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Forced Hot Air Heating

Forced Hot Air Heating

By far the most commonly encountered type of heating system, forced air relies on heating air with a furnace, distributing that air via a system of metal ducts, where it ultimately finds its way into each room through an outlet called a register.  A benefit from the duct work is that it provides the necessary infrastructure for both heating AND cooling; the central air conditioner distributes cool air through the same ducts.  This is why forced air systems are typically encountered in areas with a wide seasonal temperature range.  While convenient, there are some drawbacks that should be mentioned.

Air makes for a poor heat transport medium – due to its gaseous nature, it has very low density and mass per unit of measure, meaning it’s “heat carrying ability” is limited.  In order to distribute a respectable amount of heat, a large volume of air needs to be moved at a relatively high temperature.  The average air temperature generated by a furnace is between 120 – 140 Degrees F and the average velocity is around 300 fpm (feet per minute).  This is why ducts are much larger than pipes, which would be used to transport the heat via hot water in a hydronic heating system.  

Another drawback to forced air is the “convective quality”.  The heated air is blown through the duct work and into the room.  While doing so, the heat leaves the register, and makes it way to the ceiling (hot air rises by nature and this is exacerbated by the velocity inherent in this type of system).  Overall, this is a relatively inefficient means of heating; heating a space works best from the ground up (see radiant heat section).  Most of the energy is wasted by heating the ceiling area, and not the floor level where’s it’s needed.  The convective nature also allows for allergens, dust, and other particulate matter to be blown around the room.  

Forced air systems are notorious for their “hot and cold” spots.  This is mainly due to the difficulty in zoning such system.  Zoning refers to the ability to discretely control the heat output in various areas encompassed in the overall system.  

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