By Dave Bird, Instructor, Dyer Appliance Academy
In our last discussion we talked about the two types of gas used in the home today, natural gas and liquefied petroleum (LP). We also went over proper gas pressures, combustion and combustion by-products. We finished by touching on proper ventilation and gas burner construction.
This time we will talk about the three types of gas ignition systems commonly used in gas appliances. Then we will finish our gas fundamentals by covering converting a natural gas appliance to an LP fuel source.
Most gases appliances found in homes today use one of 3 basic gas ignition systems. They are the pilot (constant flame), hot surface ignition (glow-bar and glow-coil) and spark ignition.
The pilot ignition system has been around for many years but can still be found in a lot of homes today. It is used on both surface and oven burners. A small blue flame burns 24/7 waiting for the appliance to be needed.
In the case of a surface burner the gas valve is opened and gas from the manifold (internal gas distribution system) is allowed to flow to the desired burner after it has mixed with air to facilitate complete combustion.
The air/gas mixture enters the burner and is directed down the flash tube where it comes in contact with the pilot flame and is ignited. The flame then returns to the burner via the flash tube where it ignites the air/gas flowing from the burner head ports. Flame height at the burner is controlled by the position of the knob attached to the gas valve.
Oven burners work a little differently. When the oven temperature thermostat is set to a baking temperature, the gas supply to the oven pilot is slightly increased causing the size of the pilot flame to also increase. This larger pilot flame envelopes a thermocouple bulb which is attached to the oven safety valve. When the oven safety valve senses the correct heat level it opens and allows the gas to flow to the oven burner where the pilot flame ignites the air/gas mixture. The oven burner should light in approximately 30 to 90 seconds once the oven is turned on.
The hot surface glow-bar ignition system is probably the most common system used today. This system uses either a thermostat or electronic control, the ignitor and an oven gas valve. The control mechanism switches electrical power to the ignitor and the gas valve circuit which are in series. The ignitor starts to glow red and current then passes through it to the oven gas valve.
When sufficient current is sensed by the oven gas valve, it opens allowing gas to flow to the burner. The air/gas mixture is ignited by the red hot ignitor. When the set temperature is achieved, the control stops all electric power to the ignition system. The ignitor will dim and the gas valve will close stopping any burner flame. The oven burner will continue to cycle on and off to maintain the specific oven temperature the control is set at.
In a spark ignition system there is no pilot flame. The air/gas mixture sent to the burner is ignited by an electrical spark. An additional electronic control is used on the oven burner to monitor performance. At any point, if no flame is detected when it should be burning, the ignition system will be locked out in order to stop the flow of gas into the oven.
All gas appliances shipped from the manufacturer are configured to operate using natural gas. Should the customer decide they would like to use liquid petroleum (LP) as their fuel source, the appliance would need to be converted in order to function correctly.
I recently converted a gas dryer from natural gas to LP. I ordered a conversion kit for the make and model dryer in question. I received a kit that included a brass fitting that was screwed in the gas valve to permit proper gas pressure from the valve for LP. The kit also had a new orifice of the proper size to accommodate proper LP gas flow rate. There were also complete written instructions on how to properly perform the conversion as well as labels to document that a conversion had been performed. Once the burner assembly was exposed, the conversion only took a few minutes.
Next month we will talk about the heart of any refrigerated appliance, the sealed system. What is it and what are the components that make it work. Until then, be safe.