Help understanding the relationship between motor HP and CFM

DL_BC

Hello folks,

I have a bit of an engineering / controls related question, so this might be in the wrong category; apologies if so. I'm trying to better understand a slight headscratcher and hope someone can help provide clarity.

We have a hybrid heatpump where it was shipped with the incorrect ECM configuration, meaning the unit is programmed for a 3/4 HP motor, but it has a 1/2 HP motor.
This is a constant volume ECM configuration.

So, in this case if the program is for a larger motor, when the programming is targeting a specific CFM (say 800 CFM) would the measured actual CFM output from the lower HP motor be higher than expected, or lower?

Assume that the system was designed for the lower HP motor.

Zman

This would be a better question for the manufacturer but...
If the unit has air pressure sensors that are separate from the ECM motor, the number should be accurate. If the controller is using feedback from the ECM motor to come up with the number, all bets are off. My guess is that it would under-report the CFM but it is hard to say, that's all determined by proprietary programming that is baked into the controls.

DL_BC

Zman said:

This would be a better question for the manufacturer but...
If the unit has air pressure sensors that are separate from the ECM motor, the number should be accurate. If the controller is using feedback from the ECM motor to come up with the number, all bets are off. My guess is that it would under-report the CFM but it is hard to say, that's all determined by proprietary programming that is baked into the controls.

Thank you for the response. I agree it's better for the manufacturer to answer, but not always an easy task to get past technical support / application support to the Engineers.

This would line up with what I was seeing, where the controller reports 800 CFM, but the total CFM output for the space is measured by the balancer at ~1100 CFM. I'm just unsure if it is a separate unrelated issue and wanted some insight.

ratio

I'm not sooper convinced that the 'CFM' settings on ECM motors are all that accurate on an absolute scale. I haven't made a study of it, but I wouldn't be surprised if they were ±30% (which is in line with with what you're seeing, BTW…)

Cypher up a K factor & see if setting the airflow as adjusted by that factor works. Or just let the air flow guy close dampers until it's down where it's supposed to be. (<- no, don't let him do this, make sure he adjusts the ECM motor instead!)

Big Ed_4

I am sure you need to get into the installers program and set it for your system. This is normal in todays equipment . Either read the installers manual or do what I do, call tech support .

EBEBRATT-Ed

You probably need to get into the equipment controls program and adjust it.

This is common with VAV units on commercial jobs where you start the AHU get all the vavs wide open and then have a balancer measure the airflow at each vav. Whatever he gets for airflow will probably differ from what the controller thinks is going through the vav box so the control guy adjusts the controller to make the reading match so it will control properly.