ASHRAE Journal - May 2014 - 52

Technical FeaTure

Example Fan Power Limitation
ASHRAE 90.1-2010 Section 6.5.3.1.1
For Example VAV System A
Design Power Limit (bhp) = CFmS × 0.0013 + A
A = Sum of (PD × CFmD/4,131)
Ducted Fully Ducted Return Credit of 0.5 in.
max bhp= 10,000 × 0.0013. (0.5 × 9,500)/
4,131 = 14.15 bhp
Additional Credit for Energy Recovery
Energy Recovery Device Pressure Credit per Airstream
PD = (2.2 × Energy Recovery Effectiveness) - 0.5 in. w.c.
PD = (2.2 × 0.71) - 0.5 = 1.06 in.

Table 1 Example VAV systems. Static pressure summary.

VAV Air Handler - Supply Fan Static Pressure (in. w.g)
sysTem

a

b

C

D

Outdoor Air Intake

0.04

0.04

0.04

+0.10

Outdoor Air Filter

0.05

0.05

Energy Wheel

0.9

0.9

Return Duct Static

1.25

1.25

1.25

1.25

Return Damper

0.05

0.05

0.05

0.05

Filter

0.55

0.55

0.55

0.55

Heating Coil

0.10

0.10

0.10

0.10

Power Limit Add in bhp
A = sum of (PD × CFmD/4,131)
A = (1.06 × 5,000/4,131 + 1.06 × 4,500/4,131) = 2.44 bhp

Cooling Coil

0.70

0.55

0.55

0.55

Discharge Loss

0.25

0.25

0.25

0.25

cfms = the maximum design supply airflow rate to conditioned spaces
served by the system.

Supply Duct Static

1.50

1.50

1.50

1.50

Supply Fan TSP

4.40

4.25

3.39

4.25

VAV Air Handler - Exhaust Fan Static Pressure (in. w.g.)

of the coil. Therefore, the supply air total static pressure at design might actually decrease slightly when
energy recovery is added. For the example, total static
pressure seen by the supply fan drops from 4.4 in. w.g.
(1095 Pa) (System A) to 4.25 in. w.g. (1057 Pa) (System
B). The total static pressure seen by the exhaust fan
increases (1.46 in. w.g. [363 Pa] for System A to 2.8 in.
w.g. [696 Pa] for System B) more than the recovery
device pressure drop due to the filter which is also
added in the exhaust fan path. The net result at design
is that total system bhp increases from 11.9 bhp (8.9
kW) to only 13.3 (9.9 kW) (much less than 15.7 bhp
[11.7 kW] when the exchanger static pressure drop is
assumed to be added to the fans).
The assumption that energy recovery will add significant static pressure to the supply fan leads some to
consider using a return fan system (System C) to off load
some of this static pressure. This may remove the return
air duct static pressure and exhaust filter static loss from
the supply air fan, however the outdoor air path will
have the heat exchanger in its path, therefore, it will
only reduce 0.5 in. w.g. (124 Pa) of static from the supply
fan. This fan placement more than doubles the power
required to handle the exhaust air, so the net result is
4.4 bhp (3.3 kW) added for the example. This will also
have a negative impact on airflow control for the system. Energy recovery adds 0.8 in. w.g. (199 Pa) of static
pressure to each air path downstream of the return fan
and this will cause over 2 in. w.g. (498 Pa) of pressure
52

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M ay 2 0 1 4

sysTem

a

b

C

D

Return Duct Static

1.25

1.25

1.25

-

Filter

0.55

0.55

Energy Wheel

0.8

0.8

Exhaust Outlet

0.21

0.21

0.21

Exhaust Fan TSP

1.46

2.81

2.81

-

difference across the return damper. A small change in
damper position will change return flow greatly.
Another approach is to keep the same basic air handler
(as in System A) and add a separate energy recovery ventilator (ERV) to the system (System D). With this arrangement, the ERV's outdoor air and exhaust air paths are
typically not oversized to enable air economizing. This
can add significant fan energy to the system during hours
of energy recovery, especially to the exhaust path where
a smaller abrupt discharge and a less efficient fan are
often used. This system uses additional 5.8 bhp (4.3 kW)
at design, more than three times the added fan power
versus incorporating into the recovery exchanger into the
air handler unit. These two units are considered one "fan
system" when ducted together, and the prescribed fan
power limitation in the Standard 90.1 is exceeded.
The exhaust and supply fans for all these selections
range between 63% to 66% efficient. For the add-on ERV
unit in the example (System D), the fan efficiency and
internal static pressures are unknown as the ERV brake
ASHRAE Journal - May 2014

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