Why my heating system makes noise after it is shut off?

After my heating system is shut off, (let's say after 10 PM), my fan is still running for a while (about 15 - 20 minutes). Meanwhile, it makes some noise. 1. Why is my fan is still running for quite a while after the heating system is shut off? 2. Why it makes noise? 3. How to fix it?

Update:

Thank you all, for your answers.

The noise is definitely NOT from the fan. I went up to the attic when it made noise.

These two points sound more like the reason:

Confiort Climate Service - Three: Duct expansion/heat exchanger expansion and contraction due to rapid temperature change, ( install thicker gauge duct)

emgee - The blower operates until the reverse thermostat in the plenum is cool. The noise is the ductwork expanding and contracting of the metal.

Can you provide more information about the solution?

Additionally, my "fan/limit switch" has only one dial, I set it to 115. It was 110 and it made noise more often. I set it to 115 and it gets better. It still makes noise, but, it less often. Any advise?

5 Answers

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  • Favourite answer

    1 The fan shouldn't be on more the 5 minutes at the most after the call for heat has ended.

    This would indicate an adjustment is needed at your fan/limit switch usually located in the center of the heating furnace, ( usually having two wires in the top, two wires in the bottom and a dial in the center).

    This dial has three adjustment pins ( these are movable pins that are located inside the dial ). One pin is for fan turn off temperature. The second is for fan turn on temperature and the third pin is for high temperature limit.

    The first pin is usually set at 100 degrees. The second pin is set at about 150. The third limit pin is set at 180.

    It sounds like you only need to move the first temperature off pin to 100 degrees.It is probably set at too low of a temp causing it to run too long.

    #2 This noise after the heat has shut off could be several issues:

    One: Fan motor internal problems, (solution replace fan)

    Two: Fan blade dirt accumulation causing excessive vibration, ( solution clean blades)

    Three: Duct expansion/heat exchanger expansion and contraction due to rapid temperature change, ( install thicker gauge duct)

    Four: If the unit has a forced draft motor, it could have internal issues ( solution replace motor)

    Source(s): 12+ year HVAC Tech
  • 8 years ago

    If its gas heat the blower should only run about 120 seconds after the burners shut off. Could be the fan control contacts sticking relay and fan/limit bimetal. If the furnace has a circuit board it could be faulting out on limit and is in a lockout and you just notice it at night when u turn it down. Is the fan making noise? What noises u talking about? WhAt type of noise. If its a popping metal noise it's probably your duct work contracting as it cools down or heat exchanger cooling down. Worst case your heat exchanger in split and its popping back together as it cools down.

    Dude that answered after me the outdoor fan shuts off on a heat pump in defrost it doesn't stay on. The reversing valve energizes to switch refrigant flow into cooling mode so I'd coil will warm up to melt the ice if the fan stayed on the coil would never get warm enough to melt the ice on the coil

  • 8 years ago

    What fan? The drive fan? The induction fan? The outdoor unit fan (assuming you have a heat pump)? What fan?

    I'll assume that you have a heat pump and that the outdoor unit fan is still running after shut down. This isn't terribly unusual, it's in defrost mode. 15-20 minutes does seem long to me though. If it's the drive fan on a fossil fuel furnace, your anticipator or heat exchanger temp sensor is improperly set or broken.

    Dude above me, I've seen heat pumps that energize the fan during defrost. It's poor design and isn't normal, but some do this. I'm trying to remember one, but I can't right now. They are very cheap models. Old Haier models used to do this, but I can't remember a specific model. It's like the design team went to lunch on the defrost control board. I still don't know what fan is running on this guy's furnace. If he'd tell us, we might know the answer.

  • Anonymous
    8 years ago

    The blower operates until the reverse thermostat in the plenum is cool. The noise is the ductwork expanding and contracting of the metal.

    Want to fix it? Rip it out and instal a boiler with forced hydronic baseboard radiators.

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  • mahlon
    Lv 6
    8 years ago

    This is a poke. Gas furnaces have fan "limiters" which determine how long fans run during the heat cycle. They're adjustable I know, but I can't help you much in telling you where the limiter is.

    Why is the fan making noise? It might just need some ordinary cleaning and light oiling.

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