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 !
Best Answer
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.