Adjusting IDF motor pulley to FLA ratings of RTU

[Brraapp26]

Hello,

For years I've adjusted IDF motor pulleys to the FLA ratings of the blower motor with the theory of I'm loading my evap coil up more and getting the system to its peak performance. I've had techs tell me I was incorrect with this but never offer any true explanation as to why. So am I right with doing this and making an improvement in the system, or am I incorrect in doing this? The systems work fine when I do this with no complains, I just felt I was making a system better by doing so. I know I wont dehumidify as well increasing my airflow over the evap coil, but I felt I am loading the coil up more, hence removing more heat from the space. I just want to have some educated opinions on this... Thanks...

[rscamaro]

Hello,

For years I've adjusted IDF motor pulleys to the FLA ratings of the blower motor with the theory of I'm loading my evap coil up more and getting the system to its peak performance. I've had techs tell me I was incorrect with this but never offer any true explanation as to why. So am I right with doing this and making an improvement in the system, or am I incorrect in doing this? The systems work fine when I do this with no complains, I just felt I was making a system better by doing so. I know I wont dehumidify as well increasing my airflow over the evap coil, but I felt I am loading the coil up more, hence removing more heat from the space. I just want to have some educated opinions on this... Thanks...

I will set up a pair of sheaves/pulleys to run FLA only on initial start up. Then someone needs to do some air balancing, because someone designed the system to run at whatever CFM through each register and that's what it should have.

Other than that the sheaves when replaced get set up the same as the originals, where I hope that somewhere in the past there was a air balancing done to the system.

Just make sure that the customer doesn't have any issues with the increased noise if you do decide to increase the airflow. For your own peace of mind if nothing else, other than the callback you might get.

...Ron

[jayguy]

...I know I wont dehumidify as well increasing my airflow over the evap coil...

that is the problem isn't it? sure, you may get a greater load on the coil, but you may not satisfy the humidity conditions as well. the thermostat will satisfy but the humidity may be too high. to account for this, your customer may lower the thermostat setting to a much lower temperature to make the ac run longer. this will cause the system to use more energy. your customers may not complain to you, but their bills may be going up. on the flip side, you slow the air flow down and you don't satisfy the thermostat or cause the coil to freeze. it is a fine line some days.

[MicahWes]

You have to make sure you are taking readings with the panels on as well. A forward-inclined blower will unload at higher static.

[hands]

You need to set it up for the design CFM of the unit and the space.

[ACFIXR]

You need to set it up for the design CFM of the unit and the space.

[waregl82]

I would definitely set the blower for the design airflow. Airflow is the first thing I like to confirm otherwise most other measurements and data collection is meaningless, somewhat.

[GT Jets]

I am going to try to respond to your post without using my first prick card of the year.. Bear with me.

Hello,

For years I've adjusted IDF motor pulleys to the FLA ratings of the blower motor with the theory of I'm loading my evap coil up more and getting the system to its peak performance.

You are 100% incorrect in adjusting the IFM to FLA... There are literally dozens of reasons why.

I've had techs tell me I was incorrect with this but never offer any true explanation as to why.

Just because they don't have the exact terminology does not make them any less right...

So am I right with doing this and making an improvement in the system, or am I incorrect in doing this?

In most cases you will be "hurting" performance more than "improving" it...

The systems work fine when I do this with no complains, I just felt I was making a system better by doing so. I know I wont dehumidify as well increasing my airflow over the evap coil, but I felt I am loading the coil up more, hence removing more heat from the space.

You could be loading the coil less....

Some of the reasons for not doing this practice;

Condensate carryover due to coil face velocity
Major loss of latent performance
Potential loss of filter performance
If it is a gas pack, loss of heat performance
Potential of over speeding the blower wheel causing damage to the unit
Excessive noise
Premature failures of the heat exchanger
Potential overloading of the DX system

The performance is measured using CFM, not fan HP. Do the math. You are taking the lazy way out.

I just want to have some educated opinions on this... Thanks...

If you read the installation and maintenance manuals supplied with your machines, you will immediately understand that what you are doing is fundamentally wrong... Willing to bet nowhere in the O and M does it state "adjust the blower speed to the full load amperage of the blower motor". Why do you think that is?

Hope this was both constructive and insightful.

GT

[beenthere]

While you may be increasing the sensible capacity of the unit, you are also decreasing the latent capacity. And in some buildings that will cause them to set the stat lower and make them use more electric.

The factory original blower motor in a unit is selected to meet and exceed the units CFM requirements at rated TESP. And should only need to be running at FLA when the duct design is restrictive enough that the TESP is above the units rated TESP.

Look at the fan charts/data for the units you are working on. You'll see they often are moving the CFM they need at amp draws under the FLA of the motor.

[ryan1088]

Fan curve for sure, although everyone I work with adjusts based on amps. We don't even have a tach....