Real Estate Topics Forum :: View topic - proper sizing of a central air conditioner

ned jimmerson

Hello We will be building a 1600 square foot slab on grade home using 6 inch thick SIPS.for the walls With r-50 blown in cellulose in the attic area. the slab will also be insulated underneath and around the perimeter. I would like some information to give me some guidlines for the proper sizing for the central air conditioer The house will be built in West central Minnesota

nemmar · Site Admin

Ned,

It sounds like the house you're building will be very energy efficient and well insulated. That's great! Homeowners should insist that contractors use a correct sizing calculation before signing a contract for a new central air-conditioning system installation. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air-conditioning contractors. Manual J, published by the Air-conditioning Contractors of America (ACCA), is the most common method in use in the United States. There are also many user-friendly computer software packages or worksheets that can simplify the calculation procedure. You should make sure that the procedure used by the contractor follows Manual J or other approved standards.

Many factors affect a home's heating or cooling requirement or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the R-value of the home's insulation, windows, and building materials. A close estimate of the building's air leakage is also necessary. A blower door test is the best way to measure air leakage. A good estimate will also include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers, should be appropriate for the system type and size. The orientation of the house also affects heat gain and heat loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design outdoor temperature and humidity for your area. Using a higher summer design temperature results in over sizing air-conditioners.

Any bid should include an agreement to provide written calculations (listing the procedures and standards that will be followed), equipment and installation warranties, a payment schedule, and a firm completion date. When the contractor is finished, get a copy of their calculations, assumptions, and the computer printout or finished worksheet. This is your only proof that they did the job right.

Sizing Heaters and Air-conditioners: Quick but Inaccurate Methods - The following are some of the "quickie" methods some contractors may use to size a system. They are also somewhat useful for very rough sizing. NEVER use any of these to determine the final size.
* The contractor walks in the house, looks at the existing unit, and recommends that the replacement unit be the same size, or larger. This obviously does not take into account any improvements made to the house or mistakes made in sizing the original unit.
* The contractor asks you how many square feet of living space there are in your house, then tells you what size unit you need. This is called "sizing by square footage" and is the most commonly used inaccurate method of sizing. A typical value used for air-conditioners is one ton (12,000 Btu/hour) per 500 square feet (46 m2). This does not take into account differences among house orientation, insulation levels, design, construction, and energy efficiency or intended use of the system. You may get different answers from different contractors who use this technique. In that case, they may have a different "rule of thumb," or one of them may be using the "lowest cost" method. This involves adjusting the square footage rule so that whatever the contractor has in their warehouse becomes the right size for you. Since the "in-stock" unit costs the contractor (but not necessarily you) less to install, this becomes the "lowest cost" method.

Another rough method for sizing heating systems involves a prepared chart such as the one below. You use the chart in the following way. First, determine the floor area of all the heated rooms, and the levels of insulation in the floors, walls, and ceilings. Next, find the category that best describes the house. Then, multiply both the higher and lower numbers for heat loss in Btu per hour per square foot (from the table) by the floor area of the home to give you a rough range for the heating load.

Home Type or Characteristics ..... Heat loss (Btu/hr/ft2):
1) No insulation in walls, ceilings, or floors; no storm windows; windows and doors fit loosely .... 90 to 110
2) R-11 insulation in walls and ceilings; no insulation in floors over crawl spaces; no storm windows; doors and windows fit fairly tight. ..... 50 to 70
3) R-19 insulation in walls, R-30 in ceilings, and R-11 in floors; tight-*beep* storm windows or double pane windows. ..... 29 to 35
4) "Super insulated" house with R-24 wall insulation, R-40 in ceilings, and R-19 in floor; tight-*beep* storm windows or double pane windows; vapor barrier sealed carefully during construction. ..... 21 to 25
5) Earth-sheltered house with little exposure; well insulated. ..... 10 to 13

For example, if a home's energy-saving features are best described by #2, and the home has a heated space of 1,500 square feet (139.35 m2), then 1,500 x 50 and 1,500 x 70 is the heating load range. Roughly 75,000 to 105,000 Btu/hour (18,900 to 26,460 kilocalories/hour.) Although a chart like this looks official, not all houses fit the profile given. There is also no accounting for the other factors mentioned above.

This article in my online Real Estate Library talks about SEER (seasonal energy efficiency ratio) ratings for A/C units and how to buy an energy efficient air conditioner:
http://www.nemmar.com/real-estate-Cooling-Choosing-Central-AirConditioner.html