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

[2manyrocks]

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?

What kind of noise do you think you have? Air movement, fan noise, vibration, .... Fan noise by itself won't be helped by iso mounts.

You might post a pic. Possibly the wood platform is acting like a drum or sound board to amplify the noise.

There is fiberglas duct board which is intended for use in HVAC duct, but you may not want ordinary fiberglass particles blowing through your ducts. There are those who even question the use of fiberglas duct board, FWIW.

 

[CADplans]

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

That is correct, normally,,
but the fan speeds of the air handler are dictated by the air handler instructions.
Heat mode needs a minimum CFM to move the heat from the heat pump coil, as well as TWO resistance heating units.
Cooling is only dealing with the temperature difference caused by the heat pump alone.

IIRC, you want higher CFM on cooling, so that the body senses cooling from air movement.
Air movement results in comfort, with less energy used.

 

[Doughknob]

……Air movement results in comfort, with less energy used….

Interesting you say that. I have said the same thing. I am surprised someone hasn’t come up with a smart home HVAC setup that is “always on” but blowing low and slow.

 

[CADplans]

Interesting you say that. I have said the same thing. I am surprised someone hasn’t come up with a smart home HVAC setup that is “always on” but blowing low and slow.

I didn't come up with that,,
that is what they taught us in HVAC school back in 1969,,
so, it is an old concept.

The blower on my heat pump has been "ON" since November 2020. (when it was installed)

When the fan setting of the thermostat is set to "ON", the blower will run at a very low speed when the heat pump is not operating.
This fan speed is probably 1/3rd (maybe only 1/6th?) the CFM's of the cooling fan speed.

The air coming out of each register is more like a slight draft from a leaky window,
rather than the air flow you would expect from a furnace.

I think the slight air movement makes the entire house more comfortable, and less "stuffy",,,

 

[mapper]

Interesting you say that. I have said the same thing. I am surprised someone hasn’t come up with a smart home HVAC setup that is “always on” but blowing low and slow.

We replaced our furnace and heat pump a couple of years ago, new system is Trane.
Had a few noise problems that the installer worked to fix, most of it was small air leaks around some of the seams in the air handler, amazing how much noise a minute leak can make! Another issue was condensate running out of the ductwork under the air handler, tuns out the factory default setting for the fan speed was too high for the size of the heat pump and was blowing the condensate out of the coil rather letting it drain down into the collection area to drain properly.
We use a Honeywell smart thermostat that does have a circulate setting that runs the fan on low to keep the air in the house circulating all the time.

 

[CADplans]

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.

I am just thinking out loud, nothing firm, yet,,
I think I have come up with a novel way to install the block off plate.

I went to Lowes, and bought a 24"x36" piece of galvanized sheet.
It is so thin, (WAY thinner than 30 gauge) I am sure I could cut it with a pair of scissors. (I have WISS snips if necessary.)

After getting the sheet the correct shape, (maybe by cutting a cardboard pattern first)
I will cut the block off plate so that it is 1" too narrow.

I also purchased a 6 foot length of slip on black pipe insulation (it looks like a "pool noodle")
I will put a piece of the slip on pipe insulation on each side of the block off plate.
The "springiness" of the foam pipe insulation will hold the block off plate in place.
The inside of the plenum is insulated, so the foam pipe insulation will not be pushing against the steel plenum.

If I can not easily screw the block off plate in place, I plan on bonding the block off plate in place at the bottom with a can of Great Stuff spray foam.
That foam sticks to anything.

I will bond the bottom first, only using a small amount, so that it does not expand into the air handler.
Then I will bond the top in place using the Great Stuff, and maybe a few sheetmetal screws,,

That galvanized is so thin, I am sure I could easily bend the sheet into a 4" diameter tube, effortlessly.
The problem is that the metal is so thin, without backup, the metal might flap around when the fan revs up,,

If necessary, I can easily drill the plenum, and fill the entire void behind the block off plate with Great Stuff foam.

The black foam pipe insulation will also help smooth the transition from the side to the block off plate.
Everything needs to be smooth,,,,,,,,,,,,,,,,,,

I have to find the temp limit of Great Stuff foam.
I have to figure out how hot the plenum gets,,
What temp is the pipe foam "pool noodle" good for?

I could just cut the block off plate, screw it to the air handler,
then, stuff the void with rockwool, and then screw the block off plate to the plenum transition at the top.
Other than rockwool, I have fiberglass,,

Thinking about options is as much fun as doing the project!!

 

[Doughknob]

Cool. So my thoughts are spot on with others. I did not know heat pumps did this (always on). And while the “circulate” function is good, i envisioned a little more “smartness.”

A furnace with variable flame so it will self-adjust to real low once at temperature, then just burn low while the fan blows slow all while house sensors give feedback to the burners and fan so as to keep a steady temp.

i purposefully undersized my woodburner so that it can run constantly yet still feel very comfortable because the ”warm” (not hot) air is constantly coming out of the registers.

 

[CADplans]

OOPS, Great Stuff is NOT great above 240 degrees F,, so that foam is out,,

maybe I will look for some sort of silicone gasket to use,,

 

[ponytug]

With low frequency rumbles, I think that thin is not your friend. Stiffness will limit the amount of travel from turbulence.

Personally, since ducts are next to impossible to actually clean, I try really hard not to put anything other than clean sheet metal into them. If it were me, I would do all of your shape management with tin snips (scissors) and screws or rivets.

YMMV...

All the best,

Peter

 

[CADplans]

I try really hard not to put anything other than clean sheet metal into them. If it were me, I would do all of your shape management with tin snips (scissors) and screws or rivets.

I do have some industrial gauge siding left from building my shed,, maybe 22 gauge, or even possibly thicker
I could cut a piece of that, and pop rivet the siding to the back of the shaped block off panel,

Smooth on the air side, ribbed in the void area,, Hmmm,,,
If I can not easily get in the plenum to install screws, maybe a strong magnet could hold the block off in place??

 

[2manyrocks]

You could possibly buy some thicker gal. sheet metal at a HVAC supplier or a cut off from a sheet metal shop. Some HVAC suppliers won't sell to anyone who doesn't have an HVAC license, though but some will.

I was thinking that you'd remove the transition, fab and screw in the new diverter, and then reinstall the transition using foil tape.

 

[2manyrocks]

Interesting you say that. I have said the same thing. I am surprised someone hasn’t come up with a smart home HVAC setup that is “always on” but blowing low and slow.

Inverter heat pump mini split systems are essentially designed to do this and vary the compressor output depending on the heat/cooling load on the system. The indoor air handler for many mini splits is quieter than a normal home refrigerator. They are very suitable for open living room spaces, but the blowers on them aren't generally powerful enough to blow air through a duct system that you'd normally use for heating and cooling bedrooms and bathrooms. Part of their energy efficiency is because they aren't having to push air through a duct system.

Which brings us back to the OP's air handler which is having to push air through a duct system. Air weighs 14.7 pounds per square inch at sea level. This means there is a respectable amount of weight to be moved in any HVAC duct system by the air handler. So it isn't surprising that there can be noise associated with air handlers because of the amount of air they are moving.

 

[Sigarms]

I went to Lowes, and bought a 24"x36" piece of galvanized sheet.
It is so thin, (WAY thinner than 30 gauge) I am sure I could cut it with a pair of scissors. (I have WISS snips if necessary.)

If you haven't cut the metal yet, I'd take it back and cruise a local HVAC shop and see if they have a scrap piece of 26 or 24 gauge laying around. Don't know if I'd want 30 gauge or thinner in an airflow. Most shops will have break and it will be nothing for them to put a crisp 90 degree edge on it.

That said, what most local guys do if they don't have a metal shop is use a local custom metal shop. If you can find one, generally the pricing is the same, and do it right the last time. Measure the duct opening to the plenum opening along with the distance, and it should fit like a glove. Heck, they should be able to either wrap or line it for you as well.

The biggest question because this could be a airflow issue is was a start up sheet ever completed on start up with the pertinent info of static pressure and CFM's recorded on what the system is doing vs what is should be doing?

Most times if a guy has a airflow issue and you ask him what the CFM's and static pressure are, the general response is "GOOD!" but dang, he'll have his gauge readings all ready.

 

[2manyrocks]

If you have a metal shop fab a new transition to the proper dimensions, it's going to have the best airflow because it won't have all the additional screws and edges inside the transition that would be involved with adding a diverter. You also avoid the risk that the diverter won't fit, rattle or have air leakage. If the shop fabs it up wrong, then they don't have the excuse that they were making something to salvage someone else's prior foul up.

 

[ponytug]

I do have some industrial gauge siding left from building my shed,, maybe 22 gauge, or even possibly thicker
I could cut a piece of that, and pop rivet the siding to the back of the shaped block off panel,

Smooth on the air side, ribbed in the void area,, Hmmm,,,
If I can not easily get in the plenum to install screws, maybe a strong magnet could hold the block off in place??

Just screw it in from the outside. It is quite common. To put it in, you will need a way to align and support the inside piece while fastening it, which will mean disassembling the existing plenum to get access, either from on top or below. It is not going to be quick either way.

However, I do go back to my original advice and suggest that you just get a more talented HVAC person in to fix it.

All the best,

Peter

 

[ultrarunner]

Local HVAC with sheet metal fab was dirt cheap making my plenums and heavier metal.

Last one was just before Thanksgiving… I went in and he said it would be ready Monday afternoon…

I said I will let everyone coming to Thanksgiving to wear sweaters.

The owner said can I come back in an hour…?

Did a beautiful job and when I said it was heavy he said less problems…

 

[ponytug]

Inverter heat pump mini split systems are essentially designed to do this and vary the compressor output depending on the heat/cooling load on the system. The indoor air handler for many mini splits is quieter than a normal home refrigerator. They are very suitable for open living room spaces, but the blowers on them aren't generally powerful enough to blow air through a duct system that you'd normally use for heating and cooling bedrooms and bathrooms. Part of their energy efficiency is because they aren't having to push air through a duct system.

Which brings us back to the OP's air handler which is having to push air through a duct system. Air weighs 14.7 pounds per square inch at sea level. This means there is a respectable amount of weight to be moved in any HVAC duct system by the air handler. So it isn't surprising that there can be noise associated with air handlers because of the amount of air they are moving.

Ok, I agree with the first paragraph.

However, the second one is a little garbled. I am sure that you meant to write about the weight or mass of air in the system.

Yes, the pressure of air at sea level is 14.7 pounds per square inch, but that is the weight of all the air above one square inch to the end of the atmosphere hundreds of miles up. What the OP needs to worry about is the mass/weight in the volume of air in their system. The density of air at sea level under standard conditions is only 0.0765lbs/cu.ft. of volume, so air handlers aren't moving much weight (mass). That is why furnace fan motors are so small. There isn't much actual work being done.

The small pressure difference between inside the plenum versus the outside over the large area of a plenum wall isn't zero, and that's why small variations in turbulence move the whole wall, generating rumbles. Three inches of water difference is about a tenth of a pound per square inch, which is probably close to the base pressure on the plenum walls. If the turbulence generates a tenth of that, that means the turbulence would generate about 1.4lbs/sq.ft., more than enough to flex thin sheet metal. Hence, @CADplans' rumblies.

All the best,

Peter

 

[ultrarunner]

Want to burn out a blower or create backdraft just run with obstructed filter.

Last 3 blowers replaced all had plugged filters.

In to the point of leaving out the filter and sealing the access slot at one rental…

It’s the old story… you can buy a man a case of filters and show him how easy it is to change but will he do it???

Christmas I was asked to look at a furnace going “Boom” at a neighbors…

It was as simple as changing the filter something her husband always did until he passed away 3 years ago…

 

[Midniteoyl]

Want to burn out a blower or create backdraft just run with obstructed filter.

Last 3 blowers replaced all had plugged filters.

In to the point of leaving out the filter and sealing the access slot at one rental…

It’s the old story… you can buy a man a case of filters and show him how easy it is to change but will he do it???

Christmas I was asked to look at a furnace going “Boom” at a neighbors…

It was as simple as changing the filter something her husband always did until he passed away 3 years ago…

MiL had a failure once. I went over to check and found the filter was so bad that it collapsed and got sucked into the fan.

 

[Raul-02]

The noise of the turbulence is carried by the ductwork to every room in the house.

I'd cut the wall open with a zip disk and make a port in the plenum through which I could install a couple of different devices to redirect air. If it didn't work it's a simple fix seal the opening with pop rivets or sheet metal screws.
I'm thinking you've got nothing to lose, there's no warranty to void and a simple airfoil or some inline vanes to encourage laminar flow might do the job.