Chicago's climate

The weather service publishes average temperatures for each month, called normals. In the 1991-2020 normals, January averages 25.2°F, February 28.8°F and December 30.5°F. July averages 75.4°F and August 73.8°F. Spring and fall are short in between: April averages 49.7°F and October 54.0°F. Over the whole year, the average comes to 51.2°F. Those are averages, so real days run colder and hotter than these numbers. For your home, they mean a long heating season and a shorter cooling season, with only a few mild months when neither runs much.

Source: National Weather Service

Degree days are a weather tally of how cold or how warm a month ran, and so of how much heating or cooling it called for. In Chicago's 1991-2020 normals, January brings 1234 heating degree days, February 1015 and December 1069. July has only 2 and August 4. The year adds up to 5974.

Cooling degree days run the other way. July brings 326, August 276 and June 199, while the winter months have none. The year adds up to 1003. Heating does most of the work in a Chicago home, but summer cooling still matters, especially in July and August.

Source: National Weather Service

Heating equipment is not sized for an average January day. Working out how much heating and cooling a space needs at design conditions, the cold and hot extremes it is planned for, is essential. Chicago's code asks every family unit to have heating that can safely and adequately warm all habitable rooms, bathrooms and toilet rooms to at least 65°F when it is ten degrees below zero outside. In a residential building where each unit has its own heating equipment, the Heat Ordinance says the equipment must be able to keep 68°F inside under expected Chicago winter weather.

When a contractor works out how much heating and cooling a home needs, the outdoor numbers come from a standard source. Under Manual J, the designer may use the Manual J tables' outdoor design conditions or ASHRAE weather data, but may not mix the two. You do not need to pick these numbers yourself, but it is fair to ask which ones your contractor used.

Source: Northwest Energy Efficiency Alliance · Metropolitan Tenants Organization · City of Chicago · ACCA

Chicago swings between humid summers and dry, cold winters, and humidity matters as much as temperature for comfort. Keeping relative humidity at 25% in winter to 50% in summer is generally considered healthy and comfortable for people, and better for the house. In cold winter weather, relative humidity generally needs to be low to prevent condensation and possible damage to the house's structure, and to discourage mold growth. When relative humidity is more than 50%, moisture problems may occur.

Signs of too much moisture include condensation on windows, mildew, mold, peeling paint and a lot of dust mites. In summer, a properly sized air conditioner or a stand-alone dehumidifier can bring indoor humidity down. Size matters here: equipment that is too big will not run long enough to take humidity out of the air.

Source: University of Minnesota Extension · ENERGY STAR

Because the weather asks so much, it is tempting to buy the biggest furnace or air conditioner you can. That backfires. To size a system right, the heating and cooling loads must first be worked out accurately. Your contractor can find the right size by using ACCA/ANSI Manual J, or an equivalent sizing tool. Expect the contractor to gather facts about your house such as insulation, the type and size of windows, and floor area.

Rules of thumb are too often used to size systems, and they lead to equipment that is much too big. Oversized equipment runs in short cycles and does not reach efficient operation. If a unit is too large for your home, you get less comfort and higher costs. Your house may also have changed since the old equipment went in, through additions or better insulation, or the old equipment may have been too big from the start.

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

Many Chicago owners wonder whether a heat pump can handle the cold. Newer heat pump designs let some models run efficiently and near full capacity at very cold temperatures, and these cold-climate air-source heat pumps are now a workable heat source in any climate zone in the country. ENERGY STAR's certification for them requires verified performance testing down to 5°F.

A cold-climate heat pump keeps working below 5°F, but pairing it with a backup heat source heats a home most efficiently when it gets even colder. With Chicago's long winters, how the heat pump and any backup are sized and set up matters a great deal. A technician can talk through whether a heat pump alone or a heat pump with backup fits your home.

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