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New A/C, vent noise, can system be too "air tight"?
Old 12-29-2015, 01:04 PM   #1
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New A/C, vent noise, can system be too "air tight"?

In March, I purchased a 12-year old condo in SW Florida. About a month ago, I replaced the entire A/C system (2.5 ton, split system) as the compressor was failing.

I have a question about the filter portion of the new system.

The door to the utility closet where the inside portion of the a/c unit resides was built so it opens on the 'wrong side'. This makes it impossible to replace the filter on the bottom of the air handler unit without removing the closet door. (An inconvenience, to say the least.) On my "to do" list was figuring out how much effort and expense would be involved redo the bi-fold door hinge system to allow the door to open on the other side.

Last week the A/C tech stopped by to take care of a small issue that the inspector had noted. (Inspector had asked that metallic tape be put on a small cut which exposed some insulation on the inlet side of the unit). While the tech was here, I mentioned the issue of the closet door removal required to access the filter beneath the air handler, hoping to get some suggestions from a pro who sees these sorts of issues all the time. He pointed to the return air panel in the hallway and said you can simply change out the filter there as needed and then you don't even need a filter here, which means you won't need to worry about changing the door hinge arrangement. He double-checked to make sure that there was no filter present beneath the air handler unit and he then proceeded to tape up the filter access panel underneath the air handler unit, making it basically inaccessible and making everything quite air-tight.

Since then, I've had guests staying in the guest bedroom closest to the A/C unit. When I asked, they mentioned that they heard noises at night that seem to be coming from the venting system. I was in the guest room once and heard this in the daytime and initially thought it might be someone in the upstairs condo working on something, but my guests assured me that this was similar to the noises they'd heard at night.

Today I noticed quite a bit of dust on the return air vent cover in the hallway. While up on the step ladder cleaning the vent cover, I thought I'd take a look at the A/C filter right behind it (which I had installed about a month ago) to see if it was dirty and needed replacing. The filter appears to have developed a bit of a 'pot belly' due to the suction effect of the A/C blower. The A/C and fan kicked-on while I was up on the step ladder cleaning the vent cover and I can attest that the airflow is quite strong.

I've had an electronic air filter system on my furnace/A/C system in Michigan for about 25 years, so don't know what these disposable filters look like after they've been in service.

My questions to the knowledgeable folks here, before I call the A/C installer again...is this 'pot belly' effect typical or might my system be too 'air tight' (or the fan too strong) which is causing the filter to be sucked toward the air handler? I'm also wondering about the vent noises that my guests have heard overnight. I slept in the guest bedroom for a few weeks before the new A/C system was installed and never heard any vent noise (and I'm a light sleeper) Again, might this noise be due to over-suction/too much air flow or is it simply a byproduct of a new A/C? How do I get the noise to stop so my guests don't hear it in their room? Do I need a more porous/different filter?

I'd hate for the system to be so airtight that it causes the fan to work too hard, thereby shortening its life.

I'm attaching pix of the filter (top view and front view) so you can see how much it has 'belly-ed out'.

omni
Attached Images
File Type: jpg Filter - top view.jpg (256.5 KB, 15 views)
File Type: jpg Filter - front view.jpg (217.9 KB, 13 views)
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Old 12-29-2015, 01:27 PM   #2
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I've not seen that "belly" in our filters, same brand, and they showed no change when we changed our A/C unit 2 years ago. Does the unit have enough air returns?
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Old 12-29-2015, 01:37 PM   #3
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Originally Posted by omni550 View Post
In March, I purchased a 12-year old condo in SW Florida. About a month ago, I replaced the entire A/C system (2.5 ton, split system) as the compressor was failing.

I have a question about the filter portion of the new system.

The door to the utility closet where the inside portion of the a/c unit resides was built so it opens on the 'wrong side'. This makes it impossible to replace the filter on the bottom of the air handler unit without removing the closet door. (An inconvenience, to say the least.) On my "to do" list was figuring out how much effort and expense would be involved redo the bi-fold door hinge system to allow the door to open on the other side.

Last week the A/C tech stopped by to take care of a small issue that the inspector had noted. (Inspector had asked that metallic tape be put on a small cut which exposed some insulation on the inlet side of the unit). While the tech was here, I mentioned the issue of the closet door removal required to access the filter beneath the air handler, hoping to get some suggestions from a pro who sees these sorts of issues all the time. He pointed to the return air panel in the hallway and said you can simply change out the filter there as needed and then you don't even need a filter here, which means you won't need to worry about changing the door hinge arrangement. He double-checked to make sure that there was no filter present beneath the air handler unit and he then proceeded to tape up the filter access panel underneath the air handler unit, making it basically inaccessible and making everything quite air-tight.

Since then, I've had guests staying in the guest bedroom closest to the A/C unit. When I asked, they mentioned that they heard noises at night that seem to be coming from the venting system. I was in the guest room once and heard this in the daytime and initially thought it might be someone in the upstairs condo working on something, but my guests assured me that this was similar to the noises they'd heard at night.

Today I noticed quite a bit of dust on the return air vent cover in the hallway. While up on the step ladder cleaning the vent cover, I thought I'd take a look at the A/C filter right behind it (which I had installed about a month ago) to see if it was dirty and needed replacing. The filter appears to have developed a bit of a 'pot belly' due to the suction effect of the A/C blower. The A/C and fan kicked-on while I was up on the step ladder cleaning the vent cover and I can attest that the airflow is quite strong.

I've had an electronic air filter system on my furnace/A/C system in Michigan for about 25 years, so don't know what these disposable filters look like after they've been in service.

My questions to the knowledgeable folks here, before I call the A/C installer again...is this 'pot belly' effect typical or might my system be too 'air tight' (or the fan too strong) which is causing the filter to be sucked toward the air handler? I'm also wondering about the vent noises that my guests have heard overnight. I slept in the guest bedroom for a few weeks before the new A/C system was installed and never heard any vent noise (and I'm a light sleeper) Again, might this noise be due to over-suction/too much air flow or is it simply a byproduct of a new A/C? How do I get the noise to stop so my guests don't hear it in their room? Do I need a more porous/different filter?

I'd hate for the system to be so airtight that it causes the fan to work too hard, thereby shortening its life.

I'm attaching pix of the filter (top view and front view) so you can see how much it has 'belly-ed out'.

omni
I'm not an A/C guy. My system makes noises when the air flow starts, as the filter is pressed in the housing, but they don't bow like the photos show. I use mid-level filters, to avoid the airflow reduction problem of high MERV filters. And I don't use the low MERV rated filters (like fiberglass), because it will redistribute all the dust in your house, as small dust just passes the filter. I can't tell the rating for your 3M filter- here's a chart and a technical discussion. You might try a filter with a lower MERV rating.
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Old 12-29-2015, 01:43 PM   #4
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I will go back to where the filter was because you most likely have other returns that do not have a filter on it. The filter at that location is reducing the air flow to the furnace which can caused other problems. The easier thing to do is to replace the bi fold door with an accordion type of door. The other option is to put the hinges on the outside and the door will swing outwards. Low air flow will caused even greater problems in cooling mode.
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Old 12-29-2015, 02:10 PM   #5
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Had that problem in a previous house. The dog hair would get into the filter on the furnace which was difficult to replace and expensive as well. We put a cheap filter behind the intake vent to the closet, and the problem was solved except the cheap filter was getting plugged and causing problems with the furnace blower over heating (too much stress on the intake when the dog hair clogged the vent.). The ac guy said the additional filter would void the warranty if we continued to use it. We just changed it on a regular basis and removed it before the regular service visits!


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Old 12-29-2015, 02:10 PM   #6
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On my "to do" list was figuring out how much effort and expense would be involved redo the bi-fold door hinge system to allow the door to open on the other side.
You can change the hinge side on a bi-fold door in ~15 minutes... It should be a louvered door.

Use a cheap blue fiberglass filter. They will give the best air flow. Then check for noises.

You may have an intake air starved system. It may need more air intake. Try the blue filter first.

Are you sure you are installing the filter in the right direction. To me it looks like the airflow arrows are pointed to the hinge side of the bi-fold door. The arrows should be pointed TOWARDS the furnace on a filter. It is the air intake.
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Old 12-29-2015, 02:22 PM   #7
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It should be easy to reverse the door, then go back to the old filter. The pot belly and noises would indicate to me that the system is restricted.


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Old 12-29-2015, 02:28 PM   #8
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AC duct systems have to be airtight. That should not be a problem.

What is the size of the filter relative to the one that you show a photo of? If a filter is too small, its restriction of airflow will cause it to bow. That latter filter is 18"x24", which I do not think is too small for a 2-1/2 ton unit.

Perhaps you just need to change it more often. And if the airflow is too strong, you may be able to reduce the speed of the motor in the air handler. The fan motor often has 3 speeds, which is selected by a wiring change at the motor in order to accommodate different installations with different duct lengths.

PS. Just saw that your filter was only 1 month old, and the photo does not show it to be dirty. So, the airflow may be too strong.
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Old 12-29-2015, 02:37 PM   #9
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I need to add that the rule of thumb is 144 sq.in. (1 sq.ft.) of filter area per ton of AC cooling. That means at least 360 sq.in. of filter for a 2.5-ton AC, while yours is 18 x 24 = 432 sq.in.

If the fan speed is too high, it will cause wind noise. And high air velocity through the filter will cause dust bypass because the suction is too strong. That's not good. Air should move through the filter with a speed of 300 ft/min. Do you have an anemometer?
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Old 12-29-2015, 02:50 PM   #10
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Another vote for possibly reducing fan speed. You want the compressor to stay running for a good bit of time to reduce humidity and increase efficiency. If the cycle is too short it's not good.
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Old 12-29-2015, 03:30 PM   #11
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Thanks, all.

With these responses i get the feeling that I'm on the right path re: too much airflow/starved system, etc.

I've responded to your questions/comments below.

The bi-fold door is not louvered but, rather, solid. Switching sides involves drilling into the tile floor for the new hinge, either recycling the door (and drilling holes to move the hinge pins into...and moving the door knob to the other panel and somehow hiding the hole where the old knob was, etc. ) or buying a new door...which involves finding paint to match, etc etc. Not a huge project, but it will take me some time to figure out an exact plan, buy the ceramic drill needed, etc.

No, I don't have an anemometer.

Perhaps I will try a blue filter until I can get around to reversing the door.

This 18x24X1 filter tht I installed is a 3M Filtrete electrostatic air cleaning filter (There is no indication as to the MERV rating on the filter, and I've long since disposed of the packaging it came in.)

When the tech was here last week I asked him as to which filters he recommends. He said the fan-fold-type ones, which made me think I had installed the proper one.

Now that I am 'reliving' this whole experience, I definitely recall hearing the filter itself making noises for the first few days or so, whenever the A/C kicked on. (Again, this might have been an indication of a starved-for-air system.)

I installed the filter properly -- the arrow was (and now the 'belly' is) pointing towards the interior air handler unit.

The old A/C also did not have a filter installed under the air handler unit. (I checked when I first bought this place, as I was wondering how long it had been since it was replaced, and about the size...and also as to how they might have replaced it without folding the stiff filter frame in half to get it past the door hinge area....only to find that there wasn't a filter installed in the place designed for a filter.)

I don't have any pets. I'm certain there has been quite a bit more dust/lint in the air than usual...lots of new fabric items (area rugs, bedding, towels, furnishings)...plus 6 people for a while instead of just me.

I'm still wondering about the airflow. This condo is one of 168 units which all have the same ducting layout. The wall-mounted grille in the hallway (which is where this 18X24 3m filter is mounted) is the only "air return" to the air handler.

With the old A/C, I barely felt when it kicked on and off. Now, depending on where I am sitting in the condo, I can hear it kick on/off (and when its running, it sounds like I'm hearing the wind blowing outside).

I can also feel a breeze on my skin when it is running if I am sitting directly in line with a vent. For my own comfort, I prefer not to have a breeze blowing on me when the A/C is on. Does mean I can/should request a lower fan speed?

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Old 12-29-2015, 03:35 PM   #12
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No, I don't have an anemometer...

I can also feel a breeze on my skin when it is running if I am sitting directly in line with a vent. For my own comfort, I prefer not to have a breeze blowing on me when the A/C is on. Does mean I can/should request a lower fan speed?
I was just teasing about the anemometer. I do not have one either, but an AC technician should have one in his tool kit. He can check the airspeed and rewire the fan motor for you if it has provision for different motor speeds.
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Old 12-29-2015, 03:41 PM   #13
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Has the duct system been vacuumed in the recent past (1-3 years)? If not, this might resolve much of the dust.
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Old 12-29-2015, 03:57 PM   #14
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Definitely need a lower fan speed. When you described problem my first guess was you have one intake. You would not use a filter there, but maybe a very porous one. When you muck with the design of hvac, make small alterations.

Buy new doors pre- painted. Or use two knobs. Drilling a hole in ceramic tile is not so hard.
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Old 12-29-2015, 03:58 PM   #15
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By the way, changing the fan motor speed takes nothing more than untwisting a wire nut and moving a wire to one of the three choices: Hi, Med, Lo.

The wiring diagram is usually on a label taped inside the housing cover, and shows the color of the wires corresponding to the 3 speeds above. If you are handy or know someone who is, you can do this yourself without paying for service. It's easy.
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Old 12-29-2015, 04:08 PM   #16
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If you're having to change fan speed on a new system, the system was not well designed by the installer, and/or is inappropriate for the application...
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Old 12-29-2015, 04:21 PM   #17
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... or he failed to select the proper fan speed for the installation.
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Old 12-29-2015, 04:49 PM   #18
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... or he failed to select the proper fan speed for the installation.

Which would be part of "well designed"...
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Old 12-29-2015, 05:05 PM   #19
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If you're having to change fan speed on a new system, the system was not well designed by the installer, and/or is inappropriate for the application...

As a consumer, I am (we are) sort of at the mercy of professional A/C companies, no?

Further, I was in a time crunch, with a failing compressor (in daily high-80s external temperatures with high humidity). Plus I was leaving on a week-long cruise in a few days and hosting 5 guests upon my return.

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Old 12-29-2015, 05:14 PM   #20
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Here's the filter I was using. It's a MERV 8.

Filtrete 600 Dust Reduction Clean Living Filter - 18x24x1 (6-Pack)

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