1. Introduction to Defrost
HVACR318 – Refrigeration
Introduction to Defrost

2. Purpose of Defrost Systems
Defrost systems are required in low temp and extremely low temperature applications to remove the ice that builds up on the evaporator coil.
Ice is formed when warmer, humid air comes into contact with the below freezing conditions of an evaporator coil.

3. Type of Defrost Hot Gas Defrost Electric Defrost
Uses a solenoid to re-route the discharge hot gas from the compressor to the evaporator bypassing the metering device and condenser.
Electric Defrost
Shuts down the compressor and the cooling system and energizes an electric heater.

4. Defrost Controls
The controls required to run a defrost cycle are simple:
Defrost Clock/Time
Solenoid Valve

5. Defrost Timer Controls
Initiates and terminates defrost cycle.
Defrost frequency times can be changed by adding, moving, or installing more “set” pins.
Try and keep the cycles “proportional” and keep in mind customers busy periods.

6. Defrost Time Controls
The same controls are used for both hot gas and electric defrost systems. You do not need different timers as they are pretty much standard.
Some contain a termination (clutch) solenoid to prematurely take the control out of defrost when needed.

7. Defrost Timer with Solenoid
Timer Motor N 1
Source
Source
Copper
band 3 2
Defrost Heater Or
Hot gas Solenoid X 4
To Dtt
To evaporator fan motor
To refrigeration circuit (comp and cfm)

8. Defrost Timer with Solenoid
Timer Motor X 3
Source
Source N
Copper
band 1 2
Defrost Heater Or
Hot gas Solenoid 4
To Dtt
To evaporator fan motor
To refrigeration circuit (comp and cfm)

9. Defrost Timer No Solenoid With X terminal
Timer Motor N X
Source
Source 1
Defrost Heater Or
Hot gas Solenoid 3 2 4
To evaporator fan motor
To refrigeration circuit (comp and cfm)

10. Defrost Timer No Solenoid No X terminal
Timer Motor N 1
Source
Source 3 2
Defrost Heater Or
Hot gas Solenoid 4
To evaporator fan motor
To refrigeration circuit (comp and cfm)

11. Defrost Time Control Defrost Termination DTT Type
DTT/High Limit Switch: Standard DTT which is a Defrost Thermostat, set at a high temperature to open when the ice has been melted or the coil reaches a specific temperature.
The DTT de-energizes the heater.

12. Defrost Time Control Defrost Termination DTT Fan Delay Type
Terminates and locks out the defrost cycle
A Single Pole double Throw temperature actuated switch that runs the fan in refrigeration mode and will energize the release clutch solenoid (in the DTC) in the defrost mode.

13. Defrost Time Control
When the control terminates the defrost cycle the EFM will be delayed until the compressor can make the evaporator cold enough to switch the control back to the EFM circuit.
This control prevents a “Hot Pull Down” scenario on the compressor. Hot pull down means hot gas being returned to the inlet of the compressor.