Return Air Sizing...too loud!

[TABCON]

I'm a general contractor and have been using the same HVAC sub for the last 5 years, up until last week that is. It's not so much that he is not very good, just not there when i need him and installs seem to take way too long, not to mention poor follow up on any problems. Anyway, that's a different story.

My main problem right now is a new home I built last year. Inside, he installed 16 SEER Carrier Infinity units both up and down. The outside compressors are Carrier Performance Series, a 5 ton down for about 2,300 square feet of living area and a 2 ton on the second floor for about 800 sq. feet of living. 1st floor unit works great...quiet and cools very efficiently.

The problem is with the second floor unit. The home is located in the deep south with typical summer temps of 93 to 98 degrees on a daily basis. The attic usually does not get above 95 degrees. The home is well insulated with R30 in the ceilings and R13 in the wall cavities.

At night the unit cools well, but during the day, when temps rise, with the thermostat set on 70, the temp will not drop below 79 to 75. The system is about one year old and I just had my new HVAC contractor check it. Puron level was about a pound low due to a leaky connection, but he fixed that. It now cools better, but not as good as it should during the day.

The Totaline Digital thermostat is set so that the 1st stage of the compressor never kicks in. I also have it adjusted +10 degrees. The air flow being sucked into the return seems unusually loud. You can hear the sucking noise from downstairs. Could the air return be too small? It's 20"x20" for the 2 ton unit cooling approx. 800 sq. feet, which seems like it should be plenty big enough for the square footage. The owners tried one of those pleated air filters and the unit was so loud they had to go to the cheapie type filter. Also, radiant heat from windows on the second level is not an issue.

We're going to further insulate the attic space with a radiant heat barrier. Not because of the unit in particular, but because it makes such a huge difference in overall comfort of the home in areas where it gets this hot.

Any ideas on what I can do to increase the efficiency of this unit?

Thanks!

[lolson]

did you go low bid?

[AcDOCnTRAINING]

Well at that eff. I'm sure we are dealing with a R-410a unit or puron system. I always recommend that a subcooling be performed on the unit. Charging a unit by pressures is not the correct way of making sure a unit is charged properly.You may want call the tech that came out and ask if he performed a subcooling to the unit. If he tries to talk you out of it, or has no idea what you are talking about just call someone who does. About your return box its 20x20 but is there a flex connected to the box? If so whats the size of it? Also you may want to check the supply,a small supply or obstructed supply(vents shut closed)will cause just as much noise as a small return. Good luck there!

[TABCON]

Low bid? LOL, no, never. I would have used window units for that...lol.
Seriously, we only build high end custom homes with square footage costs of $200+ per sq.foot living. I don't like to be called back on problems, so we only use the best of everything.

The actual size of the rigid return air plenum box is larger than 20x20, that is just the filter size. But the rush of air going into the 20x20 hole in the wall sounds very loud. Also, no flex, all rigid wood frame with insulation panels.

I will also have my new contractor check on the charging method you mentioned.

Thanks guys.

[AcDOCnTRAINING]

:confused: You may have a control issue, maybe the dead band on the thermostat is not set right, or the t-stat is not calling the proper stage of cooling. Any how you should right down on paper your compliant and have a tech come out again. If they still can't figure it out they will call a Factory tech which knows the product by heart. GOOD LUCK:D

[skippedover]

Low bid? LOL, no, never. I would have used window units for that...lol.
Seriously, we only build high end custom homes with square footage costs of $200+ per sq.foot living. I don't like to be called back on problems, so we only use the best of everything.

The actual size of the rigid return air plenum box is larger than 20x20, that is just the filter size. But the rush of air going into the 20x20 hole in the wall sounds very loud. Also, no flex, all rigid wood frame with insulation panels.

I will also have my new contractor check on the charging method you mentioned.

Thanks guys.

I think you're actually mixing two different issues here, which could at some point intersect but right now I'd treat them as separate issues.

#1 Issue: The system is not cooling to the clients satisfaction
#2 Issue: The return air is noisy.

Understanding that you do not wish to have these issues at all upon system commissioning and also that you do high end homes, which you indicate and your equimpent selection indicate hour are not taking the 'lowest bid' for the sake of cutting corners. That all SHOULD work in your favor but sometimes the company or companies you hire, may not be up to your standards, except to supply the equipment. What I'm saying is that if the company you were using in the past or are now considering using in the future does not start work from an achitects plan and do a Manual 'J' load calculation, Manual 'D' duct design and Manual 'S' equipment selection, you're not getting what you think you're paying form.

When a proper load analysis is done, if the equipment is then not performing up to design specifications (outdoor temp/indoor temp/indoor humidity) then it should be a relatively easy task to go through the entire system, step by step and determine what parameter is outside specifications, what it will take to bring it into spec. and then do that repair/mod.

When a duct design by Manual 'D' is done, the system should not be noisy. Airflow ratings are determined by the size of the duct the air is flowing through, the total volume of air flowing through that duct and the actual speed of the air moving through the duct. As an example, when our company installs a duct system with a variable speed blower motor, experience has taught us that the face velocity throught a return air grille can set up a harmonic that will cause some 'singing' of the louvers in the grille. So as a matter of company policy, we increase the size of the grille by 25%. You say you have a 2-ton system there and normally that would be approximately 800 CFM. We'd increase that airflow to 1,000 CFM and select a return air grille that would would handle at least 1,000 CFM. I think you indicated your client is using a filter grille so I looked at a Hart & Cooley #659 filter grille and at 20 x 20 size, 700 FPM face velocity, it is rated at 1,342 CFM, more than enough to satisfy the example system. As you can see from that simple research, the finger would point to some other location in the system for the noise besides the return air filter grille.

So if your original installer did not do the load analysis and duct design, I'd hire a new company to do that. The results may not be what you want to hear but it's money well invested to both resolve the current issue and maybe to prevent future issues. Usually you'll find that companies who start properly, end properly, fully support their installations and employe the best people. Naturally, they're NEVER the lowest bid and may be higher than you've been accustomed to paying but they're worth every penny in the long run. Hope this helps. :)

[classicrock4you]

I think all the replies have missed a major problem.

As I read the origional post, Tabcon says that average summer temperature is 95 to 98 degrees and the attic temperature stays at 95 degrees? Ninety five???? 5 degrees less that 100? 3 degrees less than the outdoor temperature?

Maybe he turned the boots the wrong way and is blowing air into the attic!
(sarcasm intended)

Can you say leaky ductwork?

[ckone180]

Restriction in the return?

[a/cpro]

Why is the first stage wired so it never kicks in.??

[James Tramel]

the return air is 20"x20" whats the grill size? The grill could b too small or the path rest. in another way

[TABCON]

I think you're actually mixing two different issues here, which could at some point intersect but right now I'd treat them as separate issues.

#1 Issue: The system is not cooling to the clients satisfaction
#2 Issue: The return air is noisy.

Understanding that you do not wish to have these issues at all upon system commissioning and also that you do high end homes, which you indicate and your equimpent selection indicate hour are not taking the 'lowest bid' for the sake of cutting corners. That all SHOULD work in your favor but sometimes the company or companies you hire, may not be up to your standards, except to supply the equipment. What I'm saying is that if the company you were using in the past or are now considering using in the future does not start work from an achitects plan and do a Manual 'J' load calculation, Manual 'D' duct design and Manual 'S' equipment selection, you're not getting what you think you're paying form.

When a proper load analysis is done, if the equipment is then not performing up to design specifications (outdoor temp/indoor temp/indoor humidity) then it should be a relatively easy task to go through the entire system, step by step and determine what parameter is outside specifications, what it will take to bring it into spec. and then do that repair/mod.

When a duct design by Manual 'D' is done, the system should not be noisy. Airflow ratings are determined by the size of the duct the air is flowing through, the total volume of air flowing through that duct and the actual speed of the air moving through the duct. As an example, when our company installs a duct system with a variable speed blower motor, experience has taught us that the face velocity throught a return air grille can set up a harmonic that will cause some 'singing' of the louvers in the grille. So as a matter of company policy, we increase the size of the grille by 25%. You say you have a 2-ton system there and normally that would be approximately 800 CFM. We'd increase that airflow to 1,000 CFM and select a return air grille that would would handle at least 1,000 CFM. I think you indicated your client is using a filter grille so I looked at a Hart & Cooley #659 filter grille and at 20 x 20 size, 700 FPM face velocity, it is rated at 1,342 CFM, more than enough to satisfy the example system. As you can see from that simple research, the finger would point to some other location in the system for the noise besides the return air filter grille.

So if your original installer did not do the load analysis and duct design, I'd hire a new company to do that. The results may not be what you want to hear but it's money well invested to both resolve the current issue and maybe to prevent future issues. Usually you'll find that companies who start properly, end properly, fully support their installations and employe the best people. Naturally, they're NEVER the lowest bid and may be higher than you've been accustomed to paying but they're worth every penny in the long run. Hope this helps. :)


First of all, thanks for the time you took to really address my problem. I wish you were located around here. I'd hire your company in a NY minute. You see, since hurricane Katrina, a great deal of the expert help in this area moved on and many of the best subcontractors are booked up a year or more in advance.

A great deal of the work I do is designed and specked by architects and engineers, especially homes on the waterfront. Some of the spec books are the size of a book like War and Peace. Also, most call for rigid duct work and loads of different zones. To date, I've never had a problem with my HVAC sub. I'm still not sure what's going on now. It really bothers me that I had to go out and find a new sub, and that his service has fallen down to zip. All I can speculate is that he's tired and wants to either go on welfare or retire.

Based on what you say, the return air plenum and opening are more than large enough. So, we will look elsewhere. As a matter of course, I'm going to have all the coils and evap's cleaned. Construction and grading can cause a great deal of dust and regardless of what steps you take, some sub always cranks the ac down to 55 with the doors open.

Yea, maybe I was wrong on the attic temps. The last time I checked it was around 95, but it was a cloudy day. But the duct work is sealed properly and no leaks were found.

Also, no restrictions noted in the return. I was advised that in this heat, having the first stage kick in is useless. The thermostat is not "wired" that way, just adjusted per the menu that way. It's a Totaline digital that gives you that option.

[TABCON]

the return air is 20"x20" whats the grill size? The grill could b too small or the path rest. in another way

That is the size of the filter. The actual framed in plenum is much larger, maybe 24" x 32"?

[skippedover]

First of all, thanks for the time you took to really address my problem. I wish you were located around here. I'd hire your company in a NY minute. You see, since hurricane Katrina, a great deal of the expert help in this area moved on and many of the best subcontractors are booked up a year or more in advance.

A great deal of the work I do is designed and specked by architects and engineers, especially homes on the waterfront. Some of the spec books are the size of a book like War and Peace. Also, most call for rigid duct work and loads of different zones. To date, I've never had a problem with my HVAC sub. I'm still not sure what's going on now. It really bothers me that I had to go out and find a new sub, and that his service has fallen down to zip. All I can speculate is that he's tired and wants to either go on welfare or retire.

Based on what you say, the return air plenum and opening are more than large enough. So, we will look elsewhere. As a matter of course, I'm going to have all the coils and evap's cleaned. Construction and grading can cause a great deal of dust and regardless of what steps you take, some sub always cranks the ac down to 55 with the doors open.

Yea, maybe I was wrong on the attic temps. The last time I checked it was around 95, but it was a cloudy day. But the duct work is sealed properly and no leaks were found.

Also, no restrictions noted in the return. I was advised that in this heat, having the first stage kick in is useless. The thermostat is not "wired" that way, just adjusted per the menu that way. It's a Totaline digital that gives you that option.

You're welcome and I too am sorry we're not in your service area. But we're all still here to help.

Okay so you say you have a mechanical engineer or architect spec out the system size. That should be good news but for me it's not necessarily so. I've corrected any number of mechanical engineers for undersized systems. So we always do an in house Manual 'J' just to be sure because we don't want to be a participant in the finger pointing if things don't go right. For that matter, we always have our geo-thermal loads double checked by the engineers at the equipment suppliers for the same reason.

So assuming the calculations are correct for the moment, it then becomes a matter of going down a checklist for troubleshooting purposes. That checklist should include such things as gauge readings hi & lo, super heat & subcooling numbers, OAT & outdoor wb at the time of reading, static pressures as taken in the supply & return plenums, temperature rise across the condenser, temperature drop across the evaporator, wb & db temps indoors, motor amp ratings & actual amp readings. From these readings a good tech can tell you if the equipment is operating up to specs or if there's some reading that is NOT up to specs, indicating a problem. If the equipment all shows it's up to specs, then the Manual 'J' calc is thrown under a cloud of suspicion. I'd also recommend that you take an actual temperature reading in the attic because if it's coole than very hot up there, then there's something cooling it down.

I'm really quite suspicious of that return air noise. A temp drop across the evap would tell a lot in that regard.

[dash]

Performance Series doesn't have two stages in cooling.infinity indoor unit gas or air handler,would be better off with an Infinity control,air hanler requires one.

Models numbers would be helpful.


If it's an Infinity gas furnace,with Totalline stat,it may be running on a much higher speed then needed,impropper setup.

[TABCON]

I remember specifically asking for the Infinity control, but he told me it would not work with the unit because of the outdoor Performance units. Both interior units are Infinity as follows:

Infinity No. 38TSA060300-301 & 38TSA024340-341

He had both controllers set on a 2 stage operation program, but they work better since I turned off that option.

Any help would be greatly appreciated.

[dash]

38TSA is an outdoor unit series,they are single stage ,not two stage compressors.

Do you have gas furnaces?

Is the noiesy indoor unit close to the return?

Can you post pics of the noiesy indoor unit and connecting return duct?

[davo]

I remember specifically asking for the Infinity control, but he told me it would not work with the unit because of the outdoor Performance units. Both interior units are Infinity as follows:

Infinity No. 38TSA060300-301 & 38TSA024340-341

He had both controllers set on a 2 stage operation program, but they work better since I turned off that option.

Any help would be greatly appreciated.


If they are using variable speed air handlers, the problem could be as simple as the air handler is configured for the wrong cfm, causing noise.

Judging by those condensing unit model #'s the units are about 2005/2006 vintage.
Down here anyway, for a while during that time, Carrier was giving us R-22 air handlers with R-410a kits that had to be field installed. I have found 4 in the last 3 weeks that were never changed to the R-410a valves.

[Chris_Worthington]

Ok,,,, 20X20 grill,,, what size is the duct work for the return??? How long is it?? How many turns??? etc??? They all add to the (noise) higher static of your return. A 20X20 grill with a straw connected to the AHU will be noisy, period.

Sounds like you have the equipment to make some changes, possibly??? "If" you have someone in your area capable of doing some simple psychometric calc's then you "may" get away with lowering your coil bypass factor down to 8-9% ??? "If" you can still maintain delivery no problem.......

Is this a Thermidistat??? that you are having problems with, they need to be not only wired up correctly, but "configured" correctly as well.....

Ya need a tech with some smarts, ask some questions, techincal questions. Who cares if you dont know the answer or not, look for them to stumble and move on to the next.

[firecontrol]

Did I miss it or didn't anyone mention doing a total static check on the unit in question? This should have been done by the second contractor called in as part of his verifying that the unit being worked on had proper air flow moving across it before he charged it once the leak was repaired. Also part of this preliminary check is to veryify that the proper CFM's of air are moving across the coil for the size unit. Didn't see either mentioned.

A 2 ton unit with that large of a return should work fine looking at it from where I'm at. The noise could be from air noise or mechanical. If the entire return for the unit passes through this one grill, try pulling the grill off and see if the noise goes away. It could be as simple as changing the style/manufacturer of grill.

If removing the grill and all the other items listed so far are at manufacturers specifications and the noise still isn't reduced I'd suspect mechanical noise being reflected down the return and exiting the grill. This can sometimes be resolved "after the fact" by adding duct lining to the inside of the ductwork. This will act as a muffler by absorbing some of the noise before it can be reflected out the grill.

Good luck with the new contractor.

[Chris_Worthington]

Should it be a "grill" noise as FireControl pointed out, you could try straightening out the louvers with a pair of needle nose plyers.

Bottom line airflow is the most overlooked and most important item to check when performing any cooling service, make sure someone checks the CFM'S

[Mstrav]

we still have no idea of what size the round duct feeding the return is? when the grill is opened and you look past the square metal what size is the round pipe or is it all square / rect? is it ligned or externally wrapped? The next thought is how far is the unit from the return as the blower noise could be what you hear.

with the filter out of the return and the return grill open is the noise still there?

I oversize my retuurns and use bar type not stamped, but I design for noise and also like (2) 90* before the back of the furnace to keep it quiet.


good luck to you,

matt

[fire]

This should not be noisy (duh). The air flow is too high. I,ll bet $10.00 Can.:D

[EWallace10]

Your return should be sized as follows: 1 square foot of free return air grill area (measure louvers lenght x width, multiply time .96) per ton of AC.
You need to be using the Infinity control for the Infinity system you paid alot of money for some really great features you are not getting with the other control.
Also, you need to be using the low speed to remove humidity, an 80 degree house with a relative humidity of 48 percent will feel like a 75 degree house.
I agree with the previous posts. You need to have airflow checked with a hood, check the static across the system, and check the drop across the coil. I don't understand why they would put in a 24x32 box and install a 20x20 FF? Is this an upflow system on a platform? If so ensure you don't have a media filter or filter base installed and are using filter frame grills with filters, it is too much static pressure for the system to operate against and will directly affect blower performance and airflow.

[Chris_Worthington]

You need to have airflow checked with a hood...OR.....

With the E-heat on take the VoltsXAmpsX3.413=? and divide by TDX1.08=CFMS

Make sure the supply temp probe cannot see the electric heat or the equation will be thrown off by the radiant....;)