Gas Furnace Ignition Control Operation | HVAC Heating

• The gas furnace control board enables the induced draft motor and then waits for

• The pressure switch to signal that it has closed

• This completes all the safety circuits as long as no limit switches are open

• Once the control board has verified that all safety switches are closed it energizes the hot surface igniter or the intermittent pilot igniter

• If the system is equipped with hot surface ignition a small amount of delay time is experienced while the hot surface igniter heats up and then the gas valve opens and feeds gas to the main burners. If the system is equipped with the intermittent pilot the intermittent pilot valve opens on the gas valve at the same time the spark ignition begins sparking.

• The gas furnace control board verifies a flame whether the system is hot surface ignition or intermittent pilot. The gas furnace control board uses flame rectification to verify the flame is lit. With an intermittent pilot system once the flame has been verified the gas furnace control board signals the main gas valve to open to feed gas to the main burners.

• The main burners fire for 30 seconds to 1 minute and the gas furnace control board turns the main blower on to blow air throughout the ductwork.

• After the thermostat is satisfied and the space has reached the desired temperature the gas furnace control board shuts down all gas to main burners and cycles the system down. The main blower fan continues to run for a few minutes to dissipate excess heat from the heat exchanger.

• Some gas furnace control boards even have diagnostic abilities to help an HVAC technician troubleshoot the gas furnace.

 

 

 

AC Unit

1. The thermostat, set to cooling mode, calls for cooling. The switch inside the thermostat closes. That energizes the “Y” and the “G” circuit in the thermostat or the compressor contactor and the fan circuit.

2. The compressor contactor closes, and the condenser fan motor and the compressor turns on. That happens simultaneously as the compressor contactor controls both the compressor and the condenser fan motor. That is important because if the compressor contactor pulls in or engages and the fan doesn’t start, but the compressor does, then the technician knows there is a problem with the condenser fan motor and vice versa. If neither starts, but the contactor is pulled in, then the technician knows there is either a problem with the contactor passing voltage across the contacts or a power problem. If the contactor does not pull-in, then the tech knows there is a control problem.
   Condenser Fan Motor

3. At the same time the compressor contactor is doing its job, the blower fan relay closes (from the “G” contact in the thermostat) and energizes the blower fan. While the compressor gets the refrigerant moving through the refrigeration system, the fan begins moving air through the duct system and across the evaporator coil.

4. When the thermostat satisfies the “Y” and the “G” contacts in the thermostat open de-energizing the compressor contactor and the fan relay. The compressor and condenser fan

Heat Pumps

 

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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.

This also sends a 24-volt signal through the "O" terminal to the reversing valve pilot solenoid 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. The reversing valve pilot solenoid stays energized as long as the thermostat is set for cooling.

 

 

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.

 

 

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 and or display on the thermostat as "Emergency Heat".

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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Electric Water Heater

 

The basic operation of a two thermostat system (upper and lower) on an electric water heater of 240 volt supply is as follows:

Only one element will come on at any one time. This is known as a flip/flop system. On a 240 volt water heater, there will always be 120 volts to both

elements. The thermostat will direct the second leg of the 120-volts to the element to complete the 240 volts required to energize the element.

 

Normal operation: When hot water is being used, cold water enters the bottom of the heater (either through the bottom inlet nipple or the dip tube), the bottom thermostat closes and the element will begin to heat the cold water. When a significant amount of hot water has been used, the upper thermostat will take priority and heat up the top portion of the heater. Once heated, it will flip/switch power down to the lower thermostat and heat the lower portion.

High Limit Control: All electric water heaters are supplied with a high limit control switch. This switch is a safety device designed to shut the unit off if it overheats and the water reaches an unsafe temperature. Power to the thermostats and elements is completely cut off when it trips. The high limit control can be reset by firmly pushing on the red button above the upper thermostat. An audible click can be heard when it resets. If the high limit control trips frequently it is an indication of additional problems. Contact a qualified technician for service.