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View Rieland's profile

Shop Air Filter

by Rieland
posted 08-19-2017 12:35 AM

7 replies so far

View clin's profile


1121 posts in 1881 days

#1 posted 08-19-2017 04:29 AM

Good question. I don’t have the answer. But you might do some research online. I would think blower manufacturers would have some guidance on this.

3” sounds a bit tight, but I’d look at it this way, just to provide some perspective.

The intake on each side of the blowers is X square inches of area. While to some extent the air can come in from every direction, it of course is really going to be coming mostly straight in from one direction (end of the enclosure where the intake and filters are located).

So I would look at this as a straight shot and make sure that area is at least as large as the intake of the blower. I know that makes almost no sense without a picture.

But here are some numbers.

Let’s say the intakes of the blower are 10” diameter circles. So that’s an area of PI * r^2 (PI r-squared). So 3.14×5” x 5” = 78.5 sq-in.

So looking at the straight path of the air coming in sideways to the intakes, if you have a 3” clearance, that path would have an area of 3” x 10” (diameter of the intake) for 30 sq-in. So this would seem to be restrictive.

This of course ignores the need of the air to make a 90-degree turn. But also ignores the fact that the air will come in from several directions. Though I don’t think it would be a full 360 degrees. Or at least most of the air will come from the direction of the filters.

I’ve never seen a furnace with only 3” of clearance on the sides of the blower. But I’m also guessing the blower is on the small side for a typical furnace.

You could also do some experiments. Just cobble together some plywood to form a box and see what you can get away with. These types of blowers are a bit counter intuitive in that they actually unload with more restriction. So if you are restricting it too much, you’ll hear the blower speed up.

Another approach, just go have a look at a real furnace, scale yours to match. If your blower is 2/3 the width of the furnace, make your clearance 2/3 the same clearance, or more. That would at least give you something to hang your hat on.

While I have a Jet filter, if I were building my own, I’d consider increasing the area of the intake filters. While it is common to just use one flat side of the box for the intake, there’s no reason to restrict it to that. For example, imagine butting the intake side of your box up to another box. This box would have 5 of 6 sides available that could have filters on them.

The larger the surface area of a filter, the less it will restrict the air flow. It will also take longer to clog. But of course, once you do clean or replace them, you have more filters to deal with.

-- Clin

View Woodknack's profile


13503 posts in 3265 days

#2 posted 08-19-2017 05:03 AM

You don’t need any space, except for filters if they are on the intake side. But all the blower motors I’ve had needed to be restricted somewhat or they would overheat, which filters will do if you have enough.

-- Rick M,

View Rieland's profile


3 posts in 1166 days

#3 posted 08-19-2017 05:20 AM

Actually that all makes a lot of sense to me. It isn’t a huge blower 120v 3 speed 18×18x10 with a 9” intake on the sides. But I feel it will be more then enough for my shop. So going with that math and a straight shot I would need around a 7” clearance on the sides. Which is getting wider then I was wanting.

Rick I was actually thinking of that as another option. Figured I could keep a smaller clearance if I rotated the blower and have 2 sets of filters (one set of filters on each intake side) it would be less restrictive and could probably keep the unit closer to the size I want.

I guess I will just have to do like Clin suggested and just try and rig up a rough box with some scrap and see how the motor reacts.

Thanks for the replies will have to keerp thinking this out and try and see what workss best.


View Carloz's profile


1147 posts in 1476 days

#4 posted 08-19-2017 11:53 AM

My Jet has a regular filter first similar to those used in the house ventilation and a secondary fine filter that looks more like a bag. To accommodate that it is much longer than your design.

View GR8HUNTER's profile


7927 posts in 1597 days

#5 posted 08-19-2017 01:36 PM

not sure if you looked at mine or not …but Rick is dead on with the restriction …furnace blower motor like to be restricted …that being said you could always put a grill on the outside on exhaust side … as far as size it does not really matter …..what I did is measure my fan ….then went to Lowes online just to see what common size filters there where and ended up with 22×22 ….as far as length don’t forget to figure in all filters and spacing like my spacing is 3- 1inch filters and 4 – 1 inch spacers …hope this is clear for you ….if not I will try and explain it better :<))
Welcome 2 LJ’s

-- Tony---- Reinholds,Pa.------ REMEMBER TO ALWAYS HAVE FUN :<))

View Fred Hargis's profile

Fred Hargis

6508 posts in 3378 days

#6 posted 08-19-2017 03:41 PM

I don’t think it’s worth worrying over, the furnace cabinet it was in was probably smaller than the box you build. I used 3 filters on mine, the first was a cheapo pleated filter to catch the coarse stuff. Then A filtrete red, followed by a Filtrete Ultra Allergen. If I was doing it today (and I will be soon) I would go for one of those 3”+ thick job, preceeded by a cheapo filter.

-- Our village hasn't lost it's idiot, he was elected to congress.

View Rieland's profile


3 posts in 1166 days

#7 posted 08-20-2017 05:35 AM

Tony yours actually was one of the ones I looked at on here. And basically doing about the same thing. Rigged up a test box today with some cheap filters just to get an idea. And tested the amps to make sure they dropped. Motor didn’t sound like it was running too hard so I think I will be going with a 20×20 filter. Going to go with a 2 or 3 filter setup.

Thanks for the replies everyone it helped me figure out what I wanted to do and make sure i was going about it correctly.


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