We have all heard the horrifying story about the family that just barely escaped with their lives, or even worse the family that died in their sleep, there lives ended by a silent killer. The silent killer is CO (Carbon Monoxide). If you don’t change or maintain your furnace filter as recommended by the manufacturer you could be putting your family in danger. Your furnace filter is designed to remove dust from air as it passes through your furnace. The longer your furnace filter is installed the dirtier it becomes. This increased loading of the filter with dust causes a resistance to air flow through the filter and reduces air flow.
Gas and oil furnaces, like all heating and air conditioning equipment, are designed to have a certain amount of air flow at all times to operate properly and at highest efficiency. This air flow is measured in CFM (Cubic Feet per Minute). These appliances have a heat exchanger which has the heat source, flame, inside and the air passes over the heat exchanger on the outside. As the air passes over the heat exchanger it picks up the heat from the flame inside.
To get maximum efficiency from these systems the walls of the heat exchanger are made as thin as possible but not so thin that they cannot withstand the heat that they are subjected to during normal operation. This brings us back to the fact that these appliances are designed with minimum air flow requirements.
A furnace filter that remains in the system so long that it begins to negatively impact air flow can be dangerous. As the amount of air flowing over the heat exchanger declines, the operating temperature of the heat exchanger increases. This is when heat exchanger can crack because they are operating above design conditions. The hotter the heat exchanger gets the more it expands when heating and contracts as it cools during the off cycle. This expanding and contracting can become excessive and cause the metal of the heat exchanger warp or crack. A cracked heat exchanger can cause combustion gases to mix with the air in your home.
The good news is that properly designed furnaces have high limit switches that prevent excessive heating of the heat exchanger. These limit switches monitor the temperature at or near the heat exchanger and if the temperature reaches a predetermined set point, the unit will be shut off either until manually reset or until the limit switch senses that the temperature is within safe limits and resets automatically.
A furnace can operate a long time on an auto reset limit switch without anybody even knowing. This means that the furnace is operating at the limits of its design capabilities. Today most limit switches are the manual reset type, designed this way so that the furnace will not operate until it is serviced and the problem corrected.
Don’t rely on these safety switches to protect you from disaster. These are mechanical devices which can and do fail to operate properly at times. So reduce your risk by maintaining your furnace filter properly. This means cleaning or changing your furnace filter regularly. It is always best to follow manufacturers guidelines.