Furnace Electronic Ignition Systems: Hot Surface Igniter

Whenever we talk about electronic ignition systems we must first look at the key components which make these systems work and understand how they work. Hot surface ignition systems work in various ways depending on what the manufacturer and design engineer determined in the design. Some are direct main burner ignition while others light a pilot and the pilot lights the main burners. A vast majority of hot surface ignition systems light the main burners directly without using a pilot light. This is a simpler form of using a hot surface igniter to light a pilot light and then have the pilot light the main burner. There are fewer parts to worry about and therefore less mechanical things which can break down or go wrong to worry about in a direct main burner ignition system.

Safety Factors of Electronic Ignition: Hot Surface Ignition

On the other hand, a hot surface ignition system which lights a pilot light first and allows the pilot to light the main burner will allow less gas into the atmosphere when the pilot valve inside the gas valve opens whereas a direct main burner ignition system is going to open the valve and allow more gas into the atmosphere. That creates a hazard if something were to go wrong with igniter by allowing more gas into the atmosphere. There are safety back ups for this scenario. If the flame detector does not sense a flame within a specific period of time then the main valve shuts down. So if there was a problem igniting the main burners, or even the pilot light, the valve shuts down preventing un-ignited gas from entering the atmosphere wherever the furnace is located. A delay timer kicks in and allows a specific amount of time before a retry of ignition. Most gas controls allow 3 to 4 retries for ignition and then it shuts the system down for more than an hour. This prevents a dangerous build up of combustible gases by allowing the gases to dissipate before trying to light the burners again. Even systems with a pilot have this feature as it is imperative that a dangerous amount of gas not be allowed to build up and create a dangerous situation. The flame sensor and the timer controls in the gas control board are responsible for this safety feature in electronic ignition gas controls. These safety features offer you a safety factor in electronic ignition systems that make an explosion practically impossible with electronic ignition gas systems whether these systems are direct, indirect, hot surface ignition, or spark ignition.

Sequence of Operation for Direct Hot Surface Ignition: Direct Main Burner Ignition

• On a call for heat the thermostat closes the "W" contacts. This is the signal for the furnace to begin its sequence of operation.
• Modern electronic ignition systems including hot surface ignition systems have a forced draft or induced draft motor which creates a flow of combustion air for the fire and allows for forced venting of the combustion gases. This motor starts up and begins introducing combustion air into the main burner area.
• A pressure switch located in the system senses the operation of the induced or forced draft motor and closes contacts proving to the gas control board that the motor is on and functioning.
• The control board determines everything is good and it energizes the hot surface igniter. One can view this operation by observing the hot surface igniter glowing red hot.
• After a specific amount of time the gas burner control board opens the gas valve allowing for gas to flow to the main burners.
• When the gas reaches the main burners and hot surface igniter the gas begins to burn. Located on the burners is a flame sensor. If the flame sensor does not detect a flame within a specific period of time it shuts the gas valve off preventing raw gas from entering the atmosphere around the furnace which would create a dangerous situation. If the flame sensor senses a flame the control board stops energizing the hot surface igniter and the flame continues to burn until the thermostat is satisfied.
• When the thermostat is satisfied the "W" terminal opens up and the gas control board shuts down the gas valve. The induced draft or forced draft motor continues to run for a few minutes to dissipate excess heat and combustion gases from the system.

Hot Surface Igniter Technical

The hot surface ignition system is a good system. The hot surface igniter is made from a silicon carbide element which glows red hot when electricity is passed through it. Most hot surface igniters work off of 120 volts but there are a few out there that work off of 240 volts (Honeywell Smartvalve Hot Surface Pilot Ignition uses 24 volts). It is important not to touch the element because oil from the fingers can cause problems with the hot surface igniter element. The most common reason for failure of hot surface igniter is they develop a crack in them which prevents a closed electrical circuit when energized. By and large hot surface ignition systems are very reliable heating systems and a very good system of lighting gas.

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