Sizing an air conditioning or heat pump unit for a home isn’t magic, but it isn’t rocket science either. It takes knowledge of some arcane principles and mastery of mathematical operations (maybe magic or rocket science after all). Duct, vent, return and return register size all play a part as well. The duct work must handle the air flow and the return size must ensure good airflow even with partially obstructed filters. A local problem such as airborne dust is a major local consideration here in the Las Vegas Valley. It is better, cheaper and much easier to design right and ensure emplacement during new construction. But renovations, additions and other considerations may warrant a reassessment of the adequacy of the AC system.
AC Sizing to Prevent Short Cycling
The unit(s) must be sized to prevent short cycling. An AC or heat pump operates more efficiently if they can run for a period before being shut off. Oversized unit cool so quickly that they tend to cycle of and on in short periods of time. It takes a few moments for the unit to compress the gas after starting the compressor as pressure equalizes throughout the system when off, most refrigerant is a gas and needs to be liquified. Short cycle times never allow the system to reach peak efficiency. The AC unit also needs time to extract humidity from the air. Humidity is what gives us that hot and sticky feeling. Air conditioning is more than simply cooling and for a heat pump more that cooling and heating.
How Do I Calculate what Size Air Conditioner I Need?
On the same token you don’t want a system that is so small it essentially runs continuously through most of the daylight hours. A rule of thumb is to determine the square footage and figure 25 Btu per square foot. A 2700 square foot home will need 2700 x 25 = 67, 500 Btu capable unit. A ton of AC is equal to the cooling provided by a ton of ice. It is the amount of heat to melt that ton, one-ton AC unit is rated at 12,000 Btu per hours. It would take a 5.5 tons of AC to cool a 2700 square foot house, or it can be divided between multiple units. Factors to be considered:
• Size of the house
• Number of windows
• Type of insulation
• Height of the ceiling, the higher equates to more are to be cooled. The 25 Btu approximation is for 8-foot ceilings.
• Average and maximum summer temperatures
• Color and type of roof
• Basement or slab?
A lot of variables to be sure.
Manual J or J Load Calculation
is used to determine the heating and cooling loads of a building, home or commercial. Hand calculations are involved and tedious. Most AC pros depend on software to do the calcs. It is complicated even accounting for the direction faced by the front of the house and runs multiple pages of fill in the blanks type of considerations.
How to Determine AC Tonnage
Central AC and heat pumps are measured is tonnage. As mentioned it is the heat needed to melt a ton of ice. Water has a high latent heat of evaporation. This means water can absorb a lot of heat to change states from a solid to a liquid and liquid to vapor or gas. Most units are available in 1.5 to 5 tons for residential purposes, and for larger homes there are multiple AC units responsible for zones. But a ton of AC is rated at 12,000 Btu’s per hours. What’s a Btu? It stands for British Thermal Units and 1 Btu is the heat required to raise the temperature of 1-pound of water 1-degree Fahrenheit. An AC removes heat from the air, a heat pump either removes or adds heat depending on the season.
Why Not Buy Biggest AC Unit?
• Size costs. The bigger the unit the more the cost to acquire it. Bigger is more expensive, and could be a bunch more expensive, and if you don’t need it, well . . . you shouldn’t get it.
• Bigger eats more power. Larger units have more powerful fans and compressors all conspiring to consume more electricity. Size matters, its expensive.
• Wear and tear plus maintenance costs increase if the unit is turning off and on frequently.
• Humidity in uncomfortably high as the AC does not have the run time to remove it.
Central & More in Summerlin, North LV, Henderson, Enterprise, Paradise, Sunrise Manor, Spring Valley, Aliante, Anthem, Desert Shores, Eldorado, Green Valley Ranch, Inspirada, Mountain’s Edge, Peccole Ranch, Providence, Rhodes Ranch, Seven Hills, Silverado Ranch, The Lakes, Tuscany Village & Las Vegas Nevada
Get a pro to calculate the size of your unit, it will be much more economical in the long run. Air Supply Air Conditioning and Heating can handle all your HVAC needs. Call us to schedule your next appointment today!