Gas Furnace Types

This Gas Fired Unit Heater Serves a Large Open Area

Gas furnaces come in a variety of sizes and can be used for different applications. By and large, the most common gas furnaces use natural gas and utilize electronic ignition. Electronic ignition gas furnaces are slowly replacing the older style standing pilot furnaces where a pilot light remained lit all the time. Now, with the newer modern electronic ignition, gas furnaces only use gas when there is an actual call for heat. This adds efficiency because with the older standing pilot gas furnace the pilot remained on even in the summer months. While the heat produced by a standing pilot light is negligible, it is still added heat to the system in the summer when the whole purpose in the summer is to remove the heat from the system rather than add heat like a standing pilot would do.

A gas furnace is rated for efficiency by AFUE or Annual Fuel Utilization Efficiency rating. Furnaces installed before the nineties have efficiency ratings down to 60 percent efficient (or less in some cases). The AFUE ratings of gas furnaces is simply the amount of heat delivered by the gas furnace divided by the fuel used to make that heat. Another way to look at AFUE is to measure the amount of heat lost up the exhaust stack. If the furnace delivers 90 percent of the heat produced into the dwelling then ten percent of the heat produced is lost up the stack. Even the highest AFUE rated furnace is going to lose heat up the stack. It is nearly impossible to get 100 percent AFUE out of any gas or oil furnace simply because all oil and gas delivered to a furnace has a small amount of moisture in it. Gas typically has a moisture content of 4 to 5 percent. Oil depends on the quality of oil purchased, how many additives used in the oil, and the integrity of the oil tank and piping system. The end result of all this moisture and additives in the fuels effects the AFUE of the furnace and is the reason why no gas or oil furnace can achieve more than 96 percent efficiency.

The Standing Pilot Gas Furnace

The typical Standing Pilot Gas furnace uses a thermocouple to keep a flame lit 24 hours a day seven days a week. The pilot light is manually lit and once lit it should remain lit unless a draft blows it out or the thermocouple fails. Thermocouple failure is common but not as common as a draft blowing the pilot light out. Typically, whenever a high wind condition exists outside it is possible for the wind to find its way to the pilot light and extinguish it. If this happens to you and you are not sure how to relight the pilot light call a professional. If you follow the manufacturers safety guidelines and you feel comfortable lighting the pilot then carefully do so at your own risk. Remember, both natural gas and propane are flammable and can be explosive given the right conditions. By nature, natural gas is lighter than air and propane is heavier than air. This means that propane will settle in the lowest parts of your dwelling and natural gas will settle or get trapped in higher parts of the dwelling. Always keep this in mind and if you smell a rotten egg smell then do not light a match or lighter until the area is safely vented. Many gas fired water heaters use the standing pilot system.

The Electronic Ignition Furnace 

Electronic ignition gas furnaces are more commonly produced today than standing pilot gas furnaces. They offer added efficiency because these gas furnaces automatically light a pilot when the thermostat calls for heat while the standing pilot gas furnace requires a pilot light to be lit all the time. Most gas and oil furnaces are integrated with the air conditioning system and make use of the same blower and the electronic ignition keeps the heat out of the furnace even in the summer so there is less heat to remove by the air conditioning system. The electronic ignition gas furnace are more efficient than sanding pilot gas furnaces.

The following video demonstrates an electronic ignition gas furnace starting up. The pilot light lights first and then the main burners light after the flame has been proven. This is an older model Carrier gas furnace. One of the first Carrier furnace models on the market to utilize electronic ignition.

Gas Fired Unit Heaters

In some applications it is necessary to use gas fired unit heaters to heat a space. Gas fired unit heaters can be found in garages, mechanical rooms, electrical closets, and large open spaces in large structures like warehouses, aircraft hangers, or vehicle garages. These gas fired unit heaters are usually not ducted but they can be adapted to ductwork.

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