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A contractor quoting a furnace replacement in St. Louis County starts with a load calculation because the burner and heat exchanger will carry real work across most of the calendar. Homes in St. Louis County, MO run heat about seven months a year, from October through April. Undersized equipment cycles harder and more often to meet demand, and the ignition system takes the punishment of every short cycle. An install that looks adequate on paper starts failing components ahead of schedule when the idle months are few. Proper sizing at the point of installation is what keeps that wear from arriving early.
Ductwork laid down to serve a furnace that is no longer in the house carries a set of assumptions about airflow, trunk sizing, and return placement that a modern unit may not share. Half the homes in St. Louis County, Missouri were built in 1969 or earlier (2020-2024 ACS 5-year). That spread across the housing stock means a contractor quoting a furnace installation here is often evaluating whether existing duct runs can handle the static pressure a new system requires, and whether the electrical service at the panel will support what the new equipment draws. Both answers shape the job before the unit itself is chosen.
What Gets Installed With a New Furnace
The furnace itself sits in the basement, utility room, or closet and is the unit a homeowner sees most plainly. Inside it, a heat exchanger warms the air that moves through the house, a blower pushes that air out, and a burner assembly fed by a gas line is what a contractor connects when the unit goes in.
The flue pipe runs from the furnace to an outside wall or up through the roof, carrying combustion gases out of the house. Alongside that, a contractor installs the thermostat on an interior wall and connects it to the furnace so the two can communicate.
The drawing shows a typical furnace layout, and a licensed contractor determines exactly what applies to your home.
Get matched in St. Louis CountySt. Louis County, by the numbers
208 severe weather events were recorded in St. Louis County, Missouri between 2015 and 2024.
NOAA National Centers for Environmental Information, Storm Events Database, bulk CSV export, events 2015-01-01 through 2024-12-31.Half the homes in St. Louis County, Missouri were built in 1969 or earlier (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesSt. Louis County, Missouri holds 446,192 housing units (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesOwners occupy 69.2% of the occupied homes in St. Louis County, Missouri (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesFurnace Installation Rules for St. Louis County, Missouri4 codes, standards, and laws
What the Installation Actually Looks Like
Non-weatherized gas furnaces manufactured on or after November 19, 2015 must reach 80.0 percent AFUE.
Non-weatherized oil-fired furnaces manufactured on or after November 19, 2015 must reach 83.0 percent AFUE.
Mobile home gas furnaces manufactured on or after November 19, 2015 must reach 80.0 percent AFUE.
Non-weatherized gas furnaces manufactured on or after December 18, 2028 must reach 95.0 percent AFUE, up from the 80.0 percent minimum that applies to units manufactured before that date.
Severe Weather Events in St. Louis County
St. Louis County weather puts real load on heating equipment, and that load shapes what a furnace has to do year after year. How hard a system works is a local fact, and the record below is part of that picture.
208 severe weather events, 2015–2024. Source: NOAA National Centers for Environmental Information, Storm Events Database.