4 speed blower motor to a 3 speed blower motor

Old 01-29-14, 10:21 AM
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4 speed blower motor to a 3 speed blower motor

I was wondering if someone could help me, I recently had to replace my air blower motor which was a 4 speed 1/2HP 115V it had a black, white, red, blue, orange and 2 brown wires. The new motor that I purchased is a 3 speed 1/2HP 115V with again a black, white, red, blue and 2 brown wires. The wiring diagram for my Heil 9MPT furnace was as follows: Black - HI (Cool), Orange - MHI (Heat), Blue - MLO (Heat), Red - Low (M1), White - Neutral), Brown - capacitors.
The wiring diagram on the new 3 speed motor is as follows: Black - High, Blue - Med, Red - Low, 2 Browns. How do I wire the new motor.
Old 01-31-14, 10:30 AM
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All else aside (motor compatibility etc) the obvious reconnection would be to just wire it up the same without any orange connector...since the motor doesn't have one for the MHI speed.

You do this at your own risk...seems to me there's a motor compatibility issue here perhaps. All I'm doing here is stating the obvious. Not suggesting you should do this in any way.
Old 01-31-14, 11:43 PM
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The wiring diagram only shows how the blower motor is wired at the factory.

Every duct system is different, and it's up to the installer to adjust the speed if needed.

Wire the new motor the same way the old one was wired (cool->hi, heat->medium), then measure the temperature rise (see A DIY Test for Furnaces | Structure Tech Home Inspections).

If the temperature rise is below what's printed on the nameplate, reduce the speed and re-check it. If it's above, you've got a problem; ideally, you want it to be in the middle of the range.

As for a/c, if you've got it, high is probably the correct speed for most systems. If you have a smaller a/c (like 1.5-2 tons), a lower speed might be required. But too much airflow is better than not enough.

Of course, a 4 speed motor gives more flexibility with respect to getting proper airflow - I would only "downgrade" the furnace as a last resort.

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