Heating
The unit’s heating mode of operation is determined by the
control temperature and the heating setpoint temperature. The
unit enters the heating mode of operation by comparing the
control temperature to the heating setpoint.
The control temperature can be either the return temperature
or the space temperature.
The return temperature is typically used for VAV units and the
space temperature is typically used for CAV units.
The unit goes into the heating mode of operation when the
control temperature (return or space temperature) is below the
heating setpoint by more than ½ the deadband.
For example, a standard air conditioning unit with supplemental
gas, electric, or hot water heat with a heating setpoint of 68.0ºF
and a deadband of 1.0ºF would enter heating mode if the
control temperature reached 67.4ºF. When this takes place,
the heating mode of operation will begin and the 1st Stage of
heating operation will start.
The heating mode of operations will be slightly different for
heat pump units. It is the manufacturer’s recommendation that
all Rebel heat pump units be purchased with supplemental
gas, electric, or hot water heat. When the control temperature
drops below the heating setpoint by half the deadband the
unit will energize the four way valve and initiate mechanical
heating.
On heat pumps mechanical heating is the primary source of
heat and will always be the unit’s first attempt to meet the
application’s load. After start-up the variable compressor
will ramp up to meet the DAT Setpoint. If the mechanical
heating capacity at the ambient conditions is capable of
meeting the building load the variable speed compressor will
stabilize at some value below its maximum speed. If the heat
pump’s capacity is insufficient at the ambient conditions the
supplemental (gas, electric, hot water) heat will be enabled and
gradually ramp/stage on to make up the capacity shortage.
If the combined capacity of the heat pump’s mechanical and
supplemental heating is greater than the building load the
supplemental supply will ramp/stage down. The unit will always
seek to operate with mechanical heating as much as possible.
Periodically during heating operations the unit will need to
enter defrost to remove frost build up from the outdoor coil.
During defrost mechanical heating will be unavailable and the
supplemental heat will ramp/stage up to meet the DAT set-
point .
Defrost
Defrost is a temporary and infrequent period during normal
heating operations on Rebel heat pumps. The purpose of
defrost is to remove frost that has built up on the outdoor coil
during mechanical heating. In heating mode the outdoor coil
acts as an evaporator to “pull” heat out of the ambient air. As
a result the surface temperature of the outdoor coil is below
the ambient temperature and depending on conditions maybe
below freezing. During prolonged mechanical heating while the
surface temperature of the outdoor coil is below 32ºF frost will
form .
The defrost operation is similar to mechanical cooling. In
defrost the four way valve will de-energize and the hot gas
from the compressor will be forced into the outdoor coil,
rejecting heating to the ambient, and melting any frost formed
on the coil. To speed up the melting process during a defrost
cycle the OA damper will close and the outdoor fan will de-
energize. During this period the supplemental (gas, electric,
hot water) heat will ramp/stage up to maintain the unit’s DAT
Setpoint .
Rebel heat pump unit’s have demand based defrost control
and will operate in defrost only as long as necessary to remove
frost from the outdoor coil.
Charging
Rebel units have advanced charge management systems that
obsolete many common techniques for determining over or
under charged conditions. The charge management system
means that super heat and subcooling values will float to
achieve the peak real time energy efficiency possible at current
operating conditions (building load and ambient temperature).
Rebel units also use electronic expansion valves that can
not be adjusted manually. Refrigerant should never be added
or removed from the system based on the desire to achieve
an arbitrary subcooling value. It will always be McQuay’s
recommendation that unit’s suspected of being over / under
charged have all of their refrigerant removed, leak tested with
nitrogen, and then re-charged based on the unit name plate.
Table 10: Refrigerant Charge
Unit
Size
Cooling Model
Heat Pump Model
Standard Unit
Standard Unit
w/ MHGRH
Standard Unit
Standard Unit
w/ MHGRH
3
10 .5
12 .9
12 .0
14 .4
4
11 .1
13 .5
12 .6
15 .0
5
15 .3
18 .2
16 .8
19 .7
6
15 .3
18 .2
16 .8
19 .7
7 .5
11 .0
17 .3
26 .0
31 .2
10
34 .0
39 .8
40 .0
45 .8
12
34 .0
39 .8
40 .0
45 .8
15
37 .0
43 .8
42 .0
47 .8
IM 1125-6 • REBEL ROOFTOPS 22
www.DaikinApplied.com
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efrIgeraTIon
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DOT-ITB-21-8041-AC
FPID: 437300-3-7B-01
