# [Bldg-sim] Fan Power

Aaron Smith asmith at mreng.ca
Fri Feb 22 13:51:30 PST 2013

```Robert,

I've also run into problems obtaining combined (fan and motor) efficiency
and input power, mostly with terminal units such as heat pumps and fan coil
units but also with individual fans.  What I've often done is used the watts
listed in their electrical tables.  For example, below if you take their
size 20 at high fan speed, they classify the motor HP at 1/30HP or 25W
(output power).  I've taken the subsequent columns to mean input power so
the watts at 115V is then 57W which would make the motor is 44% efficient
and that seems reasonable to me.  The listed amps don't necessarily equate
to the same wattages, they are usually higher so my only theory is that this
is the starting amps.

I'd be interested in finding out what other have to say.  I have tried
talking to some local equipment reps without any luck.

Aaron

Aaron Smith, P.Eng

M&R Engineering

-----Original Message-----
From: RobertWichert [mailto:robert at wichert.org]
Sent: Friday, February 22, 2013 12:25 PM
To: bldg-sim at lists.onebuilding.org; Scott Blunk
Subject: [Bldg-sim] Fan Power

This is a DOE-2 question, I think, but I am applying it to EnergyPro.  I

am asking here in the hopes that some will have a more in-depth

knowledge of DOE-2 and how it relates to fan power.  I am also hoping

that some here will be able to reflect on the theory and application of

fan power and energy as it relates to actual systems and modeling.

When inputting indoor fan power, I have used many sources for the fan

power.  I sometimes use the motor nameplate, which I believe is too high

in every case.

I also sometimes use FLA or RLA, which I also do not believe are

accurate, but sometimes give more realistic numbers.

Some manufacturers give fan BHP at design conditions, which I find to be

It has been suggested that I calculate the fan BHP using the theoretical

formula *** Fan BHP = (cfm x static press "w.c.) / (6356 x fan

efficiency) *** and then use a conservative fan efficiency (maybe 50%)

for a good number.  This gives a much lower value than the other

methods, and I am trying to see if there is anything wrong with this

approach.

As I understand the usage, indoor fan power is divided by motor and

drive efficiency to come up with fan energy.  So the fan BHP is the

starting point.  Having this too high drives up fan energy, sometimes

dramatically, and can influence the proposed case quite a bit if there

are lots of fans, even small fans.

Any thoughts on this approach would be very much appreciated.

--

Robert Wichert P.Eng. LEED AP BD&C

+1 916 966 9060

FAX +1 916 966 9068

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