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I have a 1980's construction house. It has a heating furnace in the attic that uses ducts to distribute the hot air to each room in the house. Recently, I bought air purifiers and I noticed that when I turn on the furnace to heat the house, the air quality index (AQI) drops to moderate level. So I went looking for filters on the vents and I noticed that only one vent return is openable and fits a filter in it - it's the hallway one. All other rooms have vents that cannot be open without a scredriver and don't seem to be designed to fit a filter. What is the point of one vent supporting a filter if the other 5 or so don't? Is there another filter in the furnace itself that I need to replace and wasn't instructed about? The low air quality coming out of the vents concerns me. Attached is a picture of one of the return vents that don't seem to support filters.

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    That looks like a register to me, not a return. I don't recall ever seeing a return with adjustable vents—there's just no reason for it.
    – Huesmann
    Commented Mar 27 at 13:42
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    The air delivery grills are called "registers", the return air grill is called just that, whether it is designed to have an incorporated filter or not. I think using consistent terminology will help prevent confusion between the two. Commented Mar 27 at 13:43

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What is pictured and probably the other grills with no filters are the service outlets where the heater air is blown into the room.

The return duct ( where air is drawn back to the furnace. ) will be the only grill for a filter.

The air handler may have a filter, depending on the model. Provide the make and model and further information may be available.

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  • Thank you! I am not at home. Will try to look up the furnace model when I get back. Do I understand correctly that you're saying that the vent with the filter in the hallway is actually drawing air towards the furnace and that's why that's the only one with a filter? That would make sense.
    – pkout
    Commented Mar 27 at 0:24
  • Yes , that is the usual set up. Only 1 or sometimes 2 returns that would hold filters. The other grills are servicing the rooms, blowing air in.
    – RMDman
    Commented Mar 27 at 1:51
  • Gotcha. Any idea what could be causing the air quality to drop so much when heat is on then? The air is basically recirculated from inside the house. Dirty ducts perhaps? Also, I noticed that we have MERV8 filter on that air intake vent. Would MERV13 be better or is that potentially too much for the blower in terms of pressure drop. The furnace is probably 20 years old.
    – pkout
    Commented Mar 27 at 1:57
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    Sorry air quality is outside my area of knowledge. However I'm sure someone on here will have answers for you.
    – RMDman
    Commented Mar 27 at 2:01
  • Turns out that the air quality drop wasn't related to the ducting after all. I turned the furnace on again and measured air quality and it was fine. Something else was causing the decline when I first measured it. The MERV13 filter was really bending and loud, so I replaced it with MERV8 to reduce stress on the blower. All is good now. Thanks again!
    – pkout
    Commented Mar 31 at 5:17
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It is unlikely you have filters in return duct registers. You will have 1 filter for the air handler.

If air quality is a problem when the system is on then your ducts are dirty. A furnace doesn't introduce anything to the airflow, it just pumps air in a circle, heating it as it moves the air.

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  • In the southeast it is common for all return ducts to be fitted with grills that accept filters and it is recommended that the filters be in place and be replaced on a regular schedule. Often the air handler filter is discarded.
    – RMDman
    Commented Mar 27 at 11:46
  • I have a filter in one 24x14x1 vent. I assume that's the air intake vent. The filter is MERV8 very old one. It looks so "gappy" that I don't think it catches any dust. I'll replace is with MERV13 filter when it arrives. I guess I might need my ducts cleaned. It's surprising how much the air quality drops when I turn it on. I'd think that any loose dust would get blown out of the ducts continuously and not really build up. I guess not.
    – pkout
    Commented Mar 27 at 14:21
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    If the system was operated without filter(s) in place, you would expect dust to accumulate in ductwork and heat exchanger etc. But it doesn't bind there as it does in the filter medium, rather it breaks off in chunks as the air rushes over it. Replacing the filter and upping the MERV are good steps, but until the dust is gone from the ducts, it'll keep coming out. (You could add filters to the registers to catch it. The more you filter, the more you restrict airflow, it's a trade-off.)
    – MadMonty
    Commented Mar 27 at 17:03
  • Makes sense. Thanks! I might invite a duct cleaning company.
    – pkout
    Commented Mar 27 at 22:03

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