Oil furnaces overheat when the furnace doesn’t get enough air flow. This can happen when the air filter becomes dirty, if registers are blocked or if there are problems with the heat exchanger or other parts of your system. Overheating can cause your furnace to shut down, or it can start a fire. If you think your furnace is overheating, the first thing you should do is shut it down.
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Some people close vents to unused rooms in order to save fuel. However, when you close more than 60 percent of the registers, your furnace may overheat because it’s not receiving enough air flow for combustion, and it has nowhere to distribute the heat it is producing. The damage caused to your furnace could cost hundreds of dollars to repair. A similar problem is caused when furniture blocks registers.
Basic oil furnace air filters are designed to block large particles so that they don’t hurt the furnace. When the air filters become dirty or clogged, the air flow to your furnace is impeded and the furnace can overheat. Filters should be changed every one to three months, depending on your lifestyle. For instance, if you have a lot of pets or a smoker in the house, your filters will need to be changed more often.
A dirty blocked or broken blower fan can cause your furnace to overheat by reducing the supply of air to the heat exchanger. Usually, the fan limit switch will turn the burner off if the temperature gets too high. Although this is an unusual situation, it can happen if the system controls are fouled up or if the air ducts are blocked. Another problem with the blower assembly can occur when the fan belt needs to be replaced. If this is the case, you’ll hear a whining or grinding sound from the blower assembly. Shut down your furnace immediately to prevent problems.
Though not common, a crack in the heat exchanger can cause your furnace to overheat. This is a serious problem that often necessitates the purchase of a new furnace. According to furnace repair instructor Leroy Richter on the Mother Earth News website, it’s usually caused by using the wrong size of nozzle or adjusting the burner assembly incorrectly.
The evaporator coil is an important part of the furnace that absorbs heat from the air in your house. When the evaporator coil is damaged or dirty, air flow is restricted and your furnace starts cycling and may overheat. Use a vacuum cleaner and coil cleaner to remove dirt. Inspect the evaporator coil annually by shining a light through the coil or by measuring supply-fan amperage and filter/coil pressure drop. Your evaporator coil needs to be cleaned if the amps are lower and the pressure drop is higher than it was last year. Install a fresh air filter before taking any measurements.