Sizing heating and cooling

Equipment is sold in sizes, measured by how much heat it can add or remove. Larger systems are meant for larger heating or cooling loads. The load is how much heating or cooling your particular home needs, and the goal is a system that matches it.

A unit that is too large causes more trouble than most people expect. Oversized equipment runs in short cycles and never reaches efficient operation. An oversized air conditioner also does not run long enough to remove humidity from the air. Oversized units tend to have shorter lives because short cycling, turning on and off rapidly, puts heavy wear on the compressor. In heating, an oversized system cycles more often, causing wear and uneven comfort.

An oversized system can come about in more than one way. The house may have changed since the equipment went in, through additions or better insulation, or the equipment may have been too large from the start. That is why simply matching the old unit's size is not the same as sizing for your home.

Source: ENERGY STAR · U.S. Department of Energy

To size a system properly, you first need accurate heating and cooling loads. The usual method is ACCA's Manual J. Manual J 8th Edition is the national ANSI-recognized standard for working out sizing loads for single-family homes, small multi-unit buildings, condominiums, townhouses and manufactured homes. A proper Manual J load calculation is required by national building codes and most state and local jurisdictions.

A load calculation looks at your house itself. ENERGY STAR says to expect the contractor to gather details such as your insulation level, the type and size of your windows and your floor area. Your contractor can then find the right size using ACCA/ANSI Manual J or an equivalent tool that weighs these and other factors. The calculation also counts building materials, insulation and windows. A contractor who measures and asks questions about your home is doing what this method requires.

Source: U.S. Department of Energy · ACCA · ENERGY STAR

The load is the first of three steps. The model code says heating and cooling equipment shall be sized under ACCA Manual S, based on loads calculated with ACCA Manual J or another approved method. In ACCA's review forms, Manual S means the equipment was chosen from the loads and the maker's performance data and stays within the maximum sizing limits. Manual D means the duct sizes were based on the chosen equipment, the duct layout, the fittings and the duct materials.

The ducts and blower have to fit the result. Equipment, including ducts, should be chosen from the loads, and the blower must deliver the range of airflow needed. For Chicago's Express Permit Program, the city says the HVAC load calculation may be prepared by any responsible person following industry standards such as ACCA Manual J.

Source: International Code Council · ACCA · U.S. Department of Energy · City of Chicago

Quick shortcuts are common, and they tend to err on the large side. The Department of Energy says rules of thumb are too often used to size systems, which leads to systems that are far too big. That brings higher cost, wasted energy and too-frequent on-and-off cycling that can hurt comfort and efficiency. Loads in energy-efficient homes are often so small that they fall outside the range of sizes makers offer.

Replacing equipment brings its own temptation. It can be tempting to skip the Manual J calculation for a replacement when you already know the size of the old equipment. Asking whether a load calculation will be done is a fair question for any quote.

Source: U.S. Department of Energy

Chicago's climate leans heavily toward heating. From 1991 to 2020, the city's normal year had 5,974 heating degree days and only 1,003 cooling degree days. In the same normals, the average temperature was 25.2°F in January and 75.4°F in July.

A load calculation uses a set of outdoor design conditions for the location. Under Manual J, the designer may use either Manual J's own outdoor design tables or ASHRAE weather data, but may not mix them. Chicago's Heat Ordinance also requires heating equipment in each unit to be able to hold 68°F indoors under expected winter weather for Chicago, where each unit has its own heating.

Source: National Weather Service · ACCA · City of Chicago

Heat pumps raise a special question because one unit both heats and cools. Should it be sized for the home's heating needs or its cooling needs? In most of the country, sizing a heat pump right for cooling leaves it undersized for heating, and sizing it right for heating usually oversizes it for cooling. Variable-capacity heat pumps can be sized right for heating and oversized for cooling with less worry about comfort and efficiency.

The key idea is the balance point. The thermal balance point is the outdoor temperature where the home's heating load equals the heat pump's output. Below that temperature, the heat pump cannot fully heat the home without a backup heat source. If heating capacity is too small, the backup heat makes up the difference at a much lower efficiency. In a dual-fuel system with a furnace, the thermostat runs the heat pump first and switches to the backup heat only if the heat pump cannot meet the call for heat. The heat pump page goes deeper into cold-climate models and backup heat.

Source: U.S. Department of Energy · Natural Resources Canada