ASHRAE Journal - April 2021 - 25

TECHNICAL FEATURE

condition, both rejected and absorbed heat
FIGURE 1 Typical two-pipe configuration (2PC) water source heat pump (WSHP).
are equivalent; no heat should be added by the
70°F
boiler or removed by the cooling tower.
Isolation
Valve
Figure 2 shows steady-state conditions when
60°F
80°F
cooling mode or heating mode is dominant.
ON
OFF
ON
On the top, two heat pumps operate in heatAbsorb:
Reject:
Heat Added Heat Rejected
12 kBtu/h
0 kBtu/h
12 kBtu/h
ing mode, and one pump operates in cooling
Tank
0 kBtu/h
0 kBtu/h
mode. Deﬁcient heat occurs in the loop, so
2.4 gpm
0 gpm
2.4 gpm
Boiler
CT
the boiler should operate to add 12 kBtu/h
70°F
OFF
70°F
OFF
4.8 gpm
70°F
(3.5 kW) of heat in the steady-state condition
in which the water loop temperature drops
FIGURE 2 Example illustrating the steady-state conditions for two cases of 2PC WSHP operation;
below 50°F (10°C). On the bottom, two heat
two pumps operate in heating mode (top) or in cooling mode (bottom).
pumps operate in cooling mode, and one
pump operates in heating mode. Excess heat
46.6°F
occurs in the loop, and the cooling tower
40°F
60°F
40°F
should operate and reject 12 kBtu/h (3.5 kW)
ON
ON
ON
of heat in the steady-state condition when the
Absorb:
Absorb:
Reject:
water loop temperature increases above 90°F
Heat Added Heat Rejected 12 kBtu/h
12 kBtu/h
12 kBtu/h
Tank
12 kBtu/h
0 kBtu/h
(32°C). In transient periods, when the water
2.4 gpm
2.4 gpm
2.4 gpm
Boiler
CT
loop temperature stays within the upper or
46.6°F
OFF
50°F
ON
7.2 gpm
50°F
lower limits, both the boiler and cooling tower
are off, and their isolation valves are closed as
93.3°F
shown in Figure 3.
In all cases, the main loop return water
80°F
100°F
100°F
temperature is obtained from mixing water
ON
ON
ON
streams leaving the WSHPs at different temReject:
Absorb:
Reject:
Heat Rejected 12 kBtu/h
Heat Added
12 kBtu/h
12 kBtu/h
peratures. For instance, if we look again at
Tank
12 kBtu/h
0 kBtu/h
Figure 1 and assuming that the tank was ini2.4 gpm
2.4 gpm
2.4 gpm
Boiler
CT
tially at 70°F (21°C), the steady-state return
93.3°F
ON
90°F
7.2 gpm
OFF
90°F
water temperature is 70°F (21°C), obtained
from mixing the two water streams at temperatures of 60°F (16°C) and 80°F (27°C). Because
WSHPs in cooling mode. Each WSHP can receive or
the supply water temperature should be 70°F (21°C) as
deliver cold or warm water into any of these pipes using
well, this mixing reduces the efﬁciencies of WSHPs.
two-position isolation valves. For instance, if the WSHP
This could be avoided by circulating the cold water
operates in heating mode, the warm supply water valve
(e.g., 60°F [21°C]) that is leaving WSHPs operating in
(in red) and the cold return water valve (in blue) will be
heating mode directly to WSHPs operating in cooling
opened, and if the WSHP operates in cooling mode, the
mode, and circulating the warm water (e.g., 80°F [21°C]) cold supply water valve (in blue) and the warm return
that is leaving WSHPs operating in cooling mode to those water valve (in red) will be opened.
WSHPs operating in heating mode. A four-pipe system
The installation of two tanks is optional, but if installed
conﬁguration (4PC) provides that solution (Figure 4).
they should be connected with a common pipe (Figure 5).
This proposed 4PC includes two supply pipes; one
If the tanks are not installed (Figure 4), a common pipe
carries the cold water delivered to WSHPs in cooling
with minimum resistance should be installed between
mode, and the other carries the warm water delivered
the cold and warm pipes.
to WSHPs in heating mode. It also includes two return
Compare the 2PC in Figure 1 with the proposed 4PC
pipes; one carries the cold water from WSHPs in heatillustrated in Figure 4. The 2PC main loop water at 70°F
ing mode, and the other carries the warm water from
(21°C) is supplied to both WSHPs operating in different
APRI L 2021

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ASHRAE Journal - April 2021

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