Forced air heating systems are used in most homes to day to make houses comfortable when the temperature dips below zero degrees. In North America, 35 million homes are heated by these systems. Now what is a forced heating system? A forced-air heating system takes the room air through-the ducts and sends it to the central furnace for heating and filtering it. The warmed air is then forced back to the rooms through other duct work. The Ducts are actually long pipes made of metals and are wrapped with insulation to prevent escape of hot air into the surrounding. This hot air is delivered in all rooms through air registers or grills.

 Furnaces

Most of the forced air systems have a furnace to heat the room's air. Furnaces come in various models such as “up flow," "down flow" and "horizontal" models. The designs help them to fit in basement, attic or in limited-spaces. A furnace can run on various types of fuels like gas, oil or electricity.

 When you power on your furnace, the thermostat decides the amount of heat needed by sampling the external air. It then sends signal to the furnace to generate the heat. The gas or oil in the furnace reaches the burner with the opening of a valve. It is ignited which produces flames. The flames heat up the heat exchange where the heat is transferred to the air. The furnace fan pushes the air through the heat exchanges and then through the duct work. The combustion products are vented out through a flue pipe. The same thing happens in electric furnaces too, but here the electricity is used to heat up a coil to generate the heat. To reduce energy wastage, modern furnaces have inducer fan to pull the exhaust gases through the heat exchanged and induce draft in the chimney. The condensing furnace goes evens step forward. To reclaim all the heat that is being wasted through flue in form of water vapor, it sends the vapor to a second heat exchanged. Here water vapor is cooled below 140°F. In this process heat is released which is passed through the exchanged to cool the room air. The water is vented through a sidewall with a plastic pipe.

 The US Department of Energy would specify the efficiency standard for modern furnaces in 2016. The current furnace standards have not been updated since 1987. The best gas furnaces have efficiency over 90%. The efficiency of a furnace is measured in terms of the amount of useful heat produced per unit of input energy (fuel). In the U.S., furnace efficiency is regulated by minimum AFUE (Annual Fuel Utilization Efficiency). AFUE appraises the efficiency of a unit from various parameters such as, seasonal efficiency, averaging peak, and part-load situations. AFUE also takes into account the various operating losses such as start-up, cool-down, etc. that take place during operation. It also considers consumption of electricity used by the air handler, inducer fan, and controls. The higher the AFUE, the more efficient the furnace.

 A forced air system has other components to improve the quality of air you breathe inside your living space. It has an air filter to screen the air of all kinds of aerial contaminants such as dust, dirt, mites, allergens, pol lens, and even mold spores. It is very essential in houses where members are susceptible to allergies or respiratory problems. Filters can be electrostatic, electronic, or pleated media. These filters are designed to manage energy-efficiency and indoor air quality. There are humidifiers and dehumidifiers in the system to maintain an optimal level of humidity in all seasons. A house with foolproof insulation may not have proper ventilation unless specially designed to allow it. Ventilation is very important to ensure flow of fresh air from outside. It prevents the indoor air from getting stale. The design of the forced air system should be so that in spite of tight insulation, it will allow an appropriate exchange of air from outside the home. A duct system free of leaks, a well-kept furnace flue, and a quality air filter will enable the forced air heating system to run at the peak of its efficiency.