Furnace with Hot Surface Ignition

Hot Surface Ignitor - Whenever we talk about electronic ignition systems in gas furnace ignition systems we must first look at the key components which make these systems work and understand how they work. Hot surface ignitor 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 Ignitor - Ignition

Silicon Nitride Ignitor

A vast majority of hot surface ignition systems light the main burners directly without using a pilot light. That is a simpler form of using a hot surface ignitor to light a pilot light. Then the pilot light lights the main burner. There are fewer parts to worry about for the furnace.

Therefore, fewer mechanical things 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 | Gas Furnace Hot Surface Ignition

On the other hand, a hot surface igniter system that 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. In contrast, a direct main burner ignition system is going to open the valve and allow more gas into the atmosphere. That would create a hazard if something were to go wrong with the hot surface 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 surrounding the furnace.

Electronic Ignition Systems | Safety | Gas Furnace Hot Surface Ignitor

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. That 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 igniters, or spark ignition.

The Sequence of Operation for Direct Hot Surface Ignitors | Direct Main Burner Ignition

• On a call for heat, the thermostat closes the “W” contacts. That is the signal for the furnace to begin its sequence of operation.
• Modern electronic ignition systems, including hot surface igniter systems, have a forced draft or induced draft motor. That 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. That 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.

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 ignitors 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 ignitor. If you do not know and use the wrongly rated HSI, the system will malfunction, or you will burn out the new replacement igniter.

Amperage and Resistance - How to Test

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 HSI, 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 hot surface igniter. One is to make sure you are getting the proper voltage from the board.

• Hot Surface Igniter Voltage Test: You disconnect the old hot surface ignitor 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 of 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 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 addressing. Further gas furnace troubleshooting is warranted.

Hot Surface Ignitor Problems

Gas has been shut off and locked by the gas company

The biggest problem encountered with hot surface igniters is an open or cracked circuit through the igniter. You check this by using the above procedure or test to determine the Ohms or resistance through the igniter. The next problem you can encounter with a hot surface igniter 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 sometimes realize 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 is not locked for whatever reason. It is important not to touch the hot surface igniter 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 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.

Gas Furnace Electronic Ignition Systems: Hot Surface Ignitor

Resource: The Home Comfort Book: The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home