The HVAC system has an Electrostatic air cleaning system and it uses a prefilter and has a slot for a post filter. I've been reading online that only the prefilter is good enough to get the job done at the rated efficiency.

In this Covid times, I'd like to make sure the air inside our house is healthy as possible.

My question then is: Would putting a post filter help with making the air cleaner? or would it just put an unnecessary strain on the system?

3 Answers 3


The only reason or use I could see for a post-filter on an electrostatic would be to run a carbon filter for non-particulate odors and/or ozone (which can be a normal byproduct of electrostatic filter operation) reduction.

  • I agree the permissible limit of ozone is .1ppm and most units limit there output to .05 ppm. The good thing is ozone breaks back down to oxygen in time. I have a grand daughter that has serious asthma and the only way to keep her out of the hospital is an electronic filter especially with the normal fires and smoke in Oregon, it used to be field burning they stopped that and now it’s Forrest burning almost every year the smoke levels outside become hazardous but not inside. A cheap pre filter to pick up the big particulates and the electronic is all that is needed.
    – Ed Beal
    Commented Jan 7, 2021 at 15:45

It's hard to say in your specific case but generally the answer is yes, more filters equal cleaner air. If you lined up 1000 filters you would have cleaner output than 1 but there are two problems:

  1. You have to push the air through 1000 filters which is a non-trivial problem.
  2. Is 1000 filters really better than 999, or 750, or 100, or 2?

You reach a point of diminishing returns where you are adding $X worth of filtering capacity but only getting a marginal increase in effectiveness. Now for some specific use cases that's worth it. For example in semiconductor manufacturing even a typical 70-90 nm (nano-meter) virus will impact a modern integrated circuit which has features in the 10-20 nm range (or even smaller).

But for your home most electrostatic filters should removed nearly everything. The pre-filter is to get any big stuff out that may cause the electrostatic filter to arc. And most electrostatic filters are effective against virus particles. So the current setup you have is likely to be as effective as you can reasonably be.

Adding the post-filter is not going to significantly change things because anything that gets through the electrostatic is likely going to be too small to be caught by a standard filter.

Bottom line is that if it makes you feel safer, do it. But I don't think it's going to make much difference.

  • More filters if not for a specific reason usually are a bad thing! First they reduce the number of air exchanges and even hepa filters can not remove smoke particulates an electrostatic will if regularly cleaned. So the reduction in air exchanges makes the air less healthy.
    – Ed Beal
    Commented Jan 7, 2021 at 15:25

The slot that you noticed is there to hold the pre-filter if the air flow through it was reversed and is not there to be used for a post filter. Running the furnace fan 24/7 is the best option as long as the electronic air cleaner is working properly. Since my wife can smell a fly fart at 1/2 mile, I use a cut to size air filter that has a filter + a charcoal filter to filter out smells. The one I use is called "The Web Odor Control" and -comes in a cellophane package. I buy mine from Home Depot or Lowes. I cut the filter media slightly smaller than the plastic web frame and use wire ties (4) to hold the media to the plastic frame. I put this filter in the pre-filter slot. I change mine every 6 months when I clean the air cleaners cells. I don't know how you clean your cells but a good way is to lay 2-2X4's in each rack of the dishwasher and place one cell per rack onto the 2X4's. This will clean the cells. Let the cells dry and reinstall in the air cleaner. I do not know if it is true but I was told many many years that these electronic air cleaners will actually filter out and kill viruses due to the high voltage in the cells.Hope this helps.

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