Furnace Blower Velocity Survey for DIY Blower Door

This page covers how the flow rates for my furnace blower were established.  The furnace blower is used to power our homemade blower door.

homemade blower door

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In an attempt get an accurate flow rate for my furnace blower based blower door, I built a short rectangular duct that I attached to the blower outlet, and then measured the flow velocities in a grid pattern near the end of that duct.

measuring blower door flow rates
This shows the short duct attached to the outlet of the furnace blower.
The Velometer Pitot proble was used to measure the velocity at 75 points in a grid pattern.

I used an Alnor Velometer Pitot probe to measure the flow velocities in a grid pattern near the outlet of the duct.  I started out with about 25 points, but found that the velocity across the duct changed rapidly in places, and to be sure of getting a good average velocity, I went up to 75 points (5 rows  of 15 points).

Because the duct is short, the variations in velocity that result from being so close to the fan are fairly large.  The velocity profile across the short dimension of the duct (perpendicular to the blower shaft) for a typical section looks like this;

furnace blower velocity profile 
The 1 to 15 are points across the duct space about 0.6 inches apart, and the bottom axis is flow velocity in fpm.

Anyway, these 75 point profiles were done for all four blower speeds, and these are the resulting average velocities and flow rates. 

speed Vavg Airlfow
1 1427 1427
2 1650 1650
3 1946 1946
4 2540 2540

The velocities are in feet per minute and flows are in cubic feet per minute.

The duct outlet area is exactly 1 sf, so the exit velocity and flow rate are equal to each other.

If you want to see more of the details on all of the velocity profiles, download this Excel Spreadsheet...

As noted before, I compared the flow rates I measured with the published flow rates for a Fasco blower sold by Grainger.  This is how they compare:

comparison of furnace blower flow rates 

The blue diamonds are the flow rates I measured, and the red squares are the Fasco blower published flow rates.

So, this is pretty good agreement and leads me to think that there is a good chance that most half hp furnace blowers will have similar flows.

velocity survey on duct
Another view of the setup for the velocity profile.
The air velocity is measured at 15 points for each one of the 5 holes in the side of the duct.
The board with close fitting holes allows consistent placement of the Pitot tube.


Any Comments?....


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Gary February 24, 2013