Blower motor speed

This is a side issue -- a curiosity question -- that came up in the process of replacing the failing blower motor in my furnace.

My blower motor is a 5-speed. As far as I know, only two are used. I assumed that different speeds were used for heat and cooling.

This morning I noticed that, with the fan control set to AUTO, the motor runs at the lower speed. When I switch it to ON (continuous), it runs at a higher speed. Is that normal? Why is that? What's the intention?

I'd be interested (AFTER the new motor is in!) to learn how to optimize the blower speed. There must be some way to record plenum temp over time, to get the 'ideal' airflow. I'll set up the new motor like the old one -- but I'm really interested in this speed business.

Paul in East Troy

 

Your wish is my command

Most furnaces utilize only two of the motor speeds during operation. One speed for heat and one for cool. Usually the cool speed is either faster than the heat speed or is the maximum fan speed. The FAN ON speed is the cool speed of your system which is why it appears faster than the heat speed.

The heat speed is matched to your system to allow for the proper heat rise thru your furnace. The heat rise value recommended by the manufacturer is listed on the furnace ID tag.

The installer measures the air temperature leaving the furnace and subtracts the air temperature coming into the furnace. That value is the rise. It could be 35f-55f nominal.
Raising the speed lowers the heat rise and vice versa.

 

Cool.

So, I hook up the leads for the speeds that I suspect to be correct. I drill a couple of pinholes in inlet and outlet ducts (or perhaps there's an imperfect joint nearby), I put a thermocouple into each hole, run the furnace, put the meter in differential mode, and use the speed that most nearly approximates the heat rise on the nameplate.

I love objective processes! Thank you, PJmax, for continuing my education.

Paul

 

You've got the idea

 

Well, the new Fasco motor is in. R&R took just over two hours, with no special challenges. The new motor runs quietly and smoothly.

Thanks for all the good information and encouragement!

I have a somewhat subjective indication that I’m moving less air with the new motor than I was with the old. I’ll do the temperature rise measurement tonight first. Might as well be scientific about it!

Paul in East Troy

 

^Yah, generally a 50F split is decent for heating.

With respect to a/c if you've got it, high may not be the correct speed if you've got a small unit. I'm sure most installers just use the factory settings (signs of that are poor dehumidification and low temperature drop across the coil)

 

So, this job is done. The furnace is working just fine. My wife is happy.

I wanted to take a moment to 'close' this. My initial selection of blower speed was not OK. I won't go into the details of what I then did to slow down the process; suffice it to say that when I changed over the blower from low to medium speed, the temperature rise dropped right into the manufacturers nameplate range, and it's working just fine.

Thanks for all your support, pjmax!

Paul in East Troy WI