Central Air main return is filtered but not the small returns from the rooms?

In my moms house, there is a large return grill in the ceiling at the top of the stairs with a 16x20 filter that feeds directly into a large return plenum. The filter is mounted just above the grill, i.e the grill holds the filter in place. Just above the filter is a large plenum that feeds to the furnace return.
Problem is all the individual return ducts from the various rooms all converge in this same large duct. Therefore the return air from the individual rooms does NOT pass through the filter. Only the return air from the main grill passes through the filter.
Am I missing something or was this how the installers built it?
Is the filter supposed to be installed somewhere up in the plenum and not right at the intake grill or ????
I looked for but did not find any other filter at the furnace or elsewhere in the system.
thanks
Mark
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In most installations, the filter is located at the input to the furnace, often as part of the furnace. It is always before the motor to keep dirt out of the motor. Sounds like this is a half-baked installation. I would try to find a place to put a filter on the input side of the furnace, so that the entire house is filtered.
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There should be a filter before the fan motor and coils. If all the air is not going through the filter the system is not designed correctly.
My house is similar to this. There is one large return on the main floor but several other returns. However while a filter can be put in the large return, it is not there. It is located at the air handler so all the air is filtered.
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wrote:

+1
Usually there is one filter right before the furnace that filters all the air going into it. Among the good reasons for that is the fact that a decent filter that can stop particles while still offering decent airflow is going to be 4 or 5 inches thick. Or of the elecronic type. Neither of which would be easy to put at air return openings in the house. I would think the most you could put there would be one of the 1" thick cheapo ones. And even then, I'd rather have one that filters all the air, at the furnace.
As suggested, the OP could figure out how to put one in at the furnace. The housings for a good media filter aren't too expensive, maybe $200. But finding one that can mate up with what is there without doing sheet metal work could be a problem. A pro can easily knock out whatever transition is needed and if necessary that's the route I'd go.
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.
That's the bare minimum. The problem is those 1" filters don't do a very good job. You can't get a filter that is 1" thick that does a good job of trapping particles without restricting airflow too much. A filter capable of doing a good filtering job is 4x that thickness. But you raise a good point. That minimum level of filtering is better than what he apparently has now.
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Dont feel left out, Ive seen some idiot do that in a six million dollar home.
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