To determine the accurate and efficient size of HVAC system necessary for a new home or upgrades, you must first estimate heat loss through walls, windows, doors, the roof and other building components with a process called the design heat load or heat load calculation. Generally, combustion units that are 10 to 30 percent above design heat load are recommended.

The Wrong Size Costs More

The wrong size heating or air unit will cost more than an appropriately sized unit for the area. It will result in unpredictable and uneven hot and cool areas throughout the building, and less temperature control for comfort. A unit that's too big will cycle on and off more than needed and waste energy and money. A unit that is too small will not adequately heat or cool all areas of the building. The calculations used to determine heat and cool air per square foot of building can be complicated, even with specialized contractor software; be sure to use contractors experienced in applying them accurately.

Installing New or Upgrading HVAC Systems

Most people don't know how to buy or install heating and air systems and equipment and have to rely on contractors for equipment recommendations, installation and service. Designing HVAC systems involves measuring most of the surface areas of the home and taking the climate and region into account, as well as the location of windows and doors to sun and winter conditions. Measurement of walls, windows, doors and the roof should be part of any contractor's estimating process. You can take these measurements yourself as well and use them with free online HVAC calculators to compare with contractor estimates (see Reference list). To upgrade or replace an existing system, a heating and cooling contractor can do an energy survey that will outline the heating and cooling needs of the building.

Being Sure

To upgrade or replace existing equipment, contractors will be more limited than when planning systems for new construction, because they will usually have to work with existing ductwork and structures versus designing the whole system. It may be reasonable to use the same size unit as the one being replaced, but a heat load calculation and an energy survey should still be done. It is a good idea to get written estimates from at least three different contractors who have been recommended and checked out.

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HVAC for Beginners

Free Online HVAC Calculators