Condenser Defrost (Heat Pumps Only); Demand Defrost Control - Trane SYMBIO 700 Troubleshooting Manual

Condenser Defrost (Heat Pumps Only)

Demand Defrost Control

The Symbio 700 Demand Defrost sequence matches the sequence on Reliatel controls, with the
exception of changes in diagnostic names:
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 (ΔT), a defrost cycle is initiated. While defrosting, the switchover 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 switchover valve is de-energized. This
reduces stress on the compressor and makes for a quieter defrost.
During the defrost cycle, a stage of auxiliary heat is turned ON if not already operating.
The defrost cycle is terminated based on the termination temperature calculation using the outdoor
temperature (OAT) +47°F. The defrost termination temperature (DTT) will be limited between 57°F and
72°F.
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