As we are now getting to the point to where all furnaces and air handlers are starting to come with ECM motors we will be of course replacing them. This is typically going to play out as a size for size swap out. We would normally think that it should be fine and we should not have any issues.

Well....

 

Based on numerous job site visits it has occurred to me that we are not understanding the differences these motors make. The PSC motor would typically start to slip in higher static systems. We "got by" with it, but when the replacement happened we notice that things were not all of a sudden working...

 

ECM Motors:

 

Standard - Constant Torque - Goes to a speed setting and doesn't fall from it.

Upgrade - Variable Speed or Constant CFM- Set the CFM desired and it goes there.

 

SO... with that in mind, the duct systems were typically too small and the constant torques motors would push higher static to maintain speed. Two problems.... number one...NOISE!, and two... higher static which means less cfm and more energy needing to be exerted by the motor. So we will need to take a static pressure reading of the system and verify on the blower chart what cfm we are getting.

 

Constant CFM gets you the airflow but again at the cost of noise plus high energy usage and potential pre-mature breakdown of the motor itself.

 

So be careful with your quoting, be sure to do your due diligence by making sure you have the proper amount of ductwork to make it work well.

 

For Heating:

CFM = BTU INPUT/1.08*^T

 

For Cooling:

400 CFM Per Ton

ex. 3 Ton AC Needs 1200CFM

 

In all change outs I try to tell people that you have one of two choices....

1. Make the new furnace/ac fit the old duct system - This could have implications of the the system not being big enough

2. Make the existing ductwork fit with the desired equipment selection.

 

Then consider doing a startup sheet to verify the first time everything is working.

 

Lets push on and do better.