When building a new house, one of the more important decisions you'll make is the size of the heating and cooling system. While proper sizing is especially critical in cooling climates, getting the right-sized unit is important in any part of the country. The following Q&A is based on information about air-conditioning systems published by the Florida Solar Energy Center, but most of the points apply to heating systems, too.
Why is proper sizing so important?
Systems should be sized to meet peak heating and cooling demand. A system that's too small can't always meet the load. It may not keep a house at the desired temperature during hot summer afternoons or cold winter mornings.
Why not just install a big system?
Aside from the fact that initial costs are higher, comfort may not be guaranteed by a "monster" system. Oversized systems run in short spurts. That means the system removes less moisture from the air than it should, which is not a good thing during hot and humid summers.
Does that mean that short runs are also less energy efficient?
Oversizing an air conditioner by 50 percent will cause a 10 percent increase in cooling energy use. It also increases peak load on the electric utility.
How do I estimate the right size system?
Beware of estimates. Builders sometimes make a rough guess--usually based on the building's floor area--and then add an extra half-ton as a safety measure. But in today's energy-efficient homes, this old guideline creates more problems than it solves. Instead, you need to carefully calculate the home's heating and cooling load.
What factors do I need to consider?
- In addition to the area to be heated and cooled, you need to factor in:
- heat generated by appliances and people
- solar exposure, glass area and shading
- heat loss through doors, windows, walls, ceilings and floors
- air and humidity infiltration from the outside
One of the most widely used load calculation methods is provided by the Air Conditioning Contractors of America. Their Manual J procedure is relatively easy to use. A number of good energy analysis software programs exist that allow you to quickly calculate energy use, equipment size and even economics. (A complete list can be found in the Oikos Product Directory.)
The easiest method is to let your air-conditioning contractor do the calculations for you. Proper air conditioner sizing saves money, improves system performance and enhances comfort.
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