The idea of stacking air filters is a tricky proposition. With all these HEPA filters on the market , one may think a better flow can be achieved and better quality of air as a result of finer air-filtration in an existing system. Not unless you have a clean duct system that includes a clean return duct to the furnace/Air blower unit. You can over heat the motor and blow a circuit if you’re not doing it properly. My question is : why are there so many systems with so poor return ductwork ? The filter at the blower exchange on the system picks up all the crap in the walls ! Cobwebs, dust bunnies, insects, hairs, plaster powder, you name it ! You can’t improve air quality by just putting a HEPA filter in that Return environment! What are these installers thinking . Just as long as the Mfg. unit has its filter in place , that meets the warranty requirement ? The HEPA filter is going to clog up , like tomorrow ! With a closed system, wouldn’t it be a whole lot cleaner ? The mfg required particle filter size at the unit could be maintained while a second closer monitored HEPA filter could be at the register upstairs. Wouldn’t that be efficient and cleaner air all around ?

My friend said her installer poopahed that and said she’s jeopardizing the system. Meanwhile the return filter gets all this dirt from inside her open wall duct channeled through her house ? That’s a pretty powerful blower too. I say this because we put a filter upstairs and it’s cleaner than the filter at the unit downstairs. The problem was the unit filter clogged anyway and the breaker blew !

  • The system needs a clean return channel to make it efficient so all the return air is actually only from the inside living area of the house and not it’s walls. That way a series of filters can be put in place, with the smaller particle filter being the one needing monitored and changed the most frequently. ( every 3 months) Dec 27, 2020 at 18:34
  • I think air filters are also used for the occupants comfort. As you stated there's alot of air-born irritants floating around. Rather than having them endlessly recycled through the system the filters trap them keeping them from offending eyes and noses.
    – ojait
    Dec 27, 2020 at 18:48
  • Really short answer: Laziness and apathy.
    – lordadmira
    Dec 27, 2020 at 21:02
  • I thought there would be some codes regarding air quality on these install; but, it seems it’s only about the units functional warranty . The mfg. can’t depend on the various installers to protect their product, so they require the filter attached on the unit itself to be changed. That filter is about 3 micron to10 micron. Nothing sub micron or HEPA level is recommended because it would clog very quickly and affect the system’s function. That submicron dust etc that gets through 3 microns gets blown out into the living space via the room registers. It’s like someone sneezing on you ! Dec 27, 2020 at 22:20
  • If the filter is not on the intake side preventing debris from entering the heat exchanger and turning into ash / shortening the exchanger life the filters don’t d much good. When a filter is plugged the fan speed increases because there is not much load on the motor the high speed can damage some motors but not normally squirrel cage motors. HEPA filters on most air handlers actually leave the air dirtiest because of less exchanges some HVAC techs don’t have this training but understand higher density do more harm than good unless the system is designed for it.
    – Ed Beal
    Dec 28, 2020 at 1:38

1 Answer 1


Filtering is not only a science but a big cost impact on system design. I install filters at the entrance to the trunk duct then add electrostatic at the furnace entrance, some put high MERV or even HEPA at the furnace and here are the reasons;

The problem with HEPA filters is the pressure drop across them if used at the entrance trunk ductwork will oil can or pop because of the pressure, flex duct will pull away from the vents so it is not lazy that they are not put at the entrance it is for system performance and longevity at the furnace location, High MERV filters change the load on the fan motor by not moving air the speed actually increases the speed and reduces motor life.

HEPA is almost as good as electrostatic but after installing the electrostatic is much cheaper and will take even smoke particles out that HEPA can not.

Electrostatic / fiberglass entrance equals a lower combined pressure drop a simple fiberglass filter at the entrance and electrostatic at the furnace provides the cleanest air especially if the system fan is left on for constant cleaning. Allowing the fan unit to move air efficiently more exchanges per hour.

To maintain a electrostatic filter the elements need to be rinsed off I do mine monthly or any time I hear popping in the filter (dirt bridging the insulators)

I mentioned after install the electrostatic is less expensive but the cost is higher up front it takes a few years of replacing high end filters for the electrostatic to be less expensive and this cost is the reason the best filtration is not installed in all systems. Making a opening in the duct at the furnace is simple and cost almost nothing for the filter 1” to 4” no difference (Installing the electrostatic is just as easy on a new system). So again it comes down to cost.

Where lazy or a lack of knowledge comes in is not knowing the air handlers “flow” curves. Installing a filter with a higher pressure drop can reduce the motor life as suggested above but locating filters close to the fan filter reduces the high pressure on the trunk line, to maintain dust & dust bunny free trunk lines the inexpensive fiberglass filters at the entrances work quite well without the negative effect of a high pressure drop.

  • All that said, would you not want a cleaner pathway to the return of the unit ? My furnace unit is under my house in a crawl space. I’m not going to crawl down under the house to change filters because of proximity placement to the exchanger. It seems logical that cleaner filtered air coming through the return is going to give cleaner air back to the living areas. Dec 28, 2020 at 18:15
  • Electrostatic feature seems like an excellent idea especially for heated air and for those of us who have wall to wall carpets too. Dec 28, 2020 at 18:21
  • Gerry a high MERV filter at the inlet in your case may collapse the ducts if metal trunks they will buckle and “oil can” with each cycle creating quite a racket. If flex duct the ducts usually break away because the vacuum deforms and pulls the duct cutting the plastic covering then the air from that space is pulled in. I know of 1 person that was a clean room operator that did just this in an attic, less than a year later the heat exchanger failed and although a top brand it was not covered by warranty because of the fiberglass was the cause of the failure all from a HEPA filter on flex duct.
    – Ed Beal
    Dec 28, 2020 at 19:00

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