Should I reduce the furnace fan speed?


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
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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.

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.

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

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.
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spamme0 wrote:

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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If its new you shouldnt mess with it, let the installer do it, at least until you learn more. Im no pro but ive done what you describe with mine. First you cant just lower blower speed without knowing the temp the heat exchanger is at. You need a probe thermometer in the exchanger to measure it, your manual gives the exact range that is safe to operate it at, go higher than the range by lowering blower speed and you can kill the exchanger in a few years and void the warranty. On mine a lower speed made no difference in sound. Get a taylor digital probe, its used for cooking and often for cooking meat. I drilled a hole just above the heat exchanger to measure my temp. But odds are its incorrect now as your installer never tested and set everything up right, so you could get it set for free after you verify what the temp is.
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ransley wrote:

Now you can have a DVM with temp. probe. With my meter I can read temp. any where within a minute.
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Pro: Less noise
Con: Reduced air filtration Heat exchanger gets hotter, more thermal stress
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spamme0 wrote:

Read the installation manual for the specs for your unit. You want to keep the temp rise within specs. Get the manual from the manufacturer if you don't have it. You may be able to download it from their website, or call and ask them for it. The manual will detail how to change the speed.
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responding to http://www.homeownershub.com/maintenance/Should-I-reduce-the-furnace-fan-speed-400119-.htm gkgtchey wrote: If the fan noise you are hearing is from a new Lennox furnace, you may want to check a couple things. Over the past few years I have had several blowers that have been noisier than most. It is in all Lennox units with a 5 ton drive, look for a 60 followed by a C or D in the model number. This furnace uses a full 1 hp motor, and while Lennox wont admit there is a problem, they will provide free of charge a new motor (that uses a 1/2 inch shaft rather than the 5/8) and a new blower wheel to accommodate the new shaft. I have changed alot of them and it always solves the problem. This doesn't apply to variable motors. spamme0 wrote:

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