ASHRAE Journal - May 2024 - 54

TECHNICAL FEATURE
FIGURE 8 Outdoor air tempering using heating coils in series.
72°F Intermediate Heating Hot Water Temp.
OA
0°F
OA In
49°F
Water Out
OA
47 °F
After
First
Coil
OA
Pump
85°F
Water In
the energy penalty of incorporating SER is minimized,
since one or both coils would be present in the baseline
DOAS unit. In unusual cases, dampers that allow
air to bypass one or both of the series coils may be
economically justified.
Higher First Cost. For hot climates where a heating
coil is not required, the addition of a precool coil
and a circulating pump adds considerably to the first
cost. The same is true for installations that would not
otherwise require a reheat coil. Also, adding a coil will
add a foot or two (0.3 m to 0.6 m) to the length of the
air-handling unit.
More Controls. Controls must be installed and
programmed to activate the energy recovery system
and to modulate its capacity to avoid overheating the
delivered air. Reference 4 provides extensive guidance
on the control of air-to-air energy recovery systems.
More Maintenance. The circulation pump will
require additional maintenance, since it will run in both
dehumidification/cooling modes and freeze-protection
mode (where the climate requires such). Additional
coils or more robust coils will require more extensive
periodic cleaning.
Economic Considerations
Table 1 summarizes the estimated annual energy
savings and economic payback for a hydronic-based
SER system in five representative climate zones. For
a given installation, payback depends on several
factors: climate, the design dehumidification
setpoint, system operating schedule, energy costs
and potential rebates based on energy savings. The
table accounts for the additional electrical energy
to operate the circulation pump in energy recovery
mode, the increase in fan power to force air through
54
ASHRAE JOURNAL ashrae.org MAY 2024
72°F
Tempered Air to
Building
CA
Fan
the SER coils and includes an annual allowance for
additional coil and pump maintenance.
SER is economically attractive in Climate Zone 0
through Climate Zone 4. Even if the operating schedule
is reduced to normal working hours, or if demandcontrol
ventilation is adopted, SER may still be costeffective.
However, in Climate Zones 5 and up, SER
probably only makes financial sense if the baseline
DOAS already has both a heating coil and a reheat coil.
For instance, for the Zone 5 site (Denver), if the baseline
DOAS has no reheat coil, then the simple payback
extends from about 10 years to about 25 years.
For simplicity, some assumptions and approximations
were made. For instance, no credit was taken for
downsizing the cooling plant and/or the distribution
piping, based on peak load reduction. And no credit was
taken for possible energy rebates. On the other hand, it
is assumed that the fan motor size is unchanged despite
the increase in total static pressure. Also, the calculation
assumes a flat-rate electrical tariff of $0.10 per kWh and
an unvarying unit cost for cooling and heating energy.
When calculating the equipment cost for warm
climates (Zones 1 and 2), it was assumed that the
baseline DOAS includes a reheat coil. For Climate Zones
3 through 5, it was assumed that the baseline DOAS
includes a heating coil and a reheat coil. In all Climate
Zones, the cost of the circulation pump was added to
the cost of the proposed SER system. In other words,
no credit was taken for the fact that, in cold climates,
a freeze protection pump is often specified and can
double as a SER circulation pump.
DOAS/SER operating strategy is as follows:
* 5,000 cfm (2360 L/s) flow of outdoor air, not
modulated, operates 8,760 h/yr.
* When OA dry-bulb is below 60°F (15°C), heat the air
(heating cost not included in this study).
* When OA dew-point is above 55°F (13°C), cool and
dehumidify to 50°F (10°C).
* When OA dew-point is below 55°F (13°C), and OA
dry-bulb is below 75°F (24°C), do nothing.
* When OA dew-point is below 55°F (13°C), and OA
dry-bulb is above 75°F (24°C), cool to 70°F (21°C).
* When dehumidifying, reheat to 65°F (18°C).
First-Cost Adders:
* $1,200 circulation pump (added in all climate
zones);
* $1,000 connecting piping and additional valves
Coil
Precool
Cooling
Coil
Coil
Reheat
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ASHRAE Journal - May 2024

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