Gas Furnace Electronic Ignition Systems: Hot Surface Igniter

Furnace with Hot Surface Ignition

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. This all depends on what the electronic ignition manufacturer and design engineer determined in the design. Furthermore, some are direct main burner ignition while others light a pilot and the pilot lights the main burners.

Safety Factors of Electronic Ignition: Hot Surface Igniter - Ignition

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. Then the pilot light lights the main burner. There are fewer parts to worry about. Therefore. less mechanical things which can break down or go wrong in a direct main burner ignition system. Furthermore, these electronic ignition systems work with both natural gas and propane gas.

Direct Ignition Versus Intermittent Pilot

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 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.

Electronic Ignition Systems: Hot Surface Igniter | Safety

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. Then it shuts the system down for more than an hour. This prevents a dangerous buildup of combustible gases by allowing the gases to dissipate. To dissipate before trying to light the burners again. Even systems with a pilot have this feature. 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. These safety features offer you a safety factor in electronic ignition systems that make an explosion practically impossible. 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. This 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/module 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 it shuts the gas valve off. This prevents 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. The flame continues to burn until the thermostat is satisfied.
  • Lastly, 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. Consequently, this dissipates excess heat and combustion gases from the system.

Hot Surface Igniter Technical: How to Check if Furnace Ignitor is Bad

Supply Volts

When replacing a hot surface igniter it is important to know the supply volts needed for the new igniter. Most hot surface igniters work off of 120 volts. However, there are a few out there that work off of 240 volts. The Honeywell SmartValve Hot Surface Pilot Ignition uses 24 volts. Before purchasing a replacement make sure you know exactly the amount of voltage supplied by the control board that powers the hot surface igniter. If you do not know and use the wrongly rated hot surface igniter the system will malfunction or you will burn out the new replacement igniter.

Amperage and Resistance - Testing an Existing Hot Surface Igniter - How to Test the Hot Surface Igniter

If the furnace is not igniting and you suspect it is the hot surface igniter then you need to test the igniter for continuity and resistance. There are many different brands of hot surface igniters and each has a unique resistance and amperage operating amps and different resistances. While some have very similar test results no two are the same especially for the resistance or number of ohms you will read from the hot surface igniter when testing it with a multimeter or ohmmeter. Most of the time there are no specifics given with the old HSI (hot surface igniter) device so you will need to go by two methods of testing the HSI. One is to make sure you are getting the proper voltage from the board.

  • Hot Surface Ignition Voltage Test: You simply disconnect the old HSI and place your meter leads on the wires coming from the board. You energize the furnace on a call for heat and make sure you get the proper amount voltage from the board. To be safe I would turn the gas supply off to ensure no gas is released during the test. Additionally, make sure you take care not to get shocked during this process. After all, this is a live voltage test.
  • Using an Ohmmeter or a multimeter set to Ohms you disconnect the hot surface igniter and give it a quick continuity test. Different HSI’s have different resistance targets as you will see from the following chart.

Hot Surface Igniter Test Chart
Model#Brand/Type/ManufacturerResistance Range @ Room TemperatureAmps/Volts
UnknownWhite-Rodgers/Used in Nordyne Equipment18 to 50 Ohms2 to 3 Amps @ 240 volts
767 SeriesWhite-Rodgers Nonspecific/Silicon Carbide40 to 75 OhmsUnknown
768 SeriesWhite-Rodgers/Used in Trane and Lennox/Silicon Nitride11 to 20 OhmsUnknown
21D64 SeriesWhite-Rodgers/Universal Generic Replacement6 to 15 Ohms3 amps @ 120 volts
201Norton/Robertshaw/Silicon Carbide45 to 90 Ohms4 to 4.75 Amps @ 120 volts
231Norton/Robertshaw/Silicon Carbide40 to 90 Ohms4 to 4.75 Amps @ 120 volts
271Norton/Robertshaw/Silicon Carbide40 to 75 Ohms4 to 4.75 Amps @ 120 volts
Q 3400A for SmartValvesHoneywell4 to 12 OhmsAmps Unknown @ 24 volts
Q3200 SeriesHoneywell45 to 90 OhmsAmps Unknown @ 120 volts

Hot Surface Ignition Test Results

If you get infinity on the meter then the hot surface igniter is bad and needs to be replaced. If you get a value way over or way under the listed resistance in the chart then it needs to be replaced with a new hot surface igniter. If the hot surface igniter is glowing red hot when in operation for the ignition then it is good. With no ignition then you have another issue other than the hot surface igniter that needs to be addressed. Further gas furnace troubleshooting is warranted.

Hot Surface Ignitor Problems

gas meter locked

Gas has been shut off and locked by the gas company

The biggest problem encountered with hot surface ignitors is an open or cracked circuit through the ignitor. This is checked by using the above procedure or test to determine the Ohms or resistance through the ignitor. The next problem you can encounter with a hot surface ignitor is not getting voltage from the board to power the igniter. The ignition problem could be one of many problems that are caused by either a malfunctioning part in the gas furnace or a problem with the fuel supply for the gas furnace. One of the first checks to make is to make sure the furnace is calling for ignition or the thermostat is set to heat. The next thing to check is to make sure you have the fuel. As noted in other articles, people do not realize sometimes their gas has been shut off for various reasons including a potential gas leak or they forgot to pay the bill.

Always make sure you check to make sure your gas meter has not been locked for whatever reason. It is important not to touch the hot surface ignitor element when handling it. If you handle it, make sure to touch the porcelain part and the not ignition portion. This is because oil from the fingers can cause problems with the hot surface igniter element. I’ve been told by some that that is an old wives tale but better to be safe than sorry. The most common reason for the failure of the hot surface igniter is they develop a crack. Consequently, a crack that prevents a closed electrical circuit when energized. Furthermore, hot surface ignition systems are very reliable heating systems and a very good system of lighting gas.


High Performance HVAC

Gas Furnace Electronic Ignition Systems: Hot Surface Igniter

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