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

[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