Chicago’s official all-time low is −27°F, recorded at O’Hare on January 20, 1985. The National Weather Service still lists that as the record. On January 30, 2019, O’Hare fell to −23°F, the coldest official reading in the city since 1985, with wind chills near −50°F. Schools closed, flights were canceled, and a lot of furnaces ran without a break for two days.
Those two winters are the stress test people in this region actually mean when they say “polar vortex.” The phrase got popular in January 2014, during a long stretch of below-zero weather. The 2019 outbreak was colder.
What continuous runtime exposes
At a more ordinary 25°F, a furnace cycles. At −20°F it runs. A dirty filter, a weak blower capacitor, a boiler circulator that has been groaning since November, a heat exchanger that was already cracked: all of those can survive a mild month and fail when the equipment is asked to run for 48 hours. The failure mode that matters most is combustion. A furnace or boiler working that hard has to vent completely. A cracked exchanger or a blocked flue in that weather is a carbon monoxide problem in a house where people are home with the windows shut.
Pipes fail next. An unheated attached garage, a crawl space, a powder room on an exterior wall, and any line that runs through a cabinet on an outside wall are the usual casualties. If the heat is keeping up, those pipes usually stay liquid. If the heat stops, they do not get a grace period at −20°F.
Heat pumps and gas heat in this climate
A standard air-source heat pump loses capacity as the outdoor temperature falls, and it leans on backup heat in a cold snap like 2019. That is a design fact, which is why so many Chicago-area homes still heat with a gas furnace or a boiler. Cold-climate heat pumps have improved, and they can be the right equipment in the right house. They still need their backup checked before January, and the backup needs to light. If you are not sure what would carry the house at −15°F, that is worth knowing before the forecast includes it.
Use the anniversary as a deadline
January 20 is a poor day to schedule the first look of the winter. The useful date was in the fall. If this winter has already been kind and the equipment has not been checked, call now, on a mild day, rather than on the morning the record is challenged. Zima HVAC handles furnaces and boilers from Mundelein across Lake County. (847) 323-8314. The seasonal checklist lives in our winter maintenance note, and boiler homes can start with the pre-winter boiler tune-up.