By Dave Bird, Instructor, Dyer Appliance Academy
There are literally millions of homes in this country that use gas appliances. People use gas for cooking, drying their clothes and heating their water. As an appliance repair technician, it is very important that you have an understanding of the types of gases used in the home, as well as the different types of ignition systems used in these appliances. You need to be comfortable with both the process of combustion, gas appliance conversion and the importance of good ventilation.
When working with gas, SAFETY is of paramount importance. Not only for you but also for your customers. Be sure to always comply with local codes and follow the use and care manual for that appliance. Combustible materials should always be kept away from any gas appliance and it should be free of any food, grease or soot. You should also verify that the appliance is properly ventilated to remove any combustion by-products from the residence.
Gas is a form of chemical energy. When gas is burned, it becomes heat energy. This heat energy is normally expressed in British Thermal Units (BTU). One BTU is equal to the amount of heat needed to raise one pound of water one degree Fahrenheit.
There are two types of gas used in homes today. The first is natural gas which is a natural product extracted from the ground. It is odorless, colorless and tasteless and is lighter than air. The second type of gas is liquid petroleum gas (LP) which is man-made. LP is also odorless, colorless and tasteless but it is heavier than air. LP will accumulate in low lying areas of the floor and create a possible hazard if it comes in contact with a spark or open flame. Both natural gas and LP have an odor agent added to them to alert us of leaks.
Gas pressure is measured in inches of water column pressure (WCP). It takes 28 inches of WCP to equal one pound per square inch. In most homes natural gas is regulated down to about 5 to 9 inches of WCP entering the home while LP gas is regulated to 10 to 12 inches of WCP. The individual appliances will further regulate that pressure down to the level required for proper operation.
If it should become necessary to measure these pressures, one of two instruments are normally used. A “U” shaped glass tube known as a manometer or a magnehelic gauge. Both will provide a gas pressure reading in inches of water column pressure.
Combustion is the rapid burning of gas when mixed with air to produce heat energy and light. Any flame must have heat, air and fuel to burn. This is known as the Fire Triangle. Take away any one of these three and combustion cannot exist. Combustion by-products must be properly vented outside the home. Proper ventilation as well as adequate fresh air is crucial to the proper operation of the appliance as well as the safety of our customers in the home.
Carbon Monoxide (CO) is a by-product of incomplete combustion. It is a very toxic gas that can kill if inhaled in large amounts. The maximum acceptable level of carbon monoxide in a gas appliance is 9 parts per million. When inhaled, CO inhibits the blood from oxygenating. I recommend the use of carbon monoxide detectors to any of my customers that have gas appliances. CO is lighter than air. When properly installed, CO detectors will warn the customer before they are exposed to hazardous levels.
Flame color is an important indicator of proper combustion. When a sufficient amount of air is mixed with the gas, the result is a nice blue flame. A blue flame indicates that complete combustion is occurring and the unwanted by-products of combustion are being completely burned up by the flame. A yellow flame or a flame with yellow tips indicate an insufficient air-gas mixture and incomplete combustion. Incomplete combustion results in the release of unwanted by-products such as carbon monoxide, aldehydes and soot.
Common appliance burner components include the orifice, gas valve, air shutter, venturi throat and mixing tube.
The gas orifice, while not technically part of the burner assembly plays an important role in burner operation. The purpose of the orifice is to regulate the flow of gas into the burner. The size of the hole in the orifice is dependent on the type of gas used in the appliance. The heat density of LP is higher than natural gas so the size of the hole for an LP orifice will always be smaller than the orifice for natural gas.
The gas valve controls the gas flow to the individual burners. Some gas valves have a “push to turn” feature that helps to prevent them from being accidentally turned on.
The air shutter, also known as an air gate, is an adjustable sleeve on the outside of the burner that used to increase or decrease the primary air supply to achieve complete combustion.
The venturi throat is a slight restriction in the burner that aides in the proper blending of the gas and primary air in the mixing tube prior to combustion.
A properly ventilated appliance sends the combustion by-products including CO and water vapor out of the home. A good ventilation system also provides good air circulation and an adequate oxygen supply for the appliance.
Common ventilation issues include too many bends or elbows in the vent that restrict air movement. Disconnected or crushed venting lines as well as physical obstructions such as lint in a dyer vent can restrict or even block airflow through a vent.
Next time we’ll discuss the three types of basic gas ignition systems as well as converting a natural gas appliance to a liquid petroleum gas source. Be safe out there.