Forced Air Distribution Systems
Homes incorporating forced-air heating systems are very efficient at distributing heat around a home, preventing stagnation of air and moving heat from different sources to the overall space. They also work well with mechanical ventilation systems. The central heat source could be a fuel-fired furnace. Unfortunately the smallest sizes usually available are in the 50 to 60,000 Btu/hr range (18 kW ) and this heating capacity is often much too large for an energy-efficient home. Using such a large heat source is inefficient in terms of fuel consumption. For example, a 50% oversized furnace will use 20% more fuel in heating the same space than a correctly sized unit.
Good heat distribution, air movement, filtering capability, humidity control and low maintenance are some of the advantages with a properly designed and installed forced-air heating system.
A forced-air distribution system works well if a home receives abundant solar energy. Since passively heated spaces can easily overheat when the window area is too large or if there is not enough mass to absorb and store the solar energy, having continuously circulating air with the forced-air distribution fan running at a slow speed helps prevent overheating. Passively heated air is distributed to all the spaces in the home, not just those on the south side.
High Efficiency Gas Furnace
High efficiency (condensing) forced-air gas furnaces offer efficiencies of 90% or better. These units use electronic ignition, induced draft fans and condensing heat exchangers. Ductwork and installation is similar to a standard furnace with the exception of the chimney and condensate drain. Condensing furnaces require a drain pipe connected to a floor drain to allow condensation (water) from the heat exchanger to drain. A standard chimney is not required because the exhaust air temperature is reduced to the point that high temp plastic pipe can be used as an exhaust vent out of a side wall.
The diagram shows a fresh air duct from outside, ducted directly into the cold air return. Combustion air is separately ducted from the exterior to the front of the furnace. Note that both air ducts are insulated. Although not shown, an air to air heat exchanger should be installed to maintain indoor air quality.
Radiator (Fan Coil Unit)
A good heating solution for an energy-efficient house is to use the advantages of a forced air system (such as good air and heat distribution, filtering capability, low maintenance) and add a small auxiliary heat source to it. This could be an electric heating element, a hot water heated coil unit, a heat pump or simply a separate wood or electric unit providing heat that the system picks up and distributes via the forced air system.
This example shows how hot water heating can be combined with a forced air system. Hot water from a boiler is circulated through a radiator placed in the ducting of a forced air system. A fan forces the air through the radiator where the heat is picked up and distributed to the entire house. The duct work design is the same as any forced air system.
An air to air heat exchanger helps to maintain indoor air quality by suppling pre-heated fresh air to the cold air return, which is then distributed to the rest of the house by the forced air system.