High pitched whine from Furnace - HVAC-Talk: Heating, Air & Refrigeration Discussion

pcestrada · #1 03-04-2010, 11:40 AM

We recently moved into our new house and I've started to notice a high pitched whine coming from the furnace. The house has 2 zones, and I only notice the whine when zone 2 (the 2nd floor) of the house is heated. There is no whine when zone 1 is heated. The furnace is a Lennox G61MPV gas furnace with a variable speed blower.

I had the HVAC installer come out this morning to take a look. He verified that the whine only happens when heating zone 2 and when the blower kicks up to its highest speed. He said that the reason I don't hear it for zone 1 was that the blower doesn't have to run as fast since the air is only going to the first floor. He examined the blower, tested the amps output, and observed the fan wheel and everything seems to be operating correctly so he can't replace the blower since there is nothing technically wrong with it. I'm a bit frustrated since I believe it shouldn't make that noise at high speed. They are going to follow up but he said that it could be an airflow issue for zone 2 and that they may need to replace some vents with grills to increase air flow in zone 2.

Any thoughts or advice would be great! Thanks.

usskaufman3 · #2 03-04-2010, 11:43 AM

That's what it sounds like to me. You getting a whine from the velocity of air going through a vent. You could take some vents off when it's on high, I would do this one at a time to see if it's just one vent.

surenuff · #3 03-04-2010, 11:48 AM

If taking vents and registers don't help, try to speed, slow motor and see if that does. Some of these newer units are developing a frequency harmonic at certain speeds. It is different for each unit, and it will sound kind of like a tuning fork.

tocss · #4 03-04-2010, 11:56 AM

i found a pice of duct that was not connected correctly. that would make a loud vibrating/humming sound.

pcestrada · #5 03-04-2010, 11:58 AM

Quote:

Originally Posted by surenuff

If taking vents and registers don't help, try to speed, slow motor and see if that does. Some of these newer units are developing a frequency harmonic at certain speeds. It is different for each unit, and it will sound kind of like a tuning fork.

That's exactly what it sounds like, a tuning fork. What can be done about it? Replace the blower?

btuhack · #6 03-04-2010, 12:06 PM

I agree with the posts that direct you to the supply and return grills. High face velocity can easily be the cause of your noise. You may need to have the grills/ cans increased in size or perhaps just a better quality grill may do the trick.Not all grills are created equal and are rated for different volumes. Have your preferred contractor help you pick the right grill that matches your volume/velocity.

big sky hvac · #7 03-04-2010, 12:06 PM

Is this noise present when both zones are calling for heat/cool? Is there a bypass damper installed? It sounds like the furnace is oversized for the zone 2 ductwork(too small) which is causing the high velocity noise.

surenuff · #8 03-04-2010, 12:23 PM

These guys are steering you right bud. Most likely it is going to be a velocity issue in a duct. But I have run into this twice now on my installs, and helped a couple of other friends who were about to kill themselves or the customer over this issue. The best way I can describe it is that it sounds like when you run your finger around the top of a whine glass, or when you strike and hold a tuning fork. Yes, the way I fixed one is take out blower and use a tape to keep the metal housing of the blower from actually coming in contact with its track on the air handler. But that was just because I could not fix duct issue that was causing the ecm to run at that high rpm. The others have been fixed by fixing static pressure problems that allowed the motor to ramp down and run at different rpm. P>S> most times this ringing sound or noise will be just whistling thru ducts. But the sound I am talking about is generated from the unit itself.

Todd S 2 · #9 03-04-2010, 01:01 PM

Quote:

Originally Posted by pcestrada

We recently moved into our new house and I've started to notice a high pitched whine coming from the furnace. The house has 2 zones, and I only notice the whine when zone 2 (the 2nd floor) of the house is heated. There is no whine when zone 1 is heated. The furnace is a Lennox G61MPV gas furnace with a variable speed blower.

I had the HVAC installer come out this morning to take a look. He verified that the whine only happens when heating zone 2 and when the blower kicks up to its highest speed. He said that the reason I don't hear it for zone 1 was that the blower doesn't have to run as fast since the air is only going to the first floor. He examined the blower, tested the amps output, and observed the fan wheel and everything seems to be operating correctly so he can't replace the blower since there is nothing technically wrong with it. I'm a bit frustrated since I believe it shouldn't make that noise at high speed. They are going to follow up but he said that it could be an airflow issue for zone 2 and that they may need to replace some vents with grills to increase air flow in zone 2.

Any thoughts or advice would be great! Thanks.

You need to add more ductwork to the second floor or add a by-pass damper. Or you possibly could "dump" some air somewhere. I put in a "wild" zone when I did mine. The ECM motor will ramp up (more airflow) if there is restriction to the airflow. This, of course, results in air noise and/or vibrations somewhere.

usskaufman3 · #10 03-04-2010, 01:08 PM

Their's nothing wrong with your furnace or blower moter.

btuhack · #11 03-04-2010, 05:00 PM

So if you have 1 return, the volume under full volume conditions appears to overwelm the return grill velocity. Have your guy measure the velocity if he hasn't already.

bribone · #12 03-04-2010, 10:53 PM

I have had a high pitch sound call before, found out the vains on the blower wheel were not tight. Hard to tell with your fingers, but any movement of the vains will cause that sound also. Put new blower wheel on, sound all gone. Only happened when blower was in high speed.

pcestrada · #13 03-05-2010, 09:18 AM

Removed all vents from the second floor and cranked up the heat on the second zone. The whine started again. Observing the furnace closely it is definitely coming from somewhere inside the furnace. So I don't think it's lack of airflow, it seems mechanical.

The really weird part is that there is no whine at all when the furnace comes on to heat the first zone, only the second. And we only started to notice it a week ago.