A.C. filters

I have noticed that the A.C. filters in many houses is in the ceiling.
What the fire truck is up with that that ?
At my daughter's house, she had two A.C. units.
I found one filter in the ceiling, but could not find the other filter.
Where can I look for the other air filter ?
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On 7/6/15 10:54 PM, Andy wrote:

My filter is at the air-handler end of the return ducts, but most, I believe, are at the living-space end. If I were looking for filters, I'd go to the air handler to see where the return ducts originated.
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On Monday, July 6, 2015 at 10:22:05 PM UTC-5, J Burns wrote:

I think the air handler is in the attic.
Could you be more specific ?
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On 7/7/15 12:31 AM, Andy wrote:

That explains why you found a filter in the ceiling.
AFAIK, the air handler is always on the return side of the evaporator.
Your return duct must run from the air handler to the ceiling filter you found. If there are any forks, each duct leads to a different filter.
For example, I have floor returns in two rooms. Their ducts combine and go to the air handler. My neighbor has a ceiling return in one room and a return low in the wall in another room. Their ducts combine and go to the air handler.
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On Monday, July 6, 2015 at 11:22:05 PM UTC-4, J Burns wrote:

All the ones I've seen here in NJ are like yours, at the air handler. That includes systems from the 50s through ones in new construction today. One reason, would be there are multiple returns in a house that would each need a filter, while at the air handler/furnace you only need one. Also a lot of systems are going in with electrostatic.
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On 7/7/15 9:09 AM, trader_4 wrote:

The OP found a filter in the ceiling. That's why I assumed any other filters would be accessible from living spaces, like my neighbor's.
I've looked closely at hers because she asks me to change filters. The only other filter I remember is in mine. My house was heated by coal grates for decades. Then oil stoves were added. Then a furnace took over. Then an evaporator was put in the duct. I figured hers was more likely to be typical, but with my lack of experience, it was a guess.
I was thinking of going to an electrostatic filter. Lately I read that an HVAC filter doesn't make much difference in air quality because dust spends most of its time on a surface like the floor, waiting to be disturbed if you don't get it first with a vacuum.
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On Wednesday, July 8, 2015 at 1:49:58 AM UTC-4, J Burns wrote:

The purpose of the HVAC filter is to protect the air handler from dust. You need to keep the coils and fins clean or they don't transfer heat properly.
It has nothing to do with air quality or protection of people. You can get that kind of filter, but it is not standard.
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On Wednesday, July 8, 2015 at 11:01:20 AM UTC-4, TimR wrote:

Yes, if by HVAC filter you mean only the minimal filter that's included with the airhandler. But all kinds of filters are offered by the manufacturers and installed as part of the system. IDK any install I've seen around here that just use the 1" minimal one that's in the air handler.

I guess that depends on your definition of standard. On new homes going up here, electrostatic is widely used now.
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On Wednesday, July 8, 2015 at 12:18:17 PM UTC-4, trader_4 wrote:

The more filters you put in the path, the larger the pressure drop across them.
And that means your fan must be sized for that drop. I'd be cautious about adding something that might reduce capacity.
We did an operating room install some time back and that air did have to be clean.
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On Wednesday, July 8, 2015 at 2:23:04 PM UTC-4, TimR wrote:

You don't typically put more filters, you just put one good one of whatever type you want.

The air handlers I've seen on typical furnaces have been capable of handling 5" media type filters, electrostatic (which have little pressure drop). They are designed so they don't cause too much pressure drop. I have a 5" one here on my 3 year old furnace, nothing special about the blower. They are thick, but pleated with a lot of surface area.
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