What size furnace do I need? Furnace sizing, load calculations and Florida's sizing rules

What a furnace's size means

The U.S. Energy Information Administration (EIA) says a British thermal unit (Btu) measures the heat content of fuels or energy sources. It defines one Btu as the heat needed to raise one pound of water by 1 degree Fahrenheit. The EIA adds that one Btu is about equal to the energy released by burning a match.

A furnace's size is how many Btu of heat it can deliver in an hour. Carrier, a furnace maker, says that when homeowners ask how big a furnace they need, they are usually referring to the Btu rating. It lists a second rating, annual fuel utilization efficiency (AFUE), which measures efficiency and is a separate question. For efficiency, see furnace types and efficiency.

Florida's energy code states its sizing tolerance for electric resistance furnaces in kilowatts (kW), a unit of electric power. The EIA says one kilowatt is equal to 1,000 Watts. It says one kilowatthour is one kilowatt generated for one hour, and that 1 kilowatthour equals 3,412 Btu.

Why a square-foot chart is not the answer

A 2011 Department of Energy guide says Manual J, a method published by the Air Conditioning Contractors of America (ACCA), estimates the heat loss and heat gain of homes.

A 2002 Department of Energy (DOE) fact sheet, written for it by the Florida Solar Energy Center, warns against casual sizing estimates based on home floor area or contractor experience. It says Manual J figures the loads from wall, ceiling, window and floor area, their insulation, and air leakage from the house and its ducts. Building orientation, roof surface color and occupancy can also make a difference.

The fact sheet gives an example: a hypothetical 2,000-square-foot home in Raleigh, North Carolina. Built conventionally, its Manual J design heating load was 46,100 Btu/hr. With the fact sheet's energy efficiency upgrades, it fell to 21,300 Btu/hr. Same floor area, less than half the heat. Raleigh is not Tampa Bay, so do not use those numbers for a Florida home.

Carrier recommends treating generic furnace size charts, which suggest Btu from climate zone and square footage alone, only as rough planning tools. It says they cannot account for the full range of factors that set a home's actual heating load.

What a load calculation is

A 2011 Building America guide for the Department of Energy says the Air Conditioning Contractors of America (ACCA) published its guidelines in Manuals J, S, and D. It says Manual J outlines the procedures to estimate the heat loss and heat gain of homes. Manual S gives guidance for choosing equipment based on those load calculations.

The DOE fact sheet calls Manual J and Manual S the residential sizing procedures recommended for use in the United States. The 2011 guide says that in the past, many contractors used rules of thumb, and as a result, many installed oversized systems.

The fact sheet says not to alter the suggested design temperatures for your location, because using the annual extreme temperatures will lead to over-sizing. It also says Manual J generally recommends 70°F indoors for heating. It warns that exaggerated indoor settings can have large impacts on required equipment size.

Ask for the calculation in writing. The DOE fact sheet tells homeowners to insist that documented sizing calculations be performed on your home using the ACCA Manual J procedure. For the cooling side of the same calculation, see A/C tonnage and sizing in Florida.

What Florida's codes say about heater size

The general rule in the Florida Building Code, Residential, section M1401.3 is that heating and cooling equipment and appliances shall be sized by ACCA Manual S or another approved method. The loads must come from ACCA Manual J or another approved calculation method. An exception says sizing is not limited to Manual S capacities in two cases: multistage or variable refrigerant flow equipment whose calculated loads are within the manufacturer's published capacities, or where the manufacturer's published capacities cannot satisfy both the total and sensible heat gains and the next larger standard size unit is specified.

The Florida Building Code, Energy Conservation, section R403.7.1 requires sizing by ACCA Manual S, based on loads calculated by ACCA Manual J or another approved method for the building's directional orientation. It adds that this code does not allow designer safety factors, provisions for future expansion or other factors that affect equipment sizing.

The same energy code then sets rules for three kinds of heater:

The 2011 DOE guide says that in the heating season, heat pumps move heat from the outdoor air to warm your house. The code sizes a heat pump on the home's cooling needs. The code says "the heat pump total cooling capacity shall not be more than 1.15 times greater than the design cooling load even if the design heating load is 1.15 times greater than the design cooling load."

An electric resistance furnace must be sized within 4 kW of the calculated design requirement. The 2011 DOE guide says electric furnaces provide heat through resistance, not by burning fuel, and send it through ducts like gas furnaces.

The capacity of fossil fuel heating equipment with natural draft atmospheric burners may not be less than the design load calculated under R403.7.1. See heat pump vs furnace for how these heaters compare.

What Florida's low heating degree days mean for size

The National Weather Service bases heating degree days on the assumption that no heating is needed at 65°F outside. It says that if the temperature mean is below 65°F, "we subtract the mean from 65 and the result is Heating Degree Days," and that the daily degree days "are accumulated so that we can compare months and seasons."

The DOE fact sheet puts Florida among the sunbelt climates, along with southern California, where there are less than 2,000 heating degree days. It also says that as a home's efficiency is improved, the recommended size of the HVAC system should drop. HVAC stands for heating, ventilation and air conditioning.

We found no published figure for a typical Tampa Bay heating load, so we do not give one.

The 2011 DOE guide says all-electric air-source heat pumps come equipped with electric resistance strip heaters for supplementary heat if needed. The DOE fact sheet calls these heat strips. For sunbelt climates like Florida's, the DOE fact sheet says to consider installing the smallest amount of electric resistance heat (the heat strips). For more, see how a gas furnace compares with electric heat strips.

What goes wrong when the size is wrong

Carrier says oversized furnaces heat spaces too quickly and turn on and off often, which is called short cycling. It says short cycling can contribute to uneven temperatures, higher maintenance costs, lower efficiency and more wear on parts.

The DOE fact sheet agrees that the overall efficiency of an air conditioner, furnace, or heat pump is reduced by excessive cycling. It says more starts and stops accelerate wear and tear on the equipment, which can lead to premature failure. It also says oversized systems use more fan power and often show more duct leakage.

Too small has its own problem. Carrier says undersized units run continuously without adequately heating the home, making it difficult to reach the desired thermostat setting.

The DOE fact sheet says ACCA methods already have enough built-in safety factors for most needs. That is why it says to follow Manuals J and S as written, with precise measurements. If you are replacing a furnace, see furnace replacement in Florida.