HVAC Calculator: Size Your AC and Water Heater to Fit Your Home

How to calculate BTU correctly is one of the most important steps to installing an HVAC system in your home. While the process is straightforward, you might need an HVAC calculator to figure out the right size AC and heater for your home. If your HVAC unit is too small, you might have problems with temperature regulation; while an oversized unit will be inefficient and may not last as long. Your air conditioner and water heater should ideally be just the right size, which can be determined by taking time to factor in the following considerations:

HVAC Calculator Considerations

Square Footage

The square footage of your home identifies its total amount of floor space. It may be documented somewhere, but you can also take measurements of each room, using a tape measure for length and width, multiplying both numbers and, once you have the square footage of each, adding them together. You’ll then have the total square footage of your home.

Understand BTUs

A British Thermal Unit, or BTU, measures the energy used by a heating and cooling system. It takes about 25 BTUs to cool one square foot. You can calculate BTU by multiplying the square footage number by 25, you’ll get the base BTU measurement for selecting the appropriate HVAC system. But it’s not only about square footage. Each room has volume, and high ceilings increase the conditioned space. If you have ceilings over 8 feet, multiply the base BTU number by 25%.

A typical 1,500 square foot home with standard ceiling heights generally needs 37,500 BTUs. You can figure out tonnage by dividing the number by 12,000, so the same home would require about 3 tons.

Climate Zones

Where you live also affects the size of air conditioner you need. For example, a 1.5-ton system in Zone 1 will serve a 600 to 900 square foot home, while in Zone 5, it would accommodate a 700 to 1100 square foot home. The established climate zones generally follow a pattern from the hottest areas in the south to the cooler, more seasonal areas to the north.

Air Conditioner BTU Chart

For a reference on how climate zones impact system size vs. square footage, we’ll look at a 3-ton system:

The square footage for heating also works by climate zone. But as each zone differs from the other in terms of temperature, humidity, and seasonal variability, the amount of BTUs per square foot changes. The breakdown looks as follows:

For a more exact match you can use this free HVAC calculator tool.

Other HVAC Sizing Variables

There are other HVAC sizing variables to consider. If you’re upgrading to a modern tankless water heater, you need to look at maximum demand in gallons per minute, the usual incoming water temperature, and the temperature you expect hot water to be (considering the degree rise needed to raise groundwater temperature to the desired level). In general, sizing an AC and heater requires looking at:

  • The number of windows in your home
  • Whether newer, insulated windows are installed
  • How many stories the structure is
  • The type of construction
  • Insulation type, quantity, and condition

For example, you’d need a larger system if your home has a high number of windows, older windows are in place, and if it isn’t well insulated. Air and heat loss are the primary factors requiring a larger system to compensate, regardless of square footage.

Contact Black Hills Inc. Home Services

You can trust Black Hills Inc. for professional AC and water heater installation. We specialize in choosing just the right systems for homes in Olympia and surrounding areas, and selecting high-quality, energy-efficient models. For help with selecting and sizing as well as expert installation, schedule service online or call 888-445-0585.