Every forced-air HVAC system needs some sort of air filter or cleaner to help remove pollutants from the air. Usually that’s some type of fibrous filter media – a typical furnace filter that gets replaced and thrown away on a regular basis.
But some HVAC systems have what’s known officially as an electrostatic precipitation (ESP) air cleaner – an electronic air cleaner. This type generally starts with a washable pre-filter to remove big dust and dirt particles. Then comes the electronic part, called the cell, which starts with high-voltage wires to charge the incoming particles and then has oppositely charged plates to attract and collect these particles. There are usually two pre-filters and two cells.
Here’s a picture of one common type of electronic air cleaner. This would be installed in the return air ductwork just before the furnace. Here you’re seeing one of the pre-filters (red dot) (the other’s been removed for better viewing) and one of the cells (blue dot).
These air cleaners can have a high removal efficiency for a wide range of particle sizes, although this depends a lot on how fast the system is moving air through the cleaner. And one very big advantage is that because air moves so easily through this type of cleaner there’s a low pressure drop and so a minimal impact on the air flow that your HVAC system is trying to heat and cool.
One disadvantage is that this type of cleaner can produce ozone and nitrogen oxide, themselves being indoor air pollutants, from the electrical operation of the system.
One other disadvantage is that it’s a huge pain to clean this kind of air cleaner, but it absolutely must be cleaned on a regular basis or its efficiency will suffer. You should clean it about every six months. If you have this type of air cleaner I recommend that you get the owner’s manual for it and follow the manufacturer’s instructions. Honeywell is a big manufacturer of this type of air cleaner, and in their manual (at least all that I’ve seen) they suggest cleaning the pre-filter with a vacuum cleaner and cleaning the cells in a dishwasher or in a big tub.
Because of the hassle of cleaning, most of them that I see have been abandoned, with all of the pre-filters and cells removed. Then the homeowner just uses a typical replaceable filter that slides into the old air cleaner’s cabinet.
Whatever you decide to do, please remember to either clean or replace your filter on a regular basis as recommended by the manufacturer. Your HVAC system will thank you.
If you’d like to read more about this topic in excruciating detail you should see this document from the Environmental Protection Agency titled “Residential Air Cleaners”.