035-20401-001 Rev. A (0904)
14
Unitary Products Group
Hot Heat Pump Enable (HOT HEAT PUMP) Settings
The Hot Heat Pump Enable (HOT HEAT PUMP) jumper input does not
affect the Forced Second Stage Feature.
Hot Heat Pump Mode
The control will operate in Hot Heat Pump Mode only if the HOT HEAT
PUMP jumper on the control is placed in the ON position. The Y2 OUT
signal must also be connected to the indoor unit.
Hot Heat Pump Mode only affects the indoor blower motor speed when
the unit is operating in second-stage heating.
The control implements the Hot Heat Pump Mode by controlling the
indoor airflow level during second-stage heating operation only. Cooling
operation is not affected. By reducing the indoor airflow level, the heat
pump system will operate with increased indoor discharge air tempera-
tures. The control changes the indoor airflow level using the Y2 OUT
signal. This terminal is connected to the high speed cooling input of a
variable speed indoor unit. When the heat pump control energizes the
Y2 OUT terminal, the indoor blower runs at high speed delivering high
airflow. When the control de-energizes the Y2 OUT terminal, the indoor
blower runs at a lower speed delivering lower airflow.
If the HOT HP jumper is in the ON position and the control receives a
call for second-stage heating, the control will energize the compressor
and measure the outdoor ambient temperature. Second-stage heating
operation can result from one of two different control features. One fea-
ture is refered to as “Forced Second-Stage” while the other is a
response to a Y1 & Y2 thermostat input.
If the outdoor ambient temperature is equal to or greater than 50F, the
control will energize Y2 OUT and keep it energized until the thermostat
is satisfied (Y1 signal removed). The higher airflow is required to keep
the operating pressures low when the outdoor ambient temperature is
50F or greater.
If the outdoor ambient temperature is less than 50F, the control will start
a ten-minute timer and keep Y2 OUT de-energized. Therefore, the com-
pressor will be operating and the indoor unit will be operating with
reduced airflow.
If the HOT HP jumper is in the ON position and if
the outdoor ambient temperature is less than 50F, the indoor air-
flow at the beginning of a heating cycle will always be low.
When the ten-minute timer expires, the control will measure the liquid
line temperature and determine whether to energize Y2 OUT and
increase the indoor airflow or keep Y2 OUT de-energized and maintain
reduced indoor airflow. The control compares the measured liquid line
temperature to a pre-programmed indoor airflow curve. It continues to
keep Y2 OUT de-energized until the liquid line temperature exceeds the
curve for the given outdoor ambient temperature continually for 30 sec-
onds. If the liquid line temperature drops below the curve, the control
will reset the 30-second timer and restart it when the liquid line temper-
ature again exceeds the curve. When the liquid line temperature
exceeds the indoor airflow control curve continually for 30 seconds, the
control will energize Y2 OUT, i.e. cause high indoor airflow, until the
thermostat demand is satisfied and the thermostat signal inputs are
removed. Figure 13 describes the required behavior.
Once the control energizes Y2 OUT to create high airflow, it will keep
Y2 OUT energized until the thermostat is satisfied and the call for heat-
ing is removed. It will do this regardless of a change in outdoor ambient
or liquid line temperature. Therefore, if the control energizes Y2 OUT
because the outdoor ambient temperature is greater than or equal to
50F or because the liquid line temperature exceeds the curve for the
given outdoor ambient temperature (point within region B), it will keep
Y2 OUT energized until the thermostat is satisfied even if the liquid line
or outdoor ambient temperature falls below the curve (point within
region A).
The control of Y2 OUT to generate high indoor airflow as required will
prevent the heat pump system pressures and temperatures from
becoming too great when the compressor is running at full capacity with
low indoor airflow.
As an example, assume that the HOT HP jumper is in the ON position.
When the control receives a call for second-stage heating (Y1 & Y2), it
checks the outdoor ambient temperature. If the outdoor ambient tem-
perature is 52F, the control will energize Y2 OUT immediately causing
high indoor airflow and keep it energized until the thermostat signal is
removed. If the outdoor ambient temperature is 48F, the control will
maintain Y2 OUT in a de-energized state and begin a ten-minute timer.
The indoor unit will be running at low airflow because of the thermostat
signal being delivered to the Y1 input of the indoor unit.
When the ten-minute timer expires, the control will compare the liquid
line temperature to the indoor airflow control curve for the measured
outdoor ambient temperature. If the point is within region A (See Figure
13), the control shall maintain Y2 OUT in the de-energized state until
the liquid line temperature rises so that region B is entered. While in
region A, the indoor unit will continue to run at low airflow. When the liq-
uid line temperature point enters region B, the control will start a 30 sec-
ond timer. When the timer expires, the control will immediately energize
Y2 OUT. If the liquid line temperature drops back into region A before
the timer expires, the control will reset the timer and restart it when the
liquid line temperature again enters region B. When the liquid line tem-
perature is in region B continually for thirty seconds, the Y2 OUT signal
will be delivered to the indoor unit and cause high airflow.
Forced Second-Stage Feature
The control determines the behavior of the forced second stage feature
based on the Switch Point jumper input. Based on the Switch Point set-
ting and the liquid line temperature, the control will lock the compressor
to second stage operation.
This forced second stage feature insures that the compressor will
always be in second stage during a defrost cycle. If the compressor
were allowed to be in first stage during defrost, the outdoor coil would
not defrost as quickly as it would in second stage. The maximum
defrost cycle length timer could expire before the defrost cycle is com-
plete.
The forced second stage feature also prevents first stage heating oper-
ation at low temperature and avoids cold indoor discharge air.
Switch Point Settings
The switch point setting affects only the forced second stage feature of
the hot heat pump.
The control allows for switch point settings to be 35°, 40° or 45° F.
The factory palces the switch point jumper in the 35° F position. If the
jumper is removed, the control will behave as if the jumper is in the 35°
F position.
The minimum switch point temperature option of 35F insures that the
compressor will always be in second stage operation during defrost. If
a temperature option below 35F was allowed, the heat pump might
require a defrost cycle (based on liquid line temperature below initiate
curve) before the control forced the compressor into second stage oper-
ation based on the switch point setting.
