Should I reduce the furnace fan speed?

Posted by spamme0 on October 13, 2009, 10:26 pm

Got a new Coleman 95.5 gas furnace.
The fan is quite a bit noisier than my old unit.
Measures 73 dB in the hallway under the return vent.
That's "C" weighted.

I pulled off the vent covers and measured the airflow
coming out of the rectangular holes in the floor
with a thermocouple anemometer.

I've got 800 linear feet/minute from the 4x10" vents
and 1200 from the 2x10" vents.

With 67F indoor air temp, the air coming out of the
vents is 115F. (my 37 year-old furnace was 150F.)

I added an air conditioner evaporator that provides
additional turbulence to the air flow, but the objectional
noise sounds like blower noise coming out the return vent.

So, The objective is to lower the fan noise.

Looks like I've got plenty of room for temperature
rise due to lower air flow.
I expect I can get the installer to come change the fan motor tap.

What are the pros and cons of turning the fan speed
down?

thanks, mike

Posted by John Gilmer on October 13, 2009, 11:03 pm

> Got a new Coleman 95.5 gas furnace.
> The fan is quite a bit noisier than my old unit.
> Measures 73 dB in the hallway under the return vent.
> That's "C" weighted.
> I pulled off the vent covers and measured the airflow
> coming out of the rectangular holes in the floor
> with a thermocouple anemometer.
> I've got 800 linear feet/minute from the 4x10" vents
> and 1200 from the 2x10" vents.
> With 67F indoor air temp, the air coming out of the
> vents is 115F. (my 37 year-old furnace was 150F.)
> I added an air conditioner evaporator that provides
> additional turbulence to the air flow, but the objectional
> noise sounds like blower noise coming out the return vent.
> So, The objective is to lower the fan noise.
> Looks like I've got plenty of room for temperature
> rise due to lower air flow.
> I expect I can get the installer to come change the fan motor tap.

You can do that yourself. The A/C "tap" and the heating "tap" can be
separately adjusted. The heating "tap" is connected to the unit that
measures the heat exchanger temperature. Just find that unit and work your
way back to the fan motor.

> What are the pros and cons of turning the fan speed
> down?

The "con" is that you will cause the heat exchanger to run a little bit
hotter.

The "pro" is that is will make a LOT less noise.

You might want to look into reducing the heat input into your furnace when
you reduce your fan speed. That will keep the temperature down. 150F at
the register is on the high side; it's even on the high side right at the
heat exchanger. Frankly, it looks like the furnace is too big for the
application. If you can back the fan speed you may find that the
fan/overheat/whatever controller is turning off the gas burner during longer
heating cycles. That's not necessarily a problem. In fact you might want
to lower the temperature where it cuts off the gas.

Ideally, on a VERY cold day your furnace should be running continuously.

> thanks, mike

Posted by hr(bob) hofmann@att.net on October 13, 2009, 11:04 pm

Is there a non-metallic boot/sleeve between the input to the furnace
and the cold air register, also a boot/sleeve on the output side to
isolate any actual furnace vibrations from the plenums? You want to
make sure there is that isolation before reducing the fan speed. Do
you know for sure there is a fan speed tap or do you change the size
of the pulley? The temperature out of the registers is not only
dependent on the air speed, but also on the maximum temperature cutoff
setting.

Posted by on October 14, 2009, 1:10 am

On Tue, 13 Oct 2009 20:04:25 -0700 (PDT), "hr(bob) hofmann@att.net"

>> Got a new Coleman 95.5 gas furnace.
>> The fan is quite a bit noisier than my old unit.
>> Measures 73 dB in the hallway under the return vent.
>> That's "C" weighted.
>> I pulled off the vent covers and measured the airflow
>> coming out of the rectangular holes in the floor
>> with a thermocouple anemometer.
>> I've got 800 linear feet/minute from the 4x10" vents
>> and 1200 from the 2x10" vents.
>> With 67F indoor air temp, the air coming out of the
>> vents is 115F. (my 37 year-old furnace was 150F.)
>> I added an air conditioner evaporator that provides
>> additional turbulence to the air flow, but the objectional
>> noise sounds like blower noise coming out the return vent.
>> So, The objective is to lower the fan noise.
>> Looks like I've got plenty of room for temperature
>> rise due to lower air flow.
>> I expect I can get the installer to come change the fan motor tap.
>> What are the pros and cons of turning the fan speed
>> down?
>> thanks, mike

>Is there a non-metallic boot/sleeve between the input to the furnace
>and the cold air register, also a boot/sleeve on the output side to
>isolate any actual furnace vibrations from the plenums? You want to
>make sure there is that isolation before reducing the fan speed. Do
>you know for sure there is a fan speed tap or do you change the size
>of the pulley? The temperature out of the registers is not only
>dependent on the air speed, but also on the maximum temperature cutoff
>setting.

Pulley??? Hasn't been a furnace made with a pulley in over 10 years
that I'm aware of. MOST blower fans are now direct drive, with the
motor shaft being the shaft of the squirrel cage.

A large number are also DC motors with active electronic speed
control.

Posted by Tony Hwang on October 14, 2009, 1:17 am

spamme0 wrote:

> Got a new Coleman 95.5 gas furnace.
> The fan is quite a bit noisier than my old unit.
> Measures 73 dB in the hallway under the return vent.
> That's "C" weighted.
>
> I pulled off the vent covers and measured the airflow
> coming out of the rectangular holes in the floor
> with a thermocouple anemometer.
>
> I've got 800 linear feet/minute from the 4x10" vents
> and 1200 from the 2x10" vents.
>
> With 67F indoor air temp, the air coming out of the
> vents is 115F. (my 37 year-old furnace was 150F.)
>
> I added an air conditioner evaporator that provides
> additional turbulence to the air flow, but the objectional
> noise sounds like blower noise coming out the return vent.
>
> So, The objective is to lower the fan noise.
>
> Looks like I've got plenty of room for temperature
> rise due to lower air flow.
> I expect I can get the installer to come change the fan motor tap.
>
> What are the pros and cons of turning the fan speed
> down?
>
> thanks, mike

Hi,
For sure, heating blower speed is slower than cooling in a system.
(cold air is heavier). I don't think it is good idea to slow the speed
arbitrarily. It may upset total efficiency of the furnace.

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