You'll have to find a unit that can match the required intake air flow of the furnace.
Estimating required air flow.
To estimate the required amount of air the furnace needs, you'll have to know what type of furnace you have, and how many BTUs the furnace is rated for. Once you have this information, you'll divide the furnaces rated BTU value by 10,000.
100,000 BTU / 10,000 = 10
Next you'll multiply that value by the CFM/10,000 BTU value for your type of furnace.
- Natural Draft Furnace = 100 CFM/10,000 BTU.
- Induced Draft Furnace = 130 CFM/10,000 BTU.
- Condensing Furnace = 150 CFM/10,000 BTU.
130 * 10 = 1300 CFM (Cubit feet per minute)
So if you had a 100,000 BTU induced draft furnace, you'd need a filter with at least an output of 1300 CFM.
As the filter gets dirty, air flow will naturally be restricted. Because of this, you'll want to choose a filter system with a higher CFM rating than you need. In our example above we wouldn't want to get a unit that is rated at 1300 CFM, we'd want one rated somewhere closer to 1400 or 1500 CFM.
Measuring Air Flow
You can determine the actual air flow using a handheld Anemometer, like this one
Start by measuring the area of the duct (Length * Width). Next measure the air speed using the anemometer (ft/min). Finally multiply the area of the duct by the air speed.
(20" * 25" = 500 sqin.) / 144) = 3.47 sqft.
3.47 sqft. * 375 ft/min = 1301.25 CFM
To determine if the air filter is restricting air flow, calculate the CFM both with and without the filter connected.
Watching for signs of reduced flow.
If you notice any of the following problems after installation, you likely have restricted the air flow too much.
- Furnace has trouble producing a flame, or flame is small.
- Furnace constantly overheats.
- Furnace struggles to keep up with heating demand.
- Output air flow is weak and/or cold.
Best Answer
You will still want a particulate filter at the intake, I was taught to put particulate filters at each inlet position this helps keep the ductwork clean and prevent build up in the heat exchanger.
Electronic filters are the next best thing in my opinion but just like a 4” MERV 8 they need to be replaced regularly, the electronic take a few minutes to clean. The mesh grid helps with the particulate matter much like the dust filters at the intake but the air flowing through the mesh takes the air to a discharged level, now the air passes over the high voltage grid and the Remaining dust - pollen , mold spores and even smoke particles are trapped in the static grid. There is very little resistance to air flow and these are effective at cleaning the air as long as they are regularly cleaned. My last home I installed this system because of a granddaughter that had severe asthma, this system kept the air in the house clean and inside she never had attacks, I pulled the mesh and grid panels once a month and rinsed them in a shower, it is amazing how much dirt was on them every month. I replaced the 3 pre filters (cheap fiberglass Every 3 months) If I had not seen seen the amount of dirt coming off the electronic grid I would not have beloved how well that system worked, we really noticed when the valley was filled with smoke and my granddaughter was fine inside the house I noticed the system starting buzzing only 2 weeks after cleaning I pulled them and cleaned again they were pulling all the smoke from the fires out of the air I was impressed with how much the system removed from the air, I did run the air handler 24/7 when our granddaughter was with us and she had less need for a nebulizer and fewer trips to the ER so I recommend them for that reason but yes they have less resistance than a high MERV filter.