Heating systems come in all sizes, types and fuels. You can check out a brief history of heating. In residential systems there are basically two different distribution systems, forced air (ductwork) and radiation (baseboards). These systems can be fueled by one or more of the following: electricity, natural gas, LP propane, oil, hot water, steam, geothermal, heat pump or wood.
For most purposes you can categorize home heating systems into two broad categories, furnaces and boilers. Generally speaking furnaces heat air and use a system of fans and ductwork to move that air around the house. Boilers heat water and use a circulator pump and piping to move the water through radiators, thus heating the home.
Furnaces come in all shapes, sizes, efficiencies and fuels. The most common furnace in urban areas is a forced air, natural gas unit. In rural areas, or areas that do not have access to natural gas pipelines, furnaces often use propane or electricity to produce heat. Regardless of the fuel all residential furnaces work on the same principle. A fuel is burnt in a heat exchanger to produce heat. Air is then passed over the heat exchanger where it picks up heat, the air is then delivered to the house through a ductwork system.
Furnaces are often classified according to efficiencies. You’ll often hear furnaces being referred to as Standard, Mid and High Efficient units. The efficiency is determined by the furnace’s AFUE . According to the Ontario Ministry of the Environment and Energy a Standard furnace is one whose AFUE is below 70%, a Mid Efficient furnace is one whose AFUE is between 71% and 82% and a High efficient furnace is one whose AFUE is above 90. When a furnace is installed in a basement it is considered an “Upflow” furnace, meaning the cooler air from the home enters the base of the furnace, and exits out the top of the furnace. The furnace is connected to a series of metal boxes and pipes, which is the ductwork. The very first metal box on the top of the furnace is called the PLENUM. If you have air conditioning the evaporator coil is installed in this plenum.
If a furnace is installed on the main floor of a home and the heat comes from floor registers, it is a downflow furnace. In a downflow furnace the cool air from your home enters the furnace at the top and the warmed air exits at the bottom.