I built a DIY air filter by fitting a Merv-13 air filter inside a cardboard box. It's held in place by wood strips cut and glued to the box and I cut a hole for a 200mm case fan with a 12v power supply. The result is an air filter that operates continuously in the room I am in, but unlike other solutions is completely silent.

My question is: When should I change the Merv-13 filter? The recommendation on the box is every 3 months, but that's for a filter working over an entire house, not with a single 200mm fan pulling air slowly. If I can still feel air coming off the fan, does that mean mold spores and other allergens are being trapped?

  • Have you replaced the filter for your furnace? You know what a dirty filter looks like, when your DIY project filter looks like that, replace it. Or, maybe a bit before it looks that bad... – FreeMan Oct 15 '20 at 15:02
  • Get a magnehelic gauge and measure the actual pressure differential across the filter? – ThreePhaseEel Oct 16 '20 at 0:03
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    I agree with magnahelix but you can make a u tube manometer with a length of clear tubing and some cooking oil (water evaporates so I have used oil) once the pressure gets above 3-4” it would be time to change the filter. With a very small fan it may be a smaller difference but I have made these for furnaces I have installed so the home owner could see at a glance if there was a high back pressure on the filter, I used to use water but it evaporates , I have tried a film of oil on each side that grew things in the water but straight oil has worked for years. – Ed Beal Oct 17 '20 at 18:09
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    I couldn't find the link again, so this isn't an answer, but I read a scientific article on air particle filters that said that filters generally increase in filtration performance as they get more clogged. If this is true, the time to change a filter is just when it doesn't pass enough air (has too much back pressure) for the system it's in to operate correctly. Unfortunately, for a room air filter, you can trade off air volume with filter effectiveness (filter some air poorly many times or very well a few times), so that doesn't give us a nice easy to specify rule in this case. – Kevin Reid Oct 22 '20 at 0:40

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