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When might a repurposed furnace fan require an airflow restriction?


William R. Walsh

Jan 1, 1970
Hello all...

My apologies if this is too far off topic. I've recently come into a few
furnace fans of the "squirrel cage" type. These fans were removed from
forced air furnaces that were taken out of service.

One of the three I have does not appear to require any flow restriction.
It's the oldest unit of the bunch. The next one obviously does--without a
restriction on the flow, the little motor running the fan will labor on the
lower speeds and a draw a ridiculously large amount of current on the higher
speeds. The current draw on that unit drops significantly and the fan speed
goes up with a partial restriction on the output. With an ammeter hooked up
to it, I tinkered with the amount of restriction until I found a point where
the current draw started to go up again after hitting a figure of about 5
amps on the highest possible speed. This also resolved the laboring that was
present at low speeds, so I'm pretty confident that this fan will run
acceptably and not burn out. The motor never gets very hot, and it sits
within the airflow made by the fan. (It's mounted inside the blower wheel
and held in place by four arms that screw into the outer casing.)

The third fan is an unknown, and I'm trying to figure out how I could know
for sure whether it needs a flow restriction or not. This fan is a unique
design with two small blower wheels and housings on each side of the motor.
The motor has a drive shaft sticking out of each side, and is positioned in
the center. As such, it is not cooled by the fans. While it seems to run
fine, and putting a restriction on the output doesn't a real difference in
the current drawn while this one is operating, the motor does speed up
somewhat. The motor does get hot to the touch, but not so much that is
uncomfortable to touch it for a long time.

As such, I'd like to know how it can be determined whether or not a given
blower fan needs a flow restriction to operate properly when current draw
figures do not indicate anything interesting. I'd also like to know why it
is that a flow restriction is sometimes required.

Thanks in advance to anyone responding to this post!