If I install a furnace-mounted, electronic air filter, like this one,
will it harm my furnace due to reduced air flow? How will I know if it reduces air flow too much?
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.
Next you'll multiply that value by the CFM/10,000 BTU value for your type of furnace.
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.
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.