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]

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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.

 

[Sigarms]

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,,,

Seems like you should of been in charge of the job...

That block off plate is exactly for that air turbulence you speak of.

Every manufacturer does written instructions differently. I always remembered these.

Generally if you don't want to rip the duct work apart to get the exact size, look it up, and then pre fab a custom transition from the supply opening of the unit to a transition to the existing ductwork.

 

[ljjhouser]

I have not read all the posts, so please excuse me if someone has mentioned this.
Before making changes, I recommend using a dual port manometer to find the ESP (External Static Pressure) on you system. This tells you the actual pressure (drag) placed on the blower motor of the air handler. And if using a furnace, most residential gas furnaces are rated for only 0.5" esp when heating. This tells you the effect of the ductwork, registers and ac evaporator. If you find areas where there are restrictions, it may help reduce noise.

 

[jaxs]

Other than rushing air noise from supply vents,noise most often originates at blower assembly and the rumble you describe is a common complaint. I know the blower was tested at installation but it's easy so I'd pull the blower again before altering anything. Squirrel cage fans are factory balanced and the clips occasionally get knocked off but more often a piece of trash lodges in a fin throwing fan out of ballance so inspect closely. Clamshell sitting on the floor at high speed shouldn't vibrate, make any kind of rhythmic or pulsing sound. If fan checks out and you feel dead space is the source,a wedge shape chunk of stryafoam held in place with Great Stuff spray foam is a simple solution.

 

[2manyrocks]

You could mark the location of the air handler outlet on the existing duct transition, take it to a sheet metal shop, and get them to fab a new transition without the dead space.

If you are comfortable bending metal, you could also insert a diverter as shown. I'd bend flanges on the sides so you have something to screw or rivet to the existing transition. I'd also bend a lip on the ends for strength and to try to keep air from getting behind the diverter. The crimps in the existing duct transition will make it practically impossible to bend the metal exactly to size. You'd have to close the gap with HVAC foil tape.

You could pick up a sheet of galvanized from Lowes. https://www.lowes.com/pd/IMPERIAL-24-in-x-3-ft-Galvanized-Steel-Sheet-Metal/3234805

The other thing that may be contributing to this situation is you essentially have a "T" in the duct at the ceiling. The air flow is hitting that T and this also contributes to resistance in how the air is flowing. Also, note that the air is leaving the air handler at a angle relative to the other ductwork.

 

[CADplans]

The other thing that may be contributing to this situation is you essentially have a "T" in the duct at the ceiling. The air flow is hitting that T and this also contributes to resistance in how the air is flowing.

Yea, I think I am going to try adding that baffle similar to what you have drawn.
But, I think I am going to bend the bottom 1/3 so that it ends up a smooth radius from vertical to the angle,, with about a 4" radius.

As far as the "T" at the top,, that was part of the original install, and the original was very quiet, no rumble, at all.

The installer tried to blame everything, other than his work,
he even said that when I switched from a 20X20X1 filter to a 20X25X4 filter, I increased the air restriction.
(The new filter has about 5 times the surface area of filter media,, WAY less restriction)
So, I removed the filter, and let the unit run with NO filter,, still noisy,,

Even after removing the filter, he said the return was TOO restrictive,,
His solution was going to be to install a 20X20 unfiltered register in return duct, in the un-conditioned basement.

I stopped him,, he did not understand that the new return would suck dirt right into the air handler,,
and, sucking air from the un-conditioned basement would GREATLY increase energy usage,,

Maybe he did know,, and I would be paying for service calls, every two years to clean my evaporator coil,,,

 

[2manyrocks]

A radius might very well improve air flow and wouldn't be that much additional work.

It is shocking what HVAC installers will foul up.

 

[jaxs]

I'm looking foward to hearing what additional tools you needed besides your sheet metal break to fashion the baffle lips,flanges,curves and radius.

 

[2manyrocks]

Good tin snips, mallet + welding table works for me. For a job this small, good snips and a small seam tool will work. https://www.harborfreight.com/6-inch-jaw-straight-sheet-metal-seamer-98728.html

If this is the only air handler in the OP's house, all the air in his house is running through this duct. It's very important that the air flow at the air handler be as smooth as one can reasonably get it. I wouldn't hesitate to buy the sheet metal, HVAC foil tape, seam tool, and snips to fix this. The snips and the seamer are also very useful for metal roofing and flashing work.

 

[3Ts]

This is an interesting post for me because I have a very noisy system as well and it has been from day one. So noisy that people can't hear each other when it's running. (I'm not an HVAC person, but I do read up on them and know what a manual J and manual D are. The HVAC installer did not.) The amount of noise varies depending on the fan speed. You have a flexible black band between the transition and the T piece, mine does not and probably should. It also sits directly on a wood platform that seems to act as a sounding board.

What I have done is to line the return air chamber with fiberglass insulation as a sound absorber. It helps but could be better. I'm thinking about trying the anechoic foam to see if that works better.

What I would like to do is to lift the unit and put vibration isolators under it. That will be a fair amount of work since gas and drain lines are involved in addition to the duct work having to be shortened. Anyone think that would help?

 

[kenmac]

You saying the fan speed is higher in heat mode than cooling mode ?
I always wired heat speed lower than cool speed. low to med. speed for heat and high speed for cooling