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Comment by smmnyc | Hacker News Reader

Is there an advantage to attaching the filter to the front of the fan so air is blown through it (as shown in the video) versus taping it to the back of the fan so air is sucked through it? Just curious if that would make any difference. It might be easier to get the filter to stay if it's getting sucked to the fan.

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I would guess that it works similar to a water pump- pushing is always more powerful than sucking - but check this out for some detailed analysis (and more data on DIY air filters) https://smartairfilters.com/en/blog/where-should-an-air-filt...

I never would've guessed turbulent air would be more difficult to push but it makes total sense! Great find.

Excellent page.

I can think/guess two reasons:

1) If the filter is at the back, since the air is pulled through it, the dirty side is outside where people and kids can touch it as it builds up. If the filter is in the front, the dirty side is inside (towards the fan blade) while the clean side is outside.

2) If the filter is at the back, due to air's resistance it is possible, due to nooks/gaps in the fan, that the some of the air may be pulled from elsewhere and not all the air may pass through the filter. If the filter is in the front, it is easier to seal between the fan and the filter to make sure all air is pushed through it.

always wondered this myself... #1 - good point!

I'm curious... doesn't #2 cut both ways? i.e. if you can seal the front, you can seal the back?

I can confirm what others said about air drag. I've done this in the past, assuming I needed the filter on the input side. I had to use two filters, some cardboard, and plenty of duct tape to create a triangular input in order to get a decent amount of airflow.

I wish the guy on the video would take an air reading measurement near the side of the filter. I'm guessing that a lot of unfiltered air is escaping out the sides.

FWIW if you tape the filter to the back then the fan doesn't get dirty as quickly. [Every now and then you should clean the accumulated dust off a fan.]

BTW you need not cover the entire area of the fan with filter material. So, for instance, if your fan's breaker is shutting the motor down b/c of the load of the filter, you can slide the filter up to cover less of the fan with the filter, reducing the load on the fan motor. While not all air passing through the fan will be filtered, over time most air will eventually pass through the filter and the room will be cleared of particulates.

Similarly, it is not necessary that the filter be the same shape or size as the fan, since with each pass at least part of the airstream is cleaned. Eventually all the air in a room will pass through the filter area.

Not sure, but I'd think it would also help to keep your fan clean.

I’ve actually built this fan “mod” and when I change out the filter, I’m going to try attaching it to the back instead.

With the filter thoroughly taped to the front (nearly airtight), there’s a large amount of blowback. I suspect that this is reducing the overall efficiency of air filtration.

With it taped to the back, you don't get blowback, but there is significantly reduced airflow out the front. I used a filter of this type for ~2 weeks during the Thomas fire. Worked very well, though the proper filter I received from Amazon a week after I built this worked better. Two is better than one, though!

Placed in front it would catch particles coming off the motor like lubricant, bits of metal from bearings and carbon particles from the brushes if it's not a brushless motor.

Perhaps strain on the cheap box fan motor as it tries to pull its air through the filter. I've gone through a number of those style of cheap box fans over the years, any kind of long-term resistance very near the intake seems to cause the equally cheap motors harm and shorten their functioning life considerably. I haven't seen the same motor damage problem with resistance added to the exhaust side. In absence of that experience I would have guessed that similar resistance on either side would harm the junk motors all the same.

I'd agree with this.

I used to put the high-efficiency filters on my furnace, my HVAC guy said these filters are so heavy that they put a strain on the blower motor. It's better for the motor to use a plain blue fiberglass filter (MSRP $1). You're not really filtering that much out of a closed house, unless you run your forced-air A/C or Heat with the windows open.

We upgraded to a "media filter" when we got new HVAC at my house. IIRC they are 4 inches thick, which gives a lot more surface area due to the deep pleats. You can then run fairly tight filters without putting undue strain on the motor - plus they last quite a bit longer. It made a moderate difference in our house in regards to dust in the air, but we also have 3 pets and live in a very dry, dusty area.

I did the same thing with my furnace. Just to try it I switched to a high merv filter rating and could immediately notice the considerable reduction in air flow that it was causing (and presumably increased strain on the furnace). Seeing as it was a bit more restrictive than I expected, I ended up ditching that and going back to a weaker merv rating filter (haven't gone back to the blue fiberglass style filters, just going with a weaker merv seems to do the trick, haven't had any problems with increased wear on the furnace in the last ~8 years).

I'd peruse this research on High-MERV Filters and their impact on airflow and pressure instead of taking one HVAC technician's word for it: http://www.homeenergy.org/show/article/nav/issues/page/4/id/...

This graph of airflow vs. MERV rating isn't very conclusive in my mind: http://www.homeenergy.org/UserFiles/file/26-6_p34big(1).jpg

Granted there are about a million variables in all HVAC systems that could affect the results.

You know, I'm going probably believe the guy that replaces blower motors day in and day out for a living as opposed to the group that sets up a "test box". Sorry.

I think the answer is yes, but I'd also like to to know why.

probably makes the fan quieter.

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