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!
William