How Hot Surface Ignition Works | Safety First
Any tas furnace ignition system needs to be safe. In the safety requirements for any method or design, we need to consider the following:
For this requirement, we need to make sure we provide an igniter that will give a temperature above the ignition temperature of the gas. For natural gas or propane, that temperature is over 1200°F. When voltage is applied to a hot surface igniter, the temperature exceeds 2000° F. Therefore; hot surface ignition ensures we will light the gas.
Verification of Ignition
There must be a method of verifying the flame. That the hot surface igniter did its job and lit the gas into a flame. For flame verification, we will use flame rectification. What is flame rectification? Flame rectification uses a flame sensor to detect a minimal amount of amperage generated by the flame. The microamps pass through the flame sensor and are detected in the ignition control board.
Ignition Types and Methods | How Hot Surface Ignition Works
There are different types of hot surface ignition. First, most hot surface igniters operate off of 120-volts. There are some manufacturers the make 230-volt hot surface igniters. Additionally, Honeywell makes, what they call, a Smart Valve.
This type of hot surface ignition differs from a typical hot surface ignition system by lighting a pilot light first. Then the pilot lights the main burners after the board verifies the pilot is lit. Therefore, everything is done through the gas valve from providing the ignition source to the flame rectification process.
Direct Ignition Sequence for Hot Surface Ignition
A majority of hot surface ignition systems light the main burners directly. This simplifies the process and eliminates a pilot like the Smart Valve described above.
- On a call for heat from the thermostat, the inducer motor engages
- As soon as the control board verifies all safeties are good and the pressure switch is closed, it engages the hot surface igniter by energizing it with voltage
- There is a slight delay allowing the hot surface igniter to reach temperature. The hot surface igniter will glow red hot.
- The gas valve will open feeding the burners with raw un-ignited gas.
- When the gas hits the igniter it lights. The hot surface igniter is strategically located for ignition purposes.
- The flame sensor senses the fire and continues allowing the gas valve to feed gas to the burners. It stops feeding voltage to the hot surface igniter.
- When the thermostat satisfies, the gas valve closes. The blower runs for a time dissipating the heat in the furnace and then shuts down.
That is the basic sequence of the hot surface ignition system.
What is it made of? | How Hot Surface Ignition Works
Hot surface igniters are ceramic. There are two specific types of hot surface igniters.
- Silicon Carbide Igniters
- Silicon Nitride Igniters
The silicon nitride is considered the better of the two because it is less fragile. Hot surface igniters, being ceramic, are prone to breaking or cracking. Any hot surface igniter should be handled with care
Durability and Reliability
The largest problem with a hot surface ignition system is cracking. That is my perspective as a former technician in the field. There are some hit surface ignition systems that never have problems and work for the life of the furnace. There are others that need to be replaced every four or five years and sometimes more.
Careful handling of the hot surface igniter is necessary. Some contradictory information I’ve learned is that the oil from your hands can cause it to crack. I was told by a Honeywell engineer that it is not true. Either way, whether true or not, it is still a good idea to handle it with care. Never touch the ignition portion of the hot surface igniter.
Finally, vibration and shock need to be taken into account for the hot surface ignition system. Too much shock or vibration can cause cracks in either type of system described above. If you have problems with cracking then make sure the furnace is isolated from vibration or shock of any kind.
How Hot Surface Ignition Works