Advisability (or not) of slowing down DC motor on Heil for A/C call?

[hoos1erbud]

Hey guys,

We recently installed one of those Heil 100,000BTU furnaces with the DC fan motor. It's got 3 dip switches to set fan speed during A/C calls. It's set to the recommended speed (1 less than max) for our 3-ton compressor.

Unfortunately, the furnace is in the basement right under the family room and the fan noise is very loud when it kicks on. I'm wondering if it would be a bad idea to try cutting the fan speed back one notch? An alternative idea would be to hook up a humidistat to the Y2 terminal, which would automatically cut the fan speed back 20% if the humidity is high (and believe me IT SURE IS!).

OR...as another possiblity...are there ways to make the ducts quieter?

After scratching my head on this for a couple of weeks, I've decided that it's probably a bad idea to have the furnace where it is....but moving it seems completely out of the question.

BTW...this new furnace cools like a dream. I put in a new Honeywell electronic thermostat and it holds 76 degrees 100% of the time. In 90 degree weather, the A/C calls are about 1 every 25 minutes and they last about 5 minutes. I guess I'd like to keep the good performance and lose some of the noise. Probably impossible, but you guys would know better than me.

Thanks for any advice!

Phil

[ruud-man]

Can you describe the noise? Is it a whistle, thump (oil can), or a rumble?

I have experienced noise during start up with some 1hp ECM motors.

Thanks..

[hoos1erbud]

It's just the noise of the air moving in the plenum. The ducts in the family room are only a few feet long...so that's where it is kind of annoying.

I guess I should say that I've had the fan turned to "on" for the last few days...and that fan noise is fine. The fan speeds up during a/c calls and the air noise gets pretty loud.

Phil

[Black Adder]

Have heard that on some Heil furnaces you will get a harmonic noise from the blower wheel. Solution is the pull the blower motor and reposition it slightly on the blower wheel. (1/2" or so) changes the position of the blower in the air flow and seems to quiet them down. As far as having short runs with a lot of air moving through them, is it possible to increase the size of the duct or the registers? If they are really close to the furnace then they should be accousitaly insulated, will cut the noise down dramatically. Also sounds like the rest of the ductwork is too small to handle the increased air volume. What size is your AC unit? 5 minute run time seems a bit short at 90 degrees outdoor temp.

[hoos1erbud]

It's 3-tons. I actually downsized the furnace from the old 80+ Heil 125,000 BTU unit it replaced.

Where can I learn about accoustically insulating the ducts? That sounds like what I need to do.

Phil

[jonm]

Not sure where you're located and at what your design high temps are but it sounds like you might have an oversized system. short cycles and high indoor humidity are classic symptoms of an oversized ac system.