RT-SVD03G-EN 93
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 3-10 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 (delta 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.
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
Figure 56. Typical Demand Defrost Cycle
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