower operates after the heating demand has been satisfied) can be adjusted by moving switches 3 and 4 on the integrated control. The unit is shipped from the factory with a blower-off delay of 90 seconds. The blower off delay affects comfort and is adjustable to satisfy individual applications. Adjust the blower off delay to achieve a supply air temperature between 90° and 110°F at the exact moment that the blower is de-energized.
Longer off delay settings provide lower supply air temperatures; shorter settings provide higher supply air temperatures
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i know he has a variable speed blower fan because OP said in his post that the fan blows harder. QED, he has a variable speed blower.

how do you know he has a lennox?

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