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High Efficiency Advantage
There are four main components to a heating system:
1. A heating unit converts fuel into heat energy. Some different heating units include:
• a natural gas, propane or oil fed furnace for use with a forced-air distribution system
• a natural gas, propane or oil fed boiler for use with a hot water distribution system
• space heaters including electric baseboard heaters and wood-burning or natural gas
2. A distribution system moves heat from the heating unit to the living area, although in the case of space heaters, heat is supplied directly to a living space without this distribution network. Some distribution systems include:
• forced-air ductwork that employs an electrically driven fan to pull air from specific
   parts of the house and deliver it to a heating unit for heating before redistributing it
   back into the living space
• a hot water distribution system that uses insulated piping to deliver hot water to either
   baseboard radiators or in-floor radiant heating loops
3. A fuel source provides energy to produce heat. Some fuel sources include:
• natural gas (the most common and currently the most cost effective)
• electricity (not strictly a fuel but, nonetheless, necessary for resistance heating)
• propane
• fuel oil
• wood
4. Controls to regulate the heating system:
• thermostats
programmable thermostats
progammable thermostats in conjuntion with zone controls
1. Because of their low efficiency and, therefore, high exhaust gas temperatures, older (conventional) gas furnaces required a continuous, fire resistant liner in a masonry chimney or a metal "B" vent. Exhausted water vapour, in contact with certain exhaust gases, produced a corrosive liquid that further necessitated masonry or stainless steel chimnies. These furnaces generally achieved fuel-to-useful energy conversions no higher than 60% and are now considered obsolete.

2. A great improvement over the conventional furnace is the mid-efficiency furnace, achieving AFUE ratings from 80% to 82%. These furnaces do not always require a chimney but, instead, may use an electric fan to force a draft through a vent to the outdoors. Their design makes no attempt to extract the considerable heat left in the escaping water vapour. This, and the fact that their exhaust gas temperature is high, means quite a bit of heat escapes unused. Not recommended.

3. The most desirable gas furnace is the high efficiency model that extracts much more heat from the combustion gases before they are expelled. Efficiencies from 90% to 98% are achieved by using a larger heat exchanger made of corrosion-resistant stainless steel. Hot water vapour can condense on this exchanger, giving up its heat as it does so. The resulting condensate is either run into a floor drain or, where one is not available, pumped to another suitable drain. The exhaust gas temperature is so low for these furnaces that they can be conveyed outdoors through a narrow plastic pipe. Some of these furnaces use a fan to force the exhaust through the plastic outlet pipe.
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