Back about four years ago:
http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/2007-October/005973.html
There was a long discussion of COP and EIR and fan power and many
interesting points were made by many knowledgeable engineers and
modelers on this list. I learned a lot from this discussion, but now I
have started using EnergyPro and the topic has reared its ugly head
again for me. If anybody wants to stir the old coals of that
discussion, or point me to the definitive post on the topic, I'd be
pleased to hear from you. Brandon Nichols and Xiaobing Liu were
especially helpful before. Gentlemen, if you are still on this list,
I'd love to hear from you.
Here goes:
EnergyPro asks for fan horsepower for the AC unit. I have been entering
the nominal horsepower and getting out-of-compliance results (fan power
too high). I have also computed actual horsepower using FLA and
Voltage, but that doesn't drop it much at all. When I put FLA x Voltage
horsepower in, it's still out-of-compliance with CA T-24 CEC.
I reviewed ANSI/AHRI Standard 340/360-2007, which is the EER rating
standard, and it says:
6.1.3.2 Indoor-Coil Airflow Rate. All Standard Ratings shall be
determined at an indoor-coil airflow rate
as outlined below. All airflow rates shall be expressed in terms of
Standard Air.
a. Equipment with indoor fans intended for use with field installed duct
systems shall be rated at the
indoor-coil airflow rate (not to exceed 37.5 SCFM per 1000 Btu/h [0.06
m3/s per 1000 W] of rated
capacity) delivered when operating against the minimum external
resistance specified in Table 5 or at a
lower indoor-coil airflow rate if so specified by the manufacturer.
And then Table 5 says "0.75 in. H2O" for my size unit (25 tons).
So, I conclude from this that if I design the duct work to have NOT MORE
THAN 0.75 in. H2O pressure drop to the supply register I can omit the
fan power. IS THIS CORRECT?
OUTSTANDING QUESTIONS:
1) Is a rooftop unit "Equipment with indoor fans intended for use
with field installed duct systems".
2) If I did design for more than 0.75 in. H2O, how would I compute
the extra horsepower?
3) Or is this fan horsepower just for auxiliary fans, not associated
with the equipment?
Sorry about the deja vu of this, but it's something I have to get a grip
on today....
Cheers!
Robert Wichert P.Eng. LEED AP
