Gas Heating Systems

Gas combustion heating systems are the most common type of home heating system in this country. Gas equipment can be fueled by either natural gas or propane with some modifications to accounting for the differences between the gases. Propane is sometimes referred to as LP for liquefied petroleum, but for the purpose of this overview, we'll refer to both natural gas and propane as "gas" since the equipment is essentially the same. The most common systems for using gas heat are forced air furnaces, water and steam boilers, and room space heaters.

To operate, gas is delivered to the heating unit from a pipeline or tank. Along its way to the heating unit, it passes through a pressure regulator that keeps a slight positive pressure on the gas relative to the pressure in the burner. This assures gas flows at an even pressure and rate. To ignite the gas when heat is needed, older systems use a pilot light that burns 24-hours a day. Newer systems have automatic ignitions that eliminate the need for the wasteful, ever-burning pilot light. Once the full burner is ignited, gas combines with air producing a flame and giving off heat. When working properly the flame should be blue in color and of a well defined shape. Yellow or orange in the flame indicates combustion is not complete due to lack of air. This can cause soot to build up on the heat exchanger reducing its efficiency. Worse, it can also cause the creation of carbon monoxide, which can be deadly if it seeps into the home. Therefore it is important to keep gas combustion equipment well maintained and tuned-up.

Inside the gas heating unit, gas burns in what is called a combustion chamber. Here it gives up much of its heat to the wells of the heat exchanger as it moves along through the venting system and out of the home through a flue pipe or chimney. If the heating unit is a furnace, cool air is blown over the heat exchanger box, which carries the heat throughout the house. In a boiler, water or steam is the distribution medium.

Flue gasses have to be hot enough that they will rise out of the unit, but if they are too hot, they take expensive heat up the chimney with them. Higher heating efficiency is achieved by slowing the rise of the exhaust and adding more heat exchange surface area to collect the heat.

Gas heating systems have several controls dictating when various parts operate. The one most know well is the wall thermostat that monitors room temperature and calls for heat when it drops below a set point. There is also a fan thermostat inside the unit that tells the fan when to turn on and shut off. It comes on only after the unit has achieved some minimum temperature to avoid blowing cold air, and remains on for a few minutes after the burner goes off to extract heat that would otherwise be wasted. Boilers have aquastats to control the flow of water or steam.