ASHRAE Journal - September 2020 - 42

TECHNICAL FEATURE

Zone 3 has energy efficiency ratios (EERs) that approach
100.
Building ventilation rates would be far in excess of
the ASHRAE Standard 62.1-2019 minimum outdoor
air requirements of 15 cfm/person (7 L/s per person) at
the higher VAV delivery temperatures. Room RH in the
range of 40% to 60% would be maintained. Table 1 shows
for the northern cities that an average of 44.6% of annual
hours land in Climate Zone 2. The heat wheel is able to
transfer the heat required to maintain the 45°F (7°C) DP
delivery off the AC/H unit with 100% outdoor air through
the modulation of Dampers A and B to reach the 45°F
(7°C) WB condition.
Only 9.2% of the average annual hours reside in
Climate Zone 1, requiring additional heating by the hot
water coil to meet the 45°F (7°C) DP condition for a target value of 40% RH in the building.
Clearly, western U.S. climates and arid climates at
higher altitudes benefit the most from this VAV design
strategy for hydration of outdoor air.

Seattle Simple Payback Through
Humidification Cost Avoidance
The factory-installed cost of a high-performance heat
wheel and a 12 in. (305 mm) deep rigid media AC/H
is estimated to be $68,000 ($1.70/supply cfm) for a
40,000 cfm (18 878 L/s) custom AHU costing $267,350.
For Seattle, hours per year at each average dry bulb
(ADB) and mean coincident wet-bulb (MCWB) conditions, were used to estimate humidification energy
avoidance provided by the heat wheel in Climate Zones
1, 2 and 3.
Not included in this energy avoidance are the fan
energy savings in Climate Zone 2 and 3 when the VAV
delivery temperature to the building is below the
conventional 55°F (13°C) DB setpoint. If the VAV terminal boxes in the building are set to satisfy the room
cooling load at 55°F (13°C) delivery temperature,
temperatures lower than 55°F (13°C) will result in
reduced fan flow into and out of each core zone where
the cooling load is fairly constant. A typical or average ambient condition for Climate Zones 2 and 3 in
Seattle is the bin temperature of 47°F (8°C) ADB/45°F
(7°C) MCWB. The VAV box delivery to the core zones
would be approximately 45°F (7°C) DB at 45°F (7°C)
DP. If the room target temperature is 75°F (24°C)
DB, then the airflow to that zone would be reduced
42

ASHRAE JOURNAL

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SEPTEM BER 2020

by 33.3% compared to a 55°F (13°C) supply air temperature delivery. Since there are 6,561 hours/year
in Climate Zone 2 and 3, fan energy savings would be
significant.
Using the same Seattle bin condition of 47°F (8°C)
ADB/45°F (7°C) MCWB and assuming a 50% VAV flow
of 20,000 cfm (9439 L/s) over the 2,747 hours/year in
Climate Zone 3, given a central plant cooling energy
consumption at 0.8 kW/ton (0.2 kW/kW) of cooling,
we can estimate sensible cooling energy avoidance in
Climate Zone 3 at 40,289.33 kWh/year. At the Seattle
electrical energy cost of $0.108 per kWh, avoided cooling
energy is estimated to be $4351.24 per year.
Assuming that the average VAV supply fan flow is
50% of full VAV flow for all ambient conditions in
Climate Zones 1, 2 and 3, the boiler energy avoided
through heat recovery is estimated at 71,263 therms
when a boiler and piping loss factor of 0.8 is applied.
The approximate value of the annual avoided energy
at a Seattle energy cost of $1.127/therm would be
$80,313.40. If the factory cost for heat recovery and
adiabatic humidification components is increased by
25% to reflect the sales representative and mechanical
contractor's markup, the simple payback for a delivered heat wheel and AC/H would be 1.06 years. Other
west coast cities such as Vancouver, Portland and San
Francisco will reflect similar paybacks for avoided
humidification energy.
More difficult to assess in Climate Zone 3 is the value
to a building owner of increasing the outdoor air ventilation rate above the minimum code requirement of 15
cfm/person (7 L/s/person) required by ASHRAE Standard
62.1-2019. Studies have attempted to assign a value to
increased worker productivity and the reduction in
short-term sick leave absence due to better outdoor air
ventilation.1
As a check against the bin hour method tabulation
shown in Table 1, Seattle TMY2 hour-by-hour typical
year points may be overlaid on the psychrometric chart
(Figure 7). There are zero hours in Climate Zone 1, 3,836
hours in Climate Zone 2 and 3,231 in Climate Zone 3 or
a total of 80.7% of the annual Seattle hours. Hydration
and sensible cooling of 100% outdoor air are produced at
a very low energy cost for Climate Zones 2 and 3. Central
refrigeration plants may be shut down for Climate Zones
2 and 3 climate conditions, the majority of which occur
in the spring, winter and fall.

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ASHRAE Journal - September 2020

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