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Design considerations for commercial
air-to-water heat pumps
Advantages of using air to water heat pumps
Using air to water heat pumps (AWHPs) as the primary source of heating and cooling commercial
buildings gives architects and engineers a long-term solution to designing sustainable buildings.
The heat pumps provide all the benefits of a hydronic system with the performance and energy
efficiency of variable refrigerant flow (VRF) designs. Their many features include:
capability
AWHPs can provide energy for a building’s cooling, heating, and domestic hot water
demands at outdoor temperatures from 5°F to 105°F. AWHPs can produce water
temperatures similar to condensing boilers running at their peak efficiencies,
depending on outside air temperatures.
efficiency
Energy recovery options, including true simultaneous heating and cooling, result in
energy use comparable to VRF systems:
 Average annual energy savings of over 40% compared to a typical boiler-plus-chiller
application.
 Installation costs are generally lower (savings are site specific).
flexibility
Hydronic systems powered by heat pumps allow long-term heating and cooling
flexibility as tenant requirements and building use change. AWHPs can be the energy
source for:
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water-source heat pumps
fan coils
radiant heating and cooling
perimeter baseboard heating
central system air handlers
make-up air handlers
VAV units
a combination of systems
In addition, AWHP equipment can be sized to maximize seasonal efficiency by using a
trim boiler for the few hours a year of the coldest-design-or-below temperatures. VRF
systems must be sized to provide all of the heat at design conditions, meaning that the
equipment is oversized for the vast majority of the heating season.
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adaptability AWHP technology adapts to existing water pipe systems. Configurations are
available that provide both domestic hot water and heating or cooling for two-pipe fan
coil systems; provide simultaneous heating and cooling for four-pipe fan coil systems;
or maintain loop temperature for water-source heat pumps systems. And as technology
changes or refrigerant requirements change, the building’s energy system can be easily
retrofitted to adapt to new technologies.
safety
Hydronic systems minimize refrigerant volume and components both inside and
outside the building envelope, reducing costs and risk of leaks and eliminating the need
for leak detection systems in occupied spaces.
AWHP unit with factory-installed circulation pumps and buffer tank
AWHP basics
Two white papers on the Wescor website provide introductions to how AWHPs work, their
efficiencies, and financial comparisons:
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Heat Pump Water Heater Basics
Heat Pump Water Heaters: Design Details
This white paper goes beyond the basics of heating domestic hot water and explains how to use
AWHP technology to heat and cool buildings in a whole-building design.
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Basic operating designs
The following two sections provide schematics for designing basic two- and four-pipe AWHP
systems. For more involved system designs, call your local Wescor representative.
We’re here to help—call your Wescor representative for assistance when designing AWHP
projects.
Two-pipe systems
In a typical two-pipe fan coil system, the entire building is either in heating or cooling mode, and
seasonal change-over is usually required. (Aermec systems do not require seasonal changeovers,
because the system easily changes between heating and cooling.) In two-pipe systems, the heat
pump has two separate operations, one for each mode.
In cooling mode, the heat pump can provide all the chilled water while also capturing energy from
the chilled water loop to provide energy to the domestic hot water circuit if there is a demand.
In heating mode, the heat pump can provide energy to the building’s hot water loop while also
providing energy to the domestic hot water loop.
If a building is using water source heat pumps for heating and cooling individual zones, the AWHP
can be used to maintain the loop temperature throughout the year.
Basic internal AWHP operating production for two-pipe system + DHW
Cold water production only to system
Compressor
3
3
Compressor
2
1
Description
Functioning
1
Heat exchanger
system side
Evaporator
cold water production
2
Heat exchanger
DHW side
not running
3
Heat exchanger
source side
Condenser
heat exchange with air
Bldg cooling return
Bldg cooling supply
3
Hot water production only to system
3
3
Description
Functioning
1
Heat exchanger
system side
Condenser
hot water production
2
Heat exchanger
DHW side
not running
3
Heat exchanger
source side
Evaporator
heat exchange with air
2
1
Bldg heating return
Bldg heating supply
Production hot water only to DHW
3
3
2
Description
Functioning
1
Heat exchanger
system side
not running
2
Heat exchanger
DHW side
Condenser
DHW production
3
Heat exchanger
source side
Evaporator
heat exchange with air
DHW supply
DHW return
1
4
Production cold water to system and hot water to DHW
3
Description
Functioning
1
Heat exchanger
system side
Evaporator
cold water production
2
Heat exchanger
DHW side
Condenser
DHW production
3
Heat exchanger
source side
not running
3
DHW supply
2
DHW return
1
Bldg cooling return
Bldg cooling supply
Production hot water to system and hot water to DHW
Description
Functioning
1
Heat exchanger
system side
Condenser
hot water production
2
Heat exchanger
DHW side
Condenser
DHW production
3
Heat exchanger
source side
Evaporator
heat exchange with air
3
3
2
DHW supply
DHW return
1
Bldg heat return
Bldg heat supply
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Four-pipe systems
Four-pipe systems can handle simultaneous and independent demands for hot and cold water, and
changeover is not required. In this configuration, the advanced microprocessor control logic
operates the unit in one of the following modes.
Cooling only
The controller activates the cold water circuit, and the heat pump acts as a traditional chiller
with energy taken from the cold water loop and sent to the outside air through the finned coils.
With dual compressors on this circuit, the unit can operate efficiently at part-load conditions.
If there is a demand for domestic hot water, the energy taken from the cold water loop can be
transferred to the domestic water loop.
Heating only
The controller activates the hot water circuit, and the heat pump’s evaporator coil takes energy
from the outside air and uses the condenser’s heat exchanger to heat the water in the hot water
loop. This is different from the usual approach of a reverse cycle heat pump, because the hot
water is produced in a different heat exchanger than the one used to produce cold water. This
separates the two hot-cold sections required for four-pipe systems.
Simultaneous heating and cooling
The unit operates as a water-to-water heat pump, moving energy from the cold-water circuit to
the hot-water circuit. The on-board microprocessor controller controls the evaporator and
condenser operation of each circuit and automatically changes from one configuration to the
other depending on the building’s demand. Because it uses separate circuits for the hot-water
and cold-water loops, the unit can provide partial to full cooling while at the same time
providing partial to full heating. Having multiple circuits and compressors ensures maximum
efficiency while in part-load or full-load conditions.
Domestic hot water while heating the building system loop
A water to water heat exchanger can be added so excess energy can be transferred to the
building’s domestic water system from the building hot water system loop.
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Basic operating layout for four-pipe system
Cold water production only to system
Description
Functioning
1
Heat exchanger
cooling side
Evaporator
cold water
production
2
Heat exchanger
heating side
not running
3
Heat exchanger
source side
Condenser
heat exchange
with air
Description
Functioning
1
Heat exchanger
cooling side
not running
2
Heat exchanger
heating side
Condenser
hot water
production
3
Heat exchanger
source side
Evaporator
heat exchange
with air
3
3
2
Bldg cooling return
1
Bldg cooling supply
Hot water production only to system
3
3
2
Bldg heat supply
Bldg heat return
1
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Simultaneous hot and cold water production to system
3
3
Description
Functioning
1
Heat exchanger
cooling side
Evaporator
cold water production
2
Heat exchanger
heating side
Condenser
hot water production
3
Heat exchanger
source side
not running
Bldg heat supply
2
Bldg heat return
1
Bldg cooling return
Bldg cooling supply
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Why we chose Aermec’s AWHPs
We are impressed with Aermec AWHPs and think you will be, too.
experience Aermec, a European company with over 1500 employees, is one of the largest HVAC
manufacturers in the world. They have over 50 years experience building HVAC
systems and over 20 years building AWHPs.
quality
Aermec uses axial fans, scroll compressors, and plate heat exchangers. Each unit has at
least two compressors and refrigerant circuits for redundancy and good performance
under part-load conditions.
Aermec’s 46-person Research and Develop Department has six test chambers sized to
test units up to 250 tons at temperatures ranging from 5°F to 130°F and RH from 20%
to 95%.
Aermec test chamber
certification All Aermec systems are tested and rated in-house to Eurovent standards (including
operation during defrost conditions, included in their COP calculations) and are
certified by AHRI in the US. When an energy analysis shows a unit’s output at design
conditions, be assured that these are the equipment’s actual operating parameters.
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innovation Aermec units can provide simultaneous heating and cooling under full or part load
conditions and can provide free cooling when in simultaneous operation. On-board
microprocessor controllers provide autonomous operation with full control and
minimum management required by the building management system. Hydronic
accessories (such as pumps, buffer tanks, or a second hydronic circuit fed by a
desuperheater) can be factory installed within the same footprint.
selection
Aermec manufacturers a full range of AWHPs, water-to-water heat pumps, and
chillers. Heat pumps for commercial applications (buildings that have independent
requirements for heating and cooling) range from 12 to 262 tons. Larger tonnage units
with screw compressors are also available.
Aermec robotic assembly
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