Blower Fans Speeds

[Freon]

You need to think of the velocity of the air as it passes through a filter. The higher the velocity, the more frictional effects from the filter and therefore the more work the blower has to do to move the air through the filter. A simple example. Say you had a blower the produced a flow of 1000 cfm. Now assume you have 1 filter that's 24 inches by 24 inches... 2 feet by 2 feet. The area of the filter is 4 sq feet. Therefore to move 1000 cfm of air through that 4 sq feet of filter the air's velocity (speed) has to be 250 feet per minute. 250 feet per minute X 4 sq feet equals 1000 cubic feet per minute. Think of the air as a block 4 feet square on the front and 250 feet long. That block moves through the filter every minute.

Now say you have 2 filters, each 2 feet by 2 feet... a total of 8 sq feet. We still have 1000 cfm of air flow but now the air can travel through the filter at a velocity of 125 feet per minute. We have cut in half the air velocity. And friction is usually related by the square of velocity. So by doubling the filter area we may have cut the wasted energy of filter friction by a factor of 4. And this is only the friction due to air velocity, not the MERV rating.

Simple rule of thumb is sizing filters... 2 cfm of air flow per inch of filter area. You want a very low velocity for the air flowing through the filter. Many installers go the cheap route and only use the filter in the furnace. A filter grille is a return grille with a door that can accept a filter. The more work the blower does circulating air and not fighting frictional effects of the filter, the better your system will be.

[JTRFLASH]

ok I think I got it.

By increasing the area of filtration you have reduced the friction and than the air can move more freely. I had it reversed thinking by adding more filtration I am reducing flow, this is all very interesting.

Am I on the right track now.

[Freon]

Yes, the more filter area the better. That's why 2 filter grilles with 16x25 filters is much much better than one 16x25 filter in the furnace. Stay at MERV 8.

[JTRFLASH]

Thanks again, MERV 8 it is.

Funny I notice that a reply I sent to you is missing, I wonder if others have not be sent also.

Did you see the one at the end asking you for recommendations on filter grill vendors.

[Freon]

Here is one of the - w w w dot hartandcooley dot com

If that doesn't work google Hart and Cooley. Then search their site for filter grilles.

[JTRFLASH]

Hi Freon, I will look for them also, I found a place called HVAC something.

14 x 20 grlles are about $22 each I would need 6, they are called TRUaire 190s, also I found the MBR 600 - MERV 8 filtes for the grilles.

They will delever to Canada, perhaps the place you mention I can purchase the filters with the grilles, without the filters they are usless.

So I talked to my wife and she thinks this is a lot of work to retrofit the grilles.

I can see that more filter area means more air through the furnace better distribution of air I guess.

Makes sense that the furnace filter can only flow 800 CFM when the blower is capable of much more. I could see premature failure of the blower motor, the strain on it.

but what are the true benefits

[Freon]

You only need as many grilles (2 at most) to have the air flow through the filters be at the optimum rate. You may need filter grilles that will fit between the wall studs. So a 16x25 filter grille might work depending on your wall stud spacing. Use the 2 cfm/sq in of filter area to determine your total filter area required. 2@ 16x25 filters (800 sq in) can handle 1600 cfm of air flow.

[JTRFLASH]

HI, ok interesting.

I measured my present returns and they are all 14 x 10, between the studs is the plenum, but I thought I would expand to 14 x 20 grilles, I can expand upward and remove the drywall for the larger opening.

There is only 14.5 inches between studs on 16 inch centres. The grilles I looked at are 14 inch inside dimensions so they would work perfectly I believe.

14 x 20 would equal 280 cfm per register, 6 registers 1680 cfm, correct?

[Freon]

A 14x20 grille with a 14x20 filter is 280 square inches of filter surface. At 2 cfm per sq in of filter, each filter grille can handle 560 cfm. 3 Grilles will handle approximately 1680 cfm.

[JTRFLASH]

Thanks for everything.

So I have had a discussion with the manufacture technical support, they have helped me somewhat in saying I have a restriction (no kidding), I asked well can you tell me where the restriction is based upon my tests.

They sent me a picture of a manometer, one end on the return air side and one on the output side.

So can you tell me what I am looking for? Will this tell me where the restrcition is based upon the readings?

I will find one or rent it if I can.

I think we agree we may have a return air restriction but the tech support fellow suggested it could be forward of the blower on the output side. We have to remember when I had the blower door off the temp rise dropped 8C.

Suggestions? Thanks Jim