HVAC121 – Gas Heat Flame Rectification

Flame Rectification Every automatic (intermittent) ignition system needs to be able to sense when flame is present or not. Flame rectification is this sensing of the flame.

Flame Rectification Once the ignition process starts the spark or hot surface igniter ignites the flame. Once the flame is ignited the flame is a conductor. A little AC voltage is sent through the flame sensor rod.

Flame Rectification The flame then converts this AC voltage to DC and sends it to ground. The control senses this current flow and knows that there is a flame. This is called flame rectification.

Flame Rectification

Flame Rectification The flow of current is sensed by the ignition control. This verifies that the flame is lit When the pilot flame is verified the main burner is allowed to open. The flame goes out, the DC current stops flowing to ground. The control senses the stop of the current flow and closes the main burner.

Flame Rectification When the flame is verified the spark in the spark ignition system is shut down. When the flame is verified the HSI in the Hot Surface Ignition System is shut down.

Troubleshooting When the ignition process fails on a spark ignition system there are two parts to check. The sensor / Spark wire The ground wire

Troubleshooting Spark Wire: Just like a automotive spark wire make sure all contact points are good and make sure that there are no frayed or missing pieces on the insulation. Make sure all connections are tight.

Troubleshooting Ground wire: Replace the ground wire. Run a new ground wire from the control board, to the burner assembly and back to a true frame ground of the equipment. Make sure the equipment is properly grounded.

Troubleshooting Hot surface Ignition: Check the wire between the control and the flame sensor. Replace the ground wire, again to burner assembly and control and frame ground.

Troubleshooting Put a meter in SERIES with the ground terminal on the control and the burner ground wire. The meter should be set to Amps and will show around 2 micro amps when the flame is present.

Troubleshooting If the amperage does not show try cleaning or replacing the flame sensor. If flame sensor is clean or replaced and the ground wire and terminals are good then you have a bad control. Controls can not be repaired, they must be replaced.

Thermocouples The thermocouple is constructed of two dissimilar metals joined at one end. When heat is applied to the joint a voltage is created. A thermocouple will produce 30mv Thermopiles or Power piles can produce either 250mv or 750mv

Thermocouples

Thermocouples

Thermocouples A thermocouple is used in some furnaces to sense pilot flame. The voltage created by the thermocouple is enough to keep the pilot coil energized. The pilot coil is like a solenoid that will stay pulled in after being moved and voltage is applied to the coil by the thermocouple.

Thermocouple If the pilot flame goes out the thermocouple cools and the voltage drops to 0. The pilot coil can not stay energized and the gas valve closes.

Thermopiles What is a thermopile? Twenty-Five thermocouples put in series. This greatly increases the DC voltage output increasing current available.

Thermopiles Voltage Output: Single Thermocouple 25 to 30 Millivolts 10 Thermocouples in series 250 to 300 Millivolts 25 Thermocouples in series 625 to 750 Millivolts

Thermopiles Pilot-stat (thermopile) generates enough voltage to: Hold open the pilot valve Energize the main gas valve The holding circuit operates the same as the thermocouple circuit.

Thermopiles The thermostat is wired in series with the main gas valve. When the space temperature drops, the thermostat closes and energizes the main gas valve.

Thermopiles The thermostat with a millivolt system differs from the 24 volt system. The heat anticipator operates on a lower voltage. A milli-volt anticipator is not adjustable The 24 volt and the milli-volt thermostat will look the same but is not interchangeable.