Control Loops - Honeywell AUTOMATIC CONTROL Engineering Manual

INDIVIDUAL ROOM CONTROL APPLICATIONS
In some heat pumps, a minimum off timer prevents a
compressor restart for three to five minutes. After shutdown,
heat pump operation must not resume until pressures equalize
between the suction and discharge sides of the compressor. Short
cycling the heat pump may result in compressor damage.
A two-stage, high-speed compressor can provide capacity
control for maximum heat pump efficiency. The room thermostat
controls the individual compressor stages. The thermostat second
stage controls the auxiliary heat (for air-source heat pumps) and
compressor stage two. When the outdoor thermostat contact closes
as outdoor temperature falls to its setpoint, the auxiliary heat
energizes with the compressor (Fig. 38).
THERMOSTAT
AUXILIARY
OUTDOOR AIR
THERMOSTAT HEAT
Fig. 38. Heat Pump Staging.
Defrost cycling is typically used when outdoor temperatures
are below 45F, because the outdoor coil may operate below
freezing and frost can form on the coil when the unit is in the
heating mode. Frost inhibits airflow through the heat pump and
degrades unit performance. To remove the frost, the heat pump
momentarily switches to the cooling mode. Hot gas from the
compressor is directed to the outdoor coil, and the frost melts.
Typically the unit cycles every 90 minutes for four to eight
minutes to defrost. Instead of time-initiated defrost, some
models use demand defrost, which cycles to the cooling mode
by measuring changes in the airflow across the outdoor coil.
ENGINEERING MANULA OF AUTOMATIC CONTROL
HEAT PUMP
POWER
STAGE 1
STAGE 2
C3023
426

CONTROL LOOPS

Heat pumps can use a variety of methods to change between
cooling and heating, including a two-position room thermostat
and manual changeover.
Assuming automatic changeover between heating and
cooling, on a rise in space temperature, a two-position room
thermostat senses the temperature rise and cycles the heat pump
in the cooling mode. When space temperature falls below the
deadband, the first stage of heating cycles the compressor in
the heating mode. A further drop in space temperature brings
on additional thermostat heating stages to turn on supplemental
electric resistance heaters.
In some heat pumps, the changeover valve cycles with the
compressor in either the heating or cooling mode. In others,
the changeover valve remains in the heating or cooling position
as long as space temperature is in the appropriate range.
Where a central water plant provides heating/cooling source
water for the heat pumps, water control is often provided to
conserve water when the heat pump cycles off and when the
water temperature is excessive for the load. Valves may be head
pressure (may be furnished with the heat pump) or temperature
controlled. Temperature controlled valves must close when the
compressor is off, but must have a minimum open position
anytime the heat pump operates in order to keep water flow
across the water sensor.
Heat pump system controls must be carefully coordinated
with the heat pump manufacturer and the water plant system
(on air-to-water heat pumps).