Home Furnace Air Handler Plenum Design help,, Anyone A GURU??

[CADplans]

I had a new air handler and outdoor unit installed, basically replacing my 35 year old heat pump.

The new unit is a 2.5 ton TRANE heat pump,, the original was a 2.5 ton CARRIER.

Since the day it was installed, the air handler has been much "noisier" than the old unit.
The noise is most noticeable when the unit is in "heat" mode, as the fan speed is higher.
The unit makes way less rumbling when in "cool" mode.
The outlet of the old CARRIER air handler was 19" X 16",, so that was the size of the plenum that was built for the original install.

The new TRANE air handler has an outlet sized at 13" X 16",,, basically the plenum should be 6" narrower than the CARRIER air handler.

To simplify making the plenum, the installer built the new plenum 19" X 16",, the last 6" of the plenum simply sits on the flat top of the air handler.
The plenum was made this way to simplify connection to the existing ductwork, I would imagine. There is a 6" dead air space in the new plenum.

WAY back when,, I studied HVAC for a while,, we were taught that the plenum should be sized identical to the outlet of the air handler.
The identical sizing was to eliminate turbulent air flow, or to reduce rumbling noise as the air entered the dead space.
I have looked on line, I can not find any "rules" for plenum design,,,

My warranty period is up, I am considering remaking or modifying the plenum, to see if eliminating the dead space in the plenum will reduce the rumbling.

Am I wasting my time, or should the plenum match the air handler outlet?? Or,, are there other "rules" of plenum design??

HELP!!

Thanks,,

 

[ponytug]

That sounds annoying! I have seen lots of subpar HVAC installs over the years. Is the rumble a deep, low frequency rumble, or a higher frequency rumble?

While it may not solve the rumbling, I believe that the proper way to do the change in size is a gradually tapered truncated pyramid. You may not have the space (length). You can find louvers designed to help keep the flow laminar, but I have only seen them used in corners.

If it were me, I would find a good local HVAC installer to come fix it, as they may have even better ideas. I would also double check that the fan and motor bearings are ok, and the new fan is properly keyed / screwed onto the motor shaft as there is more than one way to have a rumble. It wouldn't hurt to double check the flow at your vents to make sure that your fan speed is correct, and not too much fan speed for your system. Your newer system hopefully has a variable speed motor and it, again hopefully, should be easily reprogrammed to different speeds as needed.

All the best,

Peter

 

[CADplans]

I have seen lots of subpar HVAC installs over the years. Is the rumble a deep, low frequency rumble, or a higher frequency rumble?

This is a low frequency rumble,, I think it is due to the dead air space, it sounds a lot like the wind noise you hear when riding in a 1957 Chevy with the windows down.

Actually during the install, the noise was SO bad that I had them pull the blower, and run it where we could see it.
The blower had a terrible wobble, they replaced the entire blower and that helped,, a little.

And, yes, I got a sub-par install, some day, I will tell the story.

 

[Midniteoyl]

Put a temp partition in there to reduce the space to 13" and see if it makes a difference. Can rivet/screw it in if it does.

 

[CADplans]

Put a temp partition in there to reduce the space to 13" and see if it makes a difference. Can rivet/screw it in if it does.

That is actually my plan.

I have seen some partitions that break up the plenum into halves, and direct the air,,
but, I never had any personal experience with that.

I decided to ask first, because gaining access to the inside of the plenum will probably require several hours work.

In trying to eliminate the noise, they really sealed up the plenum,,,,,,,,,,,,,,,

 

[2manyrocks]

Pictures would help.

Two problems: 1) the blower speed on the new air handler is faster which makes it louder. Some fan motors have jumper cables that can be moved to lower the fan speed. Check your owner's manual that came with your unit to see if you can reduce the fan speed. 2) the outlet on the new air handler is smaller than the older unit which means the air flow is constricted, also making it louder. A third possible problem is that the transition from the air handler to the ductwork has that dead space instead of a smooth transition. So the air is fighting itself to blow through making it louder. I'd say you want to clean up that transition to avoid creating air turbulence. It could be as simple as adding a piece of sheet metal to direct the air flow so it isn't fighting itself.

The other thing is the location of the air handler itself could be contributing to the noise you hear. Putting the air handler a couple of feet from the wall intake can do a lot to lower noise levels, but you may not want to incur the cost of relocating it or may not have the space.

 

[Doughknob]

Number 1 Name in Noise, Vibration and Heat Control | Dynamat

For over 30 years, Dynamat has led the noise and vibration control industry with our innovative products for automotive and home.

www.dynamat.com

put some of this on your plenum. Doesn’t have to be “complete” coverage. The idea is that it covers the majority of large flat areas of thin metal that vibrates. It is made for and used in automobiles, but will help anywhere large flat sheet metal vibrates.

 

[CADplans]

Here are some pics of my plenum.
The shiny part is the newly added TRANE adapter.

This image shows that the plenum covers the air handler opening, as well as a horizontal 6" wide steel panel.

The blue arrow points to the horizontal panel.

From what I have read, that panel is in place to offer a place to bring in electrical power and thermostat wires, etc,,

Instead of top wiring, my installer ran the wires in the side of the unit.

And,, an interesting note.
When I went to take this pic, I turned the heat on, so that the fan was running.
You can feel the shiny adapter vibrating, but, no felt vibration in the "T" above the shiny adapter.

The air handler has much less felt vibration,, so that sort of indicates the shiny adapter is the source of the vibration,,,

 

[ponytug]

I think that you are right, the plenum should have been made to fit the far side of the plate and then tapered out to meet the T of the original plenum.

If you hold your hand on the center of the X, does the rumble get reduced? If it does, a small sheet metal "L" or thin C channel screwed in around the belly at the level of the Xs might calm the vibration down to the point that you are ok. Dynamat would work, too, but I think that these are bandaids to the original issue of the plenum not being made properly.

Having futzed with sheet metal in the past, I am more inclined to get a professional in who has the tools and knowledge, but that's me.

I would talk to a local HVAC duct shop, explain what you want, then have them make the right part. I would ask about louvers to spread the airflow while keeping it laminar.

All the best,

Peter

 

[CADplans]

put some of this on your plenum. Doesn’t have to be “complete” coverage. The idea is that it covers the majority of large flat areas of thin metal that vibrates.

We have a MACK tractor factory nearby. A local fabric shop sells extras and drops from making insulation for the cabs of the trucks,, I have some. It is VERY dense, and heavy.

It is 1" thick, and sort of like memory foam,, push your hand into it, it takes 45 seconds to fully relax.
When I get the plenum rebuilt, whatever that is,, I may add that foam insulation to the plenum.

I do not think exterior insulation alone will fix the issue. (That new plenum is lined with insulation)
I feel like the air is in turbulence in the plenum, creating the noise independent of the sheetmetal.

The noise of the turbulence is carried by the ductwork to every room in the house.
It works kinda like the communication tubes on a ship, speak into one end of a pipe, the guy 150 feet away can clearly hear what was said.

I have a mechanics stethoscope,, I may have to dig it up, and try to determine where the noise is exactly coming from.

That stethoscope works so well, I could determine that a noise in my mower was the upper bearing, not the lower one, in one of the spindles.