Variable speed fans in Packaged Units over 10 tons

Variable speed fans in Packaged Units over 10 tons

On a recent job I specified two 10-ton rooftop units.  The application was a Class 1000 Clean Room. I wanted constant volume rooftop AC units—initially, air handling units were discussed but because of budget constraints we had to settle for packaged units.  Later, when I reviewed the submittal that came from the contractor I did see the wording ‘Multi speed fan’ but did not get alarmed as many manufacturers provide units with low, medium, and high speed fan settings that is just a one-time adjustment at the start-up.  It turned out that because of the latest Title 24 mandates, all AC units over 10 tons now need to have varying speed fans.  The fan speed is varied based on the staging of the compressors.  In the old days, a ten ton unit will supply, say, 4000 CFM all the time, and the supply air temperature will be changing based on the staging compressors, but now the supply CFM will vary based on the staging of the compressors and the supply air temperature will remain almost constant.

This Department of Energy documents describes the changes meant to increase the efficiency of rooftop units:

http://www1.eere.energy.gov/buildings/technologies/pdfs/emrgtech11_shen_040313.pdf

In short, the AC units we received had variable speed fan motors.  Now the challenge was to change the fan to constant speed.  The first set of instructions from the manufacturer on changing the fan speed to constant did not work.  These were the instructions.

>>Unplug the J9 off the RTRM then unplug the J8 pins 6&7.  Reset power to unit and adjust the R136 to the desired fan speed.

The contractor kept talking to the engineering department of the manufacturer.  The second set of instructions did work.  Here is what worked:

>>For standard ID Fan operation, ensure RTRM 3J9, RTDM 5J12, and RTOM 5J8 Pins 6-7 are open (no connections).

I wish AC unit manufacturers will say it very clearly on their brochures: The supply fan speed is automatically varied based on the cooling requirements.  This is the default unit that will be shipped to you unless you specifically ask for a ‘Constant Speed Fan’ option.