Ingersoll-Rand ReliaTel, Manual, Page: 124 / 176

124 RT-SVD03L-EN
Heat Pump
Demand Defrost
There are two schemes in common usage for heat
pump outdoor coil defrosting: demand defrost and
time-temperature defrost. Demand defrost is more
efficient because defrost cycles are initiated only when
necessary, compared with initiation based on
operating time below the threshold temperature. All 3
to 10 ton convertible packaged heat pumps use
demand defrost.
Outdoor coil defrosting occurs only when operating in
heating mode with outdoor ambient temperature
below 52°F and the outdoor coil temperature below 33°
F. The first defrost cycle after power-up is initiated
based on operating time at the required conditions.
Shortly after completion of the defrost cycle, the
temperature difference between the outdoor coil and
outdoor air is calculated and is used as an indicator of
unit performance at dry coil conditions.
Over time, as moisture and frost accumulate on the
coil, the coil temperature will drop, increasing the
temperature difference. When the temperature
difference reaches 1.8 times the dry coil temperature
differential (∆ T), a defrost cycle is initiated. While
defrosting, the reversing valve is in the cooling
position, outdoor fans are off, and the compressors
continue to operate.
The defrost cycle is terminated when the coil
temperature rises high enough to indicate that the frost
has been eliminated. Termination of the defrost cycle
includes a “soft start” delay. At the end of each defrost
cycle, the outdoor fan comes on 5 seconds before the
reversing valve is de-energized. This reduces stress on
the compressor and makes for a quieter defrost.
There are three conditions that must be met for
demand defrost to operate:
• Heating mode with compressor(s) operating.
• Outdoor ambient < 52°F.
• Outdoor coil < 33°F on any circuit.
Figure 74. Typical demand defrost cycle
Defrost Service Test
When a service test defrost request is received, defrost
mode is initiated immediately. The defrost cycle will
remain active for a minimum of 1 minute, after which
the normal termination will occur when the coil
temperature exceeds the defrost termination
temperature as defined below. The service test defrost
request will remain active for a maximum of 10
minutes, after which the unit will transition out of the
defrost step as described below.
Defrost Mode Operation
Defrost mode shall remain active until the outdoor coil
temperature (OCT) exceeds the defrost termination
temperature (DTT) or until ten (10) minutes have
passed, whichever occurs first. If all compressor
outputs turn off during the defrost cycle, such as during
a high pressure cutout, defrost mode will terminate.
When defrost mode has terminated, the function will
track twelve (12) minutes to assure that a dry coil
condition has been achieved. At the twelve-minute
point, D
T
will be calculated using the current values of
OAT and OCT (OAT is expected to be higher than OCT).
This value is multiplied by 1.8 to calculate the new
Initiate Value.
Sequence of Operation
Demand defrost is standard feature which permits
defrost whenever coil icing conditions begin to
significantly reduce unit capacity. To permit defrost,
the outdoor temperature must be below 52°F, coil
temperature must be below 33°F, and the ∆
temperature F must exceed a RTRM calculated value.
After 30 minutes of run time under defrost permit
conditions, the RTRM initiates a defrost cycle. Upon
termination of this cycle, the RTRM monitors the
outdoor temperature (ODT) and the coil temperature
(CT) and calculates the ∆ temperature F (ODT-CT). This
value is stored in memory and the RTRM calculates a
defrost initiate value. The RTRM continually compares
the ∆ temperature F to the defrost initiate value. Once
the ∆ T reaches the initiate value, a defrost cycle is
initiated.
During the defrost cycle, the RTRM energizes the relay
(K3), which energizes the switch over valve (SOV)
through the normally open K3 relay contact. This will
then turn the outdoor fan motor(s) (ODM) ’Off’ by de-
energizing the (K8) and (K7) relays, which de-energizes
the (ODF) relays. The RTRM energizes the auxiliary
electric heat contactor (AH), and (BH) (if applicable), if
they are not operating, while maintaining compressor
(CPR1) operation. The defrost cycle is terminated based
on the RTRM termination temperature calculation
using the outdoor temperature (ODT) +47°F. The
defrost termination temperature (DTT) will be limited
between 57°F and 72°F.