Until someone can determine the ESP (static pressure) in the ductwork between the supply and returns with the zone dampers in various positions, any solutions tried would be purely a guess.

Without knowing all your variables, number of zones, size of unit, what accessories, size of ductwork, no of outlets, size of outlets, length of runs, outlets per zone, return air inlets/no/size, any guidance is difficult to even attempt to guess on any advise

Something to consider, some zone damper systems have setting that require 2 or more zones calling to activate system. If the noise is only on occasion, with smaller rooms/zones, you could consider this option.

However, 1100-1400 cfm on your unit, I doubt 2" in bypass will make that much difference if this is happening when small zones are calling.

Sorry

Thanks for your detailed response. I can tell you this. First, I only have two zones and the whining noise is always there when one of the zones calls for maximum heat and the other zone is closed. The HVAC person manually opened my 10" bypass all the way and the noise disappeared. So, I think he probably considers a 12 inch damper to be a possible solution.

From your experience, if both options would work, which one would you choose?

Jerry

Knowing now that you only have two zones helps. The bypass size must be selected to accommodate bypassing the excess air when the smallest zone is calling for cooling. You never mentioned whether your bypass was motorized or barometric. The barometric tend to work better on modulating/variable speed furnaces in most cases since, the motorized/pressure sensing often continues searching for proper position as the variable speed blower constantly changes slightly.

The 2" upsize may be all you will need and perhaps not, not knowing your zone and duct sizes.

Does your zone system panel have a plenum temp sensor for heat & cool? If not, it should to keep the heat exchanger from getting to hot, limit cycling or temp across a-coil during AC from getting too cold while the excess air is bypassing.

HereÂs a thought; many people have solved their airflow/insufficient duct sizing by simply installing a bypass humidifier. Not that you need to install one but, you can appreciate the bypass benefits by simply adding an additional 8" or so pipe between the supply and return ducting. With a manual damper on it you can adjust it as a primary or secondary bypass in conjunction with your zone bypass damper.

You never mentioned whether your bypass was motorized or barometric.
* It is motorized *

Does your zone system panel have a plenum temp sensor for heat & cool?
* Yes *

HereÂs a thought; many people have solved their airflow/insufficient duct sizing by simply installing a bypass humidifier.
*I do have a bypass humidifier and it does make a difference in noise when the damper is open. However, in the summer when it is closed, the noise would be louder. And, I would probably keep the damper closed in the summer so that too much bypass air does not cause a problem with my AC.*

I should also mention that the contractor is on the hook to fix this problem with no charge to me. So, my options are opened to his recommended two options. Do you have a thought on which option I should choose?

Jerry

I would first go for the correct size damper.

As far as changing blower speed, the unit must stay in the perameters set by the manufacturer with both zones open.

The correct size motorized damper must be selected so, just upsizing without calculations is again just guessing.

If your motorized damper is pressure sensing/automatic, you could also request that a second one, perhaps 10" be added and they can function together. Adding a 10" versus upsizing will be the lesser cost option for contractor assuming, he has no home for the used damper. This option has worked well for me when I wished to bypass main trunks at opposite ends.

Thanks again for your thoughts. I know you have been focusing on damper issues. I was too until the HVAC looked at the problem. In fact, before the HVAC person came to my house, I played around with my damper adjustments. That is, I adjusted the damper opening at different positions to bleed off some of the air. And when the HVAC person looked at my damper, they looked okay to him.

Anyway, he said my noise was not coming from my dampers. It was a whining sound he has heard before, due to the motor working too hard. And, although my zoning caused the problem, he felt the two options I stated would solve the motor noise.

Jerry

If this individual was able to discern that the blower motor was overworking they should have also been able to figure out why.
To help you we need to know the ductwork configuration(size of main trunk line before second zone takes off size of two trunk lines after take off we know the size of the bypass and that a 3.5 ton system should generate 1400 cfm air volume so if you tell us what size ductwork that volume is going into we should be able to help

Here is a link that might be useful: How To Repair Air Conditioners

I knew it was more than likely not the dampers, but undersized ductwork, takeoffs, and/or bypass causing the blower strain.

Again as my first post stated, until the static pressure of the furnace is tested, and calculations verified, youÂre shooting for fish in the barrel to fix your problem.

Ask your tech if he knows what an incline manometer is, IÂm sure you will get that deer in a headlight stare. This is a tool for measuring static pressure across HVAC equipment. In your case, it would be the furnace/blower. .05" or more is considered high, resulting in noise. I always liked to see .025" or less, resulting in a quieter system.

Hello all helpful posters ---

You all have given me items to think about. I think at this point I will review with the tech your inputs and determine if his conclusions seem reasonable.

Thanks,
Jerry