Gas Furnace Components 3Gas Furnace Components 3 - Gas furnace ignition controls vary from furnace manufacturer to furnace manufacturer. Some are proprietary and some are from an engineering company that specializes in gas furnace ignition controls. Honeywell being a major manufacturer of controls for HVAC also makes several gas furnace ignition controls.

Gas furnace ignition controls have advanced by leaps and bounds over the last 20 years. The standard was standing pilot ignition. Solid-state ignition controls are safer and more efficient than ignition controls of yesteryear.


Gas Furnace Components 3 | Ignition Controls

Some manufacturers lag in technology. Others want to keep it simple. The new modern systems offer safety and efficiency in all their gas furnace controls. Honeywell was the pioneer as they are with much of the advanced HVAC technology. Lennox and Carrier came in just behind Honeywell with advancements in the engineering of gas furnace ignition controls. These companies changed the entire HVAC gas furnace industry with their innovative gas controls.

Gas Furnace Components 3 | Flame Sensors

A way of sensing a flame other than the thermocouple or thermophile was first introduced by Honeywell engineers. Other furnace manufacturers soon adopted it once the technology was proven safe and efficient. The modern systems proved safer and more efficient than the standing pilot system. The standing pilot system uses a thermocouple or thermophile to prove a flame.

The problem with the thermophile and the thermocouple was that it required a flame 24/7. This requires someone to manually light the pilot light anytime it goes out. With the introduction of flame rectification from Honeywell, this all changed. Flame rectification was a new method. It did not require a flame 24/7. Using electronics and printed circuit boards the technology of flame rectification began replacing standing pilot ignition control systems. The new electronic ignition systems use a flame sensor which is nothing more than a steel rod.

Electronic Ignition for Modern Times

More and more gas furnace systems came on the market with automatic electronic ignition and flame rectification technology. The newer, more efficient systems began replacing standing pilot systems. Flame rectification worked, and engineers with the major manufacturers adopted the flame rectification technology.

White-Rogers and Carrier had different methods of proving the flame. However, they never caught on as well as the flame rectification method caught on. Carrier used a bi-metal strip. When a flame was present on the bi-metal strip, it snapped to a different position closing the contacts. That proved the flame of a pilot light. Once this pilot flame was established, the main burners ignited off the pilot light.

White-Rogers had a different method. It used a pilot and a sensor that contained mercury connected to a diaphragm via a cap tube. When the pilot was lit on a call for heat, the mercury heated up. That closed the contacts on the diaphragm assembly, proving that a flame was present. This system also never caught on but as with the Carrier system. However, these systems still exist in some gas furnaces today.

Gas Furnace Components 3 | Flame Rectifiers and Modern Gas Ignition

Flame rectification takes a standard operating voltage of 120 volts and converts the voltage to micro-amps. A flame sensor senses this minuscule amperage. Then a solid-state printed circuit board processes this signal of micro-amps proving the flame. A gas valve will not feed gas to the burners unless a flame is present. The flame ignites the gas being fed to the burners.

Flame rectification proves the flame reliably, and it provides more efficiency than the standing pilot ignition system. Gas furnace manufacturers currently use two different types of ignition methods for lighting the main burners on a gas furnace. There is intermittent pilot burner ignition and there is direct burner ignition.

The intermittent pilot lights a pilot. After the pilot flame is proven, a signal goes to the gas valve. The gas valve opens the main valve to the main burners. The burners light off of the pilot light.

Direct ignition lights the main burners with either a spark or a hot surface igniter. The flame sensor using flame rectification senses that the burners are lit. As long as it senses fire, the gas valve continues to feed gas to the burners.

These flame sensors are susceptible, and if they are fouled in any way, the furnace will not fire. Occasional maintenance of the gas furnace should include cleaning this sensor by a qualified HVAC technician. The chances are that your furnace uses this technology and perhaps even your gas water heater.

Safety Controls - Gas Furnace Components 3

The flame rectifier and pressure switch are part of the safety controls. There are other controls on the gas furnace that are necessary for the safe operation of the furnace. Other safety controls include limit switches. These sense temperature and prevent the burners from operating if the temperatures exceed safe limits.

A typical gas furnace could have four or more limit switches in various locations throughout the furnace. When these limit switches trip or opens, the control board kills the gas valve. That engages the main blower fan and the induced draft or forced draft blower. These blowers run to dissipate as much heat as possible. Some of these limit switches reset themselves when the heat returns to normal. When they reset, the furnace control board will give a small time delay before allowing the gas burners to reignite.

Safety Circuits

The heat will be restored, and the gas burners will continue to provide heat until the thermostat satisfies. Or the safety limit opens again. If the limit switch remains open or is a manual reset limit, the furnace will attempt to light three to four times. That depends on the manufacturer and the engineering safety limits, which went into the gas furnace control board.

If the safety circuits are open, the gas furnace control board detects this and locks the system out for a specific amount of time (usually measured in hours) again depending on the engineering safety limits which went into the control board circuit. After the specified amount of time, the control board will still try for ignition unless the safety circuit is open. The board will stay in a permanent lockout mode until the problem is resolved, usually by an HVAC technician.

Sometimes the problem is much more than just a malfunctioning limit switch that needs to be reset. When a safety controls trip, the problem needs the attention of a qualified HVAC technician so that all possible dangerous conditions can be eliminated, and reliable and safe heat can be restored to the structure or dwelling.

The Blower Motor - Gas Furnace Components 3

A gas furnace needs to move air across the heat exchanger, so enter the gas furnace blower motor. The gas furnace blower motor is usually in the same cabinet as the gas furnace heat exchanger and other parts of the gas furnace. The gas furnace blower motor is usually controlled by the same board, which controls the burners and ignition. The same gas furnace system can also be integrated and used with an air conditioning system.

There are some high-efficiency systems that utilize a variable speed ECM blower motor. These blower motors use DC, which is supplied by a separate solid-state board, which contains a rectifier to convert AC voltage into DC voltage. Control of these blower motors and what determines the speed they run at is different from furnace manufacturer to furnace manufacturer.

These variable-speed blower motors can run at different speeds as determined by the control board and the engineering which went into the control board. The other type of blower motor which is in gas furnaces is the PSC or Permanent Split Capacitor blower motor. These blower motors offer good start-up torque and run more efficiently (except the variable speed blower motor) than other types of motors.


Whether the gas furnace has a variable speed blower motor or a PSC blower motor, most gas furnaces use a direct drive for the blower wheel. A blower wheel is attached to the shaft of the blower. This blower wheel moves the air required to be blown across the heat exchanger and into the ducts of the residence.

High Performance HVAC

Gas Furnace Components 3