ACHRNEWS

Understanding Sequence Of Operation

May 20, 2005
In order to service and troubleshoot an air-source heat pump system, a service technician must understand the unit's sequence of operation. This is the order of events the system undergoes to cycle itself on and off. Knowing how the unit operates properly aids in determining where to start troubleshooting when the system doesn't operate properly. Where the system varies from its normal sequence is a major clue to any problems.

Cooling Cycle

Mechanical: Heat pump cooling operation is similar to the operation of a standard cooling system.

1. The compressor pumps out high-pressure, superheated refrigerant vapor.

2. The vapor leaves the compressor and passes through the reversing valve.

3. It flows through the outdoor vapor line to the finned outdoor coil. Air from the outdoor fan removes heat from the refrigerant vapor. When enough heat is removed, the vapor condenses into a high-pressure liquid. The liquid temperature is slightly warmer than ambient air temperature.

4. This warm, high-pressure liquid leaves the outdoor coil, and flows through the copper refrigerant liquid line.

5. At the end of the liquid line, the refrigerant passes through a metering device, reducing its pressure and temperature.

6. As the liquid, under reduced pressure, enters the indoor coil surface, it expands and absorbs heat from the indoor air passing over the finned surface. Heat from the indoor air causes the low-pressure liquid to evaporate and cools the indoor air. The refrigerant is now a cool vapor.

7. The refrigerant vapor travels through the insulated vapor line to the reversing valve. The reversing valve directs the refrigerant into the accumulator.

8. The accumulator controls liquid refrigerant and refrigerant oil flow back to the compressor.

9. Refrigerant vapor flows through the suction line to the compressor. The cycle then repeats.

Electrical: The electrical cycle is also similar to a standard cooling system.

1. The thermostat calls for cooling.

2. This sends a 24-volt signal through the "Y" terminal to the compressor contactor in the outdoor unit. The compressor and outdoor fan start.

3. At the same time a 24-volt signal flows through the "G" terminal to the indoor blower relay. The indoor blower starts.

4. The cooling system is now in operation.

5. The thermostat satisfies and ends the call for cooling.

6. This ends the 24-volt signal to the compressor contactor and the outdoor unit stops.

7. This ends the 24-volt signal to the indoor blower relay and the indoor blower stops.

8. The system is now off.

Knowing a heat pump’s proper sequence of operation aids in determining where to start troubleshooting when the system doesn’t operate properly. (Photo courtesy of Rheem.)

Heating Cycle

Mechanical: System operation is basically the same as operation during the cooling cycle. The difference is the position of the reversing valve that reverses refrigerant flow.

1. Setting the thermostat to the heat mode automatically powers the solenoid valve in the reversing valve.

2. The compressor pumps out high-pressure, superheated refrigerant vapor.

3. The vapor leaves the compressor and passes through the reversing valve.

4. Refrigerant flows through the insulated, indoor vapor line to the finned indoor coil. Air from the indoor blower removes heat from the refrigerant vapor warming the indoor air and heating the house. When enough heat is removed, the vapor condenses into a high-pressure liquid. The liquid temperature is slightly warmer than indoor air temperature.

5. This warm, high-pressure liquid leaves the indoor coil, flows through the small copper refrigerant liquid line, and exits the building.

6. At the end of the liquid line, the refrigerant passes through a metering device in the outdoor coil, reducing its pressure and temperature.

7. As the cool liquid, under reduced pressure, enters the outdoor coil surface, it expands and absorbs heat from the outdoor air passing over the finned surface. Heat, from the outdoor air, causes the low-pressure liquid to evaporate. The refrigerant is now a cold vapor.

8. The cold refrigerant vapor travels through the larger, outdoor vapor line to the reversing valve. The reversing valve directs the refrigerant into the accumulator.

9. The accumulator holds liquid refrigerant and refrigerant oil and controls their flow back to the compressor. They flow out through a small port inside the accumulator bottom.

10. Refrigerant vapor flows through the suction line to the intake of the compressor. The cycle then repeats.

Electrical: The heating electrical cycle is similar to the cooling cycle.

1. Setting the thermostat to the heat mode automatically powers the reversing valve solenoid.

2. The thermostat calls for first stage heat.

3. This sends a 24-volt signal through the "Y" terminal to the compressor contactor in the outdoor unit. The compressor and outdoor fan start.

4. At the same time a 24-volt signal flows through the "G" terminal to the indoor blower relay. The indoor blower starts.

5. The heating system is now in operation.

6. If first stage heating is not enough to heat the building, the second stage thermostat bulb makes a call for more heat.

7. A 24-volt signal flows through the "W2" terminal to the heating relay in the indoor air handler.

8. This sequencing relay cycles on electric elements to add more heat to the indoor airstream.

9. As the building warms, the second stage call for heat ends.

10. This breaks the 24-volt signal to the "W2" terminal and de-energizes the heating relay.

11. The electric heat element(s) cycle off.

12. The first stage thermostat call satisfies and ends the call for heat.

13. This ends the 24-volt signal to the compressor contactor and the outdoor unit stops.

14. This ends the 24-volt signal to the indoor blower relay and it stops.

15. The system is now off. The reversing valve pilot solenoid stays energized as long as the thermostat is set for heating.

Defrost Cycle

Mechanical: In heating mode, the outdoor coil is the evaporator. Moisture from the outdoor air condenses on the cooler coil and normally runs off. During the colder part of the heating season, this moisture freezes and blocks air movement through the coil. The frost is removed in the defrost cycle.

1. The heat pump operates in the heating mode.

2. The defrost control detects the buildup of ice on the outdoor coil.

3. The reversing valve solenoid de-energizes, directing hot gas from the compressor to the outdoor coil to defrost.

4. The outdoor fan stops. If it didn't, cold air from the fan prevents the melting effect of the hot refrigerant.

5. As the temperature of the indoor air drops, controls energize the electric heat elements to warm the indoor air.

6. When the defrost control detects the ice has melted, it terminates the defrost mode.

7. The reversing valve shifts to the heating position and directs hot refrigerant gas to the indoor coil.

8. The outdoor fan operates.

9. The electric elements cycle off.

10. The unit is now in the normal heating mode.

Electrical: A defrost control must recognize when there is a layer of ice on the outdoor coil and when that ice must be removed. There are several different types of defrost controls. While they vary in the methods used to recognize when defrost is necessary, they all take the same action. These controls also must determine when the ice is gone and terminate defrost.

1. The defrost control initiates a defrost cycle when ice builds up on the outdoor coil.

2. The control energizes the onboard defrost relay with 24 volts.

3. The defrost relay contacts open to de-energize the reversing valve.

4. The defrost relay contacts break power to the outdoor fan.

5. The defrost relay powers the heat relay to bring on the indoor electric heat.

6. After the ice is defrosted, the defrost control terminates the defrost cycle by de-energizing the defrost relay.

7. The defrost relay contacts close, sending 24-volt power to

the reversing valve, and the valve returns to the heating position.

8. The defrost relay contacts close, sending power to the outdoor fan.

9. The defrost relay contacts open, breaking 24-volt power to the indoor heating relay.

10. The heat pump is now in the normal heating mode.

Emergency Heat

Mechanical: The emergency heat setting on the heat pump thermostat is manually selected by the equipment owner. This is usually in response to a malfunction in the outdoor unit. Doing so locks out the outdoor unit.

The indoor auxiliary heating system must provide the heat required. Setting the thermostat to the heat position allows the outdoor unit to operate.

Due to the expense of electric resistance heating compared to the efficiency of the heat pump, repairs should be made as soon as possible.

1. Manually select the emergency heat position on the thermostat subbase.

2. The outdoor unit stops all operation.

3. On a call for heat, the in-door unit becomes the sole heat source.

Electrical: Setting the thermostat for the emergency heat mode de-energizes the compressor contactor in the outdoor unit and the indoor blower relay. A call for heat energizes the heating relay in the indoor air handler. This brings on the electric heating elements.

In some cases, selecting emergency heat also powers an emergency heat relay. This relay's contacts electrically bypass any outdoor thermostats used to stage the electric heat elements. This provides the thermostat with full heat from the indoor electric elements.

1. Moving the thermostat selector to the emergency heat position breaks the electrical circuit to the compressor contactor and the indoor blower relay.

2. This action powers the red emergency heat warning light.

3. A thermostat heat call energizes the electric heat relay.

4. The electric heat relay contacts close, powering the heat elements and the indoor blower.

5. The heat call ends, and the thermostat de-energizes the electric heat relay.

6. The electric heat relay contacts open, de-energizing the electric elements and indoor blower.

7. Moving the thermostat selector to the heat position completes the circuit to the compressor contactor and indoor blower relay.

8. The red emergency heat light goes out.

Reprinted with permission from the Residential Split System Heat Pumps service manual from Rheem Air Conditioning Division, Fort Smith, Ark. For more information, visit www.rheemac.com.

Publication date: 05/23/2005