Question RE: Return Air

**everafter720** · 10-07-2009, 01:56 PM

Hi-

I just got central heat installed in my house a few weeks ago. My furnace is a two stage trane xv95. I'm wondering if it's normal for the return air to make noise when the heater is on in the high stage (did I say that right??). It's pretty quiet when the heater is running on the second or low stage, but when it's on high, it sounds like a loud-ish fan. My house is 1100 square feet and the return air is in the hallway. Any thoughts? Maybe it's supposed to sound like a bathroom fan? Thanks.

**beenthere** · 10-07-2009, 02:55 PM

May have under sized supply and or return duct.

**tipsrfine** · 10-07-2009, 03:24 PM

Where is the furnace located?

**everafter720** · 10-07-2009, 03:39 PM

The furnace is about 10-15 feet away. When I asked the HVAC contractor to come out and check why the return air filter was noisy-ish, they told me the sound was usual. So, I don't know if I'm being oversensitive or if the return air is supposed to be pretty much quiet. The hallway sounds like my bathroom does when the fan is on. Is that normal? Thanks. Also, if it's not normal, what can they do at this point to fix it? FYI, I had multiple contactors give me a quote on this job, and all suggested that the furnace and return air be located in the hallway. Hope it's not the furnace's close proximity to the return air that's causing the problem....

**beenthere** · 10-07-2009, 05:14 PM

Post pics of the furnace, and duct to the return.

**garya505** · 10-07-2009, 06:42 PM

Originally Posted by everafter720

The furnace is about 10-15 feet away. When I asked the HVAC contractor to come out and check why the return air filter was noisy-ish, they told me the sound was usual. So, I don't know if I'm being oversensitive or if the return air is supposed to be pretty much quiet. The hallway sounds like my bathroom does when the fan is on. Is that normal? Thanks. Also, if it's not normal, what can they do at this point to fix it? FYI, I had multiple contactors give me a quote on this job, and all suggested that the furnace and return air be located in the hallway. Hope it's not the furnace's close proximity to the return air that's causing the problem....

Of course it could be something else, but the sound would be "usual" if the RA is undersized and as a result the air velocity is too high.

**sktn77a** · 10-07-2009, 07:20 PM

Originally Posted by everafter720

Hi-
I'm wondering if it's normal for the return air to make noise when the heater is on in the high stage (did I say that right??). It's pretty quiet when the heater is running on the second or low stage, but when it's on high, it sounds like a loud-ish fan.

The blower speed could be set on max. Have them check its set correctly.

**tipsrfine** · 10-08-2009, 07:57 AM

Is the return ductwork a straight shot back to the furnace? In other words if you were to look into the return would you be able to see the blower wheel in your furnace? If so, that could be the problem, but should be able to be remedied pretty easy.

**everafter720** · 10-08-2009, 01:32 PM

Thanks, everyone. I'll look into your suggestions.

**everafter720** · 10-13-2009, 01:12 PM

Hi-

I checked the dimensions of my return air filter and hole last night. The filter/grill is 24x16 and the hole for the return air is around 16 x 13 (approximate since it was hard to measure from my ladder). The furance is a 2.5 ton Trane 95XV. My house is 1100 square feet with 8 vents and 1 return air (with the above dimensions) located in my hallway. Do you guys think that the return air is large enough or is the size what is causing the noise? Thanks.

**garya505** · 10-13-2009, 01:49 PM

Originally Posted by everafter720

Hi-

I checked the dimensions of my return air filter and hole last night. The filter/grill is 24x16 and the hole for the return air is around 16 x 13 (approximate since it was hard to measure from my ladder). The furance is a 2.5 ton Trane 95XV. My house is 1100 square feet with 8 vents and 1 return air (with the above dimensions) located in my hallway. Do you guys think that the return air is large enough or is the size what is causing the noise? Thanks.

You have approximately 208 square inches of RA duct. Depending on the blower settings, you could have high air velocities in the duct making the noise.

Your contractor should check the blower speeds (if they do, ask them what they are). Maybe they can be reduced and still be in spec for your house. You could ask them to review the blower settings with you. I'm sure somebody here could do the math for you if you post the speeds.

My house is 1800sf, and I had 1600 CFM moving through a 20x10 RA grille and duct. It sounded like I was standing next to a 737.

With high air velocity, sometimes the grille can make noise. Sometimes HVAC techs will open the little fins in the RA grille and that will help, or they replace the grille with one that is less noisy. That worked in my case temporarily, but I added a RA duct anyway. Sometimes the techs will use a long sheet metal bending tool to bend the fins nice and straight. Yeah, the best solution might be to increase the RA size, but replacing or modifying the grille could be way less expensive.

**beenthere** · 10-13-2009, 03:48 PM

Your grille is too small.

**udarrell** · 10-13-2009, 06:35 PM

Originally Posted by everafter720

Hi-
I checked the dimensions of my return air filter and hole last night. The filter/grill is 24x16 and the hole for the return air is around 16 x 13 (approximate since it was hard to measure from my ladder).

The furnace is a 2.5 ton Trane 95XV. My house is 1100 square feet with 8 vents and 1 return air (with the above dimensions) located in my hallway.

Do you guys think that the return air is large enough or is the size what is causing the noise? Thanks.

A 2.5-ton A/C requires 1000-CFM of airflow.

First, we'll figure the RA filter/grille area for 1000-CFM.
24X16=384 /144= 2.6-sq.ft.
2.6 * .75= 1.99-Ak sq.ft. | 1000-CFM /divided by 1.99-Ak = 502-FPM Velocity above the 300-FPM D-Standard, & - without the filter which would decrease the Ak area & further increase velocity.

Therefore, a 24X16 filter/grille is not really large enough to satisfy Manual D. Manual D says 300-FPM maximum velocity with a 1" filter installed.

[B]A 16X13 hole area = 208-sq.ins., / 144= 1.444-sq.ft., hole area.

1000-CFM / 1.444-sq.ft.= 692-FPM Velocity; above 500-fpm > noisy!

It needs a lot larger RA hole area & a larger RA grille.

1000-CFM /300-FPM = 3.33-Ak sq.ft. or * 144= 480-sq.ins. or a 20X24 RA Filter/Grille, IMO, that's not enough.

Were it me, - I'd convert the initial RA grille to an even larger sized 30 X 24 Filter/Grille & not use a filter in the furnace. I believe that would solve both the noise problem & the filter airflow problem.

It is your decision as to what you do.

- Darrell

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Question RE: Return Air

Neither the filter/Grille nor the RA grille satisfy Manual D

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