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Gas Furnace Repair

Gas Furnace Component Proper Function and Adjustments

Keeping your gas burning furnace tuned up assures that it is running in the most efficient manner. Ventilation, a clean burner, heat exchanger, and exhaust flue are the keys to a high efficiency.

Gas Furnace
Gas Furnace at Home

The Turn On Sequence

You set the thermostat to a certain temperature to heat your home. If the temperature is below your set point, natural gas is fed the the burners and ignited by a pilot light or electronic spark. The flames warm up a heat transfer chamber. When the chamber reaches a certain temperature a large blower fan turns on and pushes the heat into the house living space.

The Heat Exchanger

Gas Furnace Heat Exchanger
Gas Furnace Heat Exchanger Diagram

The carbon exhaust fumes do not enter the heat exchanger, but are exited out of a flue to the outside. There is an opening top the top side of the furnace that allows room air to be drawn in and mixed with the exhaust on its way out.

Air Flow

Proper ventilation is very important for gas burning furnaces. The furnace room must be larger than a certain size to provide adequate burn. If the room is too small, port holes must be opened either with the outside or adjacent rooms in the house. The exhaust pipe must be large enough so that there is no build up of carbon waste in the pipe.


Today's gas burning furnaces typically have quite a few safety mechanisms built in. There may be sensors near the burners, for example, that detect flames coming out of the side. If the heat exchange chamber heats up too fast, there may be clogged pathway which will cause the gas to shut off.


When the furnace is tuned up and running as it was designed, it is in its most efficient state. By making sure the above mechanisms are operating properly you are on your way to having an efficient gas burning furnace.

Efficiency is usually determined by how much heat produced by the burners goes into the home, versus how much heat is exhausted out of the chimney. Now days furnaces can achieve 96% efficiency. This means 96% of the heat goes into the ductwork fed to the home, and 4% is lost to the exhaust. Efficiency typically goes down with the age and how long ago the furnace was made. Although having a life expectancy of 40-50 years, older gas burning furnaces could only achieve up to 85% efficiency.

There are a number of reasons for reduced efficiency, and most of those fall in to one of two categories: air flow and gas burn. Any impedance in air flow from furnace entry to furnace exit, even dust build-up, will reduce the efficiency of the gas furnace. Also, if the gas pressure is not coming in at the correct, either too much or not enough, that can cause incomplete burn or excessive carbon monoxide exhaust. Both will reduce the furnace's efficiency.

Gas Pressure

The amount of gas pressure coming from the utility is very important. Too much pressure will cause excessive carbon monoxide and wastes gas. The gas pressure is also elevation sensitive. The pressure must be de-rated for higher elevations above sea level.

Gas Furnace Pressure Meter
Gas Furnace Pressure Measurement Set Up

Gas pressure at the furnace burners is measured in unit of inches of water column. The pressure can be adjusted. On the gas controller there is a gas pressure regulator screw that can increase or decrease the amount of gas pressure being fed to the burners. Adjust the pressure only if needed.

Gas Furnace Pressure Meter
Gas Furnace Pressure Meter Used to Inches of Water Column

The Blower and Motor

The blower pushes fresh cool air from the home through the furnace chambers. It is connected to an electric motor by a belt loop. The electric motor, which looks like any motor that powers rotating appliances on the home such as a clothes washer, is rated to operate a certain revolutions per minute. If the motor is not operating at the design RPMs, the blower may not be sending the proper amount of air through the furnace. If too much air is pumped in, the burners may not burn the gas completely. If not enough air goes through, there may be too much carbon monoxide produced.

Gas Furnace Blower and Motor
Gas Furnace Blower, Motor, and Belt

RPMs can be measured indirectly by measuring the electrical current driving the motor. The manufacturer's label on the motor shows the rated current that the motor operates at. The actual current you measure should be within 10% of the rated current. To measure the current place a loop type current meter around one of the leads connected to the motor. Turn the furnace on and take a current reading.

Gas Furnace Maintenance and Efficiency

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