One symptom of a blocked air filter is that the furnace stops operating. Furnaces have an over-limit cutoff switch that stops operation if the temperature in the heat exchanger becomes too high. As a result, according to the Family Handyman, the home may remain below the thermostat setting.
The clogged filter causes the system to operate at a lower efficiency. If the unit continues to operate with a dirty filter for an entire month, the home occupant often sees an increase in the utility bills. Replacing inexpensive disposable filters on a monthly basis reduces the cost of operation.
When the HVAC system operates in the cooling mode, the lack of air flow caused by a dirty filter does not allow the cooling to transfer from the coils. As moisture from inside the home passes over the cold coils, it normally condenses and drips to a tray where it drains away. However, the super-cooled coils and the blocked air flow cause the moisture to freeze. In some cases, the ice blocks all air flow through the unit, causing a warmer than normal temperature in the home.
Clogged furnace filters allow a toxic buildup of carbon monoxide in the homes they serve. Carbon monoxide is a colorless, odorless, tasteless and deadly gas. Unfortunately, this is a symptom of a blocked air filter that home occupants usually do not discover until it is too late.