Forced warm-air heaters require a filter to clean the air as it is being circulated. The filter is generally a pad of loosely packed fiberglass which traps dust and keeps it from being blown back into rooms. Permanent filters of plastic or wire mesh are washable; the throwaway fiberglass types must be replaced. Filters should be cleaned or replaced monthly during the heating season. Do not put it off. A dirty filter will block the air flow, decrease the efficiency of the system, and can even cause extensive damage.
The only visible parts of most home heating units are the registers (or radiators) which allow the heat to flow out into a room. Registers may be used with either gravity or forced warm-air systems. In the gravity type arrangements, the register outlets are usually located low in the floor or along the baseboard of a room. Gravity system outlets are usually placed along the interior walls of a home, to give better and more economical heating. Floor grills located along outside walls are generally return vents for the cool air being displaced.

Do not use an abrasive contact cleaner, since it can scratch or otherwise damage the bimetallic points. If the thermostat is a mercury vial-type, no cleaning is necessary. The mercury unit is sealed during manufacture. If heat comes out of the registers but the blowers do not operate, the trouble is in the fan or fan motor, or there is a leak or blockage in the duct work. In some combination cooling and heating systems without a heat pump, there is a plate which may be slid in or out of the duct nearest the cooler. It is removed to allow cool air to flow through the ducts during summer. In the winter, the plate is inserted in order to keep cool air from leaking into the duct system as it is being used by the heating blowers. Check your system to see if it incorporates one of these plates and, if so, that the plate is in the right position for the heater/cooler in use.