Poor design and installation of heating and ventilation equipment commonly increases energy costs 10 to 30% in affordable housing. This wastes money, creates discomfort, and can endanger the health of families. Proper design and installation of HVAC equipment is a top priority for cutting energy bills.
How Big is Too Big?
Equipment that is too big (excess capacity) costs more to purchase and operate and leads to poor comfort levels, excess noise, and greater pollution. Do not allow rules of thumb to be used to determine equipment size.
To size equipment properly, an affiliate must require calculations that consider insulation levels, window type and orientation, and air sealing measures.
It may be difficult to get a properly sized furnace because as homes are built tighter and insulated better, the currently available units tend to be too large. For many Habitat homes, 30,000 BTU is large enough to heat a whole home comfortably and efficiently. A properly sized furnace will operate nearly continuously on the coldest days of the year.
It is best to request that the HVAC professional use the Air Conditioning Contractors Association of America's Manual J calculations (be sure to request a copy of these calculations from the HVAC professional). Calculating equipment size should take less than an hour for most affordable home designs, and will prevent the purchase of costly, oversized equipment and provide significant savings to home owners for years to come. Home Energy Rating System (HERS) raters can also approximate system sizing requirements as a part of the home analysis for Energy Star certification.
Alternatives to traditional Furnaces Include:
Radiant Floors
Ductless Mini-Splits
How is Furnace Efficiency Measured?
The efficiency of a furnace is measured by the Annual Fuel Utilization Efficiency (AFUE). The minimum efficiency allowed is 78%. Mid-efficiency units range between 80-82%, and high-efficiency units between 90-98%. To achieve a true AFUE calculation, a furnace must be able to run at its maximum capacity for a sustained period of time. If an oversized unit is used, it will turn on and off quickly before the peak performance is realized.
("Air Distribution System Design: Good Duct Design Increases Efficiency". Energy Techinical Bulletin. U.S. Department of Energy. Accessed 24 July 2009)