Heat Pumps; General; Operation - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

HEAT PUMPS

GENERAL

A hea t pump is a r efr iger ation system tha t pr ovides both
heating and cooling within the same unit. In the heating mode,
the pump deli vers hea t fr om a hea t sour ce to the conditioned
space . In the cooling mode , the pump r emoves hea t fr om a
space and transfers it to a heat sink. Heat pumps use standard
r efr iger ation components (compr essor, expansion v alve,
evapor ator, and condenser) and a r ever sing valve to r ever se
refrigerant f low thr ough the coils. A refrigerant r eversing valve
switches at chang eover to con vert the condenser to an e vapor ator
and vice v ersa.
Figur e 36 shows hea t pump c ycles. In the cooling c ycle,
refr igerant f low uses the outdoor hea t exchang er coil as the
condenser to reject heat from the space, and the indoor coil as
the evapor ator. In the hea ting cycle, flow of air and r efrigerant
is rever sed making the outdoor coil the sour ce of hea t. The
indoor coil becomes the condenser and pr ovides hea t for the
space . When outdoor air temper atur es ar e too cold to pr ovide
enough hea t tr ansfer, electr ic resistance hea ting elements can
be used to pr ovide supplemental hea t. An alter native method is
to r ever se indoor and outdoor airf low acr oss the coils and
elimina te the r eversing valve.
COMPRESSOR
INDOOR
COIL
EXPANSION
VALVE FOR
COOLING
COOLING CYCLE
COMPRESSOR
INDOOR
COIL
HEATING CYCLE
Fig. 36. Refrigerant Flow in Heat Pump
Cooling and Heating Cycles.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
REFRIGERANT
REVERSING
VALVE
OUTDOOR
COIL
CHECK
VALVE
REFRIGERANT
REVERSING
VALVE
OUTDOOR
COIL
EXPANSION
VALVE FOR
HEATING
CHECK
VALVE
C3021
INDIVIDUAL ROOM CONTROL APPLICATIONS
Heat pumps ar e typicall y classified by the hea t sour ce at the
"outdoor" coil. The common air -to-air hea t pump uses outdoor
air as its hea t sour ce dur ing the hea ting c ycle. A water -to-air
hea t pump uses w ater as the hea t sour ce dur ing the hea ting
cycle. The water suppl y may be a well or a lak e. In the cooling
mode, the outdoor coil r ejects hea t and the air or w ater becomes
the heat sink.
In commer cial applica tions, a closed-loop or r unar ound w ater
suppl y ma y serve multiple units (F ig. 37). This system r elies on
load di versification. Some units ma y be cooling while other z ones
oper ate in the hea ting cycle. In this case , the water loop is a sour ce
to heating units and a sink to cooling units, transferring heat from
one to the other . Water-loop temper atur es ar e maintained betw een
21 C and 32 C to pr ovide an adequa te hea t sour ce or hea t sink. A
centr al boiler to gether with a c hiller and/or cooling to wer temper
the water in the loop dur ing peak hea ting and cooling per iods.
For a discussion of centr al r egulation and contr ol of water pump
hydr onic loops, refer to Chiller , Boiler, and Distr ibution System
Contr ol Applica tions section.
HEAT
PUMP
CHILLER OR
COOLING
TOWER
BOILER
Fig. 37. Heat Pump Closed-Loop System.

OPERATION

Small and medium-sized heat pumps usually heat and cool
indoor air and ar e contr olled fr om a space ther mosta t. Lar ge
hea t pumps usuall y pr ovide war m and c hilled w ater and ar e
contr olled by a chilled water temper atur e contr ol.
Space ther mosta ts ar e two-position and usuall y multista ge.
They typicall y pr ovide automa tic chang eover, switching the
heat pump between heating and cooling as required to maintain
space temper atur e. The first sta ge of the ther mosta t cycles the
compr essor. If the system is in a hea ting mode , ad ditional
thermostat heating stages will bring on supplementary resistance
heat if the heat pump cannot meet the load. In the cooling mode,
ther e is only one sta ge tha t cycles the compr essor. If desir ed,
chang eover betw een hea ting and cooling can be man ual r ather
than automa tic. The ther mosta t can ha ve separ ate hea ting and
cooling settings or a single setting with a f ixed deadband
between heating and cooling.
421
THERMOSTAT
PUMP
C3022