Furnace Installation in St. Louis, MO

A furnace installation is only as good as the decisions made before the furnace arrives. Fuel, efficiency, size, vent path and gas line are linked, and a quote that skips one of them will cost you later. Furnace installation in St. Louis usually means choosing between an 80% unit that fits the old chimney and a 90%-plus condensing unit that needs new PVC.

What furnace types can you choose from?

Three decisions define the equipment.

• Fuel. Natural gas suits homes with a meter and gas line. Electric furnaces avoid combustion but cost more to run. A heat pump can replace or pair with a furnace.
• Efficiency. The federal minimum for gas furnaces is 80 AFUE. Condensing models reach 90% and above.
• Staging. Single-stage burners are simple and cost less. Two-stage and modulating models run longer at lower output, which evens out temperatures.

Fixr's 2026 figures show how much fuel type moves price: electric $1,800 to $7,300, natural gas $3,800 to $10,000.

When does a house need a new furnace installed?

Install when repair stops buying useful life or when the house has no suitable furnace to repair. Four situations account for most jobs:

• The furnace has reached the end of its range. Contractor sources put furnace life at 15 to 20 years with routine maintenance, and a unit past that line with a failing part is a replacement candidate.
• A technician finds a cracked heat exchanger. It can leak carbon monoxide into the airflow, which is why CPSC recommends a yearly professional inspection of furnaces.
• The house gained space. A finished basement or a rear addition changes the heating load, and a furnace sized for the original house may now be too small.
• The heat source is changing. A switch from electric resistance heat to gas, or to a heat pump, is a fresh installation and not a swap.

Older housing makes the first two likely. The City's median home was built in 1938, against 1979 statewide, according to census survey figures. If your furnace has years left, we would rather repair it than install over it.

What changes when you install a condensing furnace?

A condensing furnace changes the vent, the drain and often the wall penetration. ACHR News explains that it extracts extra heat with a second heat exchanger, so the exhaust is cool and wet and cannot use a metal flue. It vents through PVC, usually out a side wall, and produces acidic condensate that must drain safely.

If your old furnace vented through a chimney, that flue may also have to be capped or relined for the water heater that shares it. That is the kind of detail a written scope should state.

How is a furnace sized for a St. Louis house?

A furnace is sized to the heat the house loses on a design cold day, using ACCA's Manual J method, and not to the old unit's label. We run that calculation before we recommend a size. ACCA's Manual S then matches equipment to the load, and Manual D checks the ducts.

Oversized furnaces cycle on and off too quickly, run louder and leave temperature swings. Older north county homes add their own complications: leaky return trunks, undersized supplies and additions that were never rebalanced.

What should a furnace installation quote include?

An installation covers the furnace and everything it connects to: removing the old unit, setting the new one level, connecting the plenum and return, running the vent and the drain, connecting and leak-testing the gas, and firing the furnace with a combustion reading before anyone leaves. The itemized quote is your protection. It should list:

• The heating load result and the size chosen from it
• Furnace model, fuel, AFUE and staging
• Venting material and path, combustion air and clearance work
• Gas connection, shutoff valve, sediment trap and leak test
• Condensate drain and termination for condensing models
• Thermostat, filter size and any duct or plenum changes
• Permit, old-furnace removal and what is excluded

How does the installation go, and how long does it take?

Fixr lists furnace installs at 4 to 10 labor hours, with scope setting where in that range you land. The sequence is steady.

1. Measure the house and run the heating load calculation.
2. Review the vent path, gas line, drain and ducts.
3. Settle the equipment choice, then hand you the clear, upfront quote ahead of any work.
4. Shut off gas and power, disconnect and remove the old furnace.
5. Set the new furnace level, connect plenum, return and vent.
6. Connect gas, drain and wiring, then pressure-test the gas joints.
7. Fire the furnace, measure temperature rise and combustion, and check the safety switches.
8. Walk you through the thermostat, filter and carbon monoxide alarm placement.

What does a new furnace cost?

Fixr reports natural gas furnace installations at $3,800 to $10,000 in 2026, with labor at $300 to $1,800, permits at $50 to $1,500 and removal of the old furnace at $50 to $330 (page updated January 28, 2026). Inch Calculator, another cost site, puts a gas install in a 2,000 square foot home with reusable ducts and piping at $5,000 to $7,500 (page updated September 2023). These are market figures, not our quote.

The biggest swings come from efficiency tier, new vent and drain work, duct changes and gas piping. Ask for 80% and 90%-plus options priced separately so you can see what the extra efficiency costs.

In older houses, name three line items in the quote: capping or relining the old chimney flue, the condensate drain run, and any plenum or duct change. For a wider benchmark, Angi's 2026 St. Louis range for a full heating and cooling system, air conditioner included, is $4,755 to $11,888, with labor at $1,500 to $5,000 of it.

What do the permit, gas inspection and license rules say?

The installing contractor pulls the permit, and you should confirm that in the written quote. St. Louis County requires a mechanical permit to install a heating system, and the City of St. Louis requires a City-licensed mechanical contractor, with a $25 permit application fee. Missouri has no statewide HVAC license, so each city and county sets its own. The City adopted the 2018 International Mechanical Code and answers permit questions at 314-622-3313. St. Louis County follows the 2015 International Residential Code as amended.

Gas piping may be separately inspected, depending on your jurisdiction. Frederick E., a Google reviewer, said leaks were fixed, pressure-tested and passed Spire's reinspection. Ask any contractor, including us, for the license number your address requires.

How do you take care of a new furnace?

Change the filter on the schedule the installer gives you and keep the area around the furnace clear. Test the condensate drain at the start of each season if your model has one. Install a carbon monoxide alarm, and book furnace repair at the first sign of a hard start or short cycling.

Why Kenan Corporation for a furnace installation

An install is where honest advice shows up in the quote.

• Calculated sizing. We run a load calculation before sizing, and we explain the result in plain terms.
• Options, priced apart. Candice F., a Google reviewer, describes Kenan as great at explaining things and giving options.
• Replacement only when it is earned. Wendy R., a Google reviewer, was told by another company to buy a new furnace. Kenan Corporation found a capacitor and fixed it.
• Open 24 hours, and a 4.6★ Google profile built from 29 reviews as of October 2026.

Where do we install furnaces?

Permit rules change at every city line. Each place below is paired with the rule that shapes an install there, timed from our base near Spanish Lake in normal traffic.

• Ferguson, about 20 minutes: mechanical work needs a Construction Permit from Code Compliance, and a masonry chimney built for an old boiler may not suit a condensing furnace's venting.
• Florissant, about 20 minutes: the city cites a 7 to 10 day wait for construction permits, so build that into the install date.
• Hazelwood, about 25 minutes: the city has adopted the 2021 International Mechanical Code and Fuel Gas Code, which govern the vent and gas connection.
• Maryland Heights, about 35 minutes: the city issues residential mechanical permits and allows a like-for-like furnace swap without plans.
• Clayton, about 35 minutes: furnace permits run through the city's CitizenServe portal under the 2021 mechanical code.
• Creve Coeur, about 40 minutes: a county-licensed contractor pulls the permit and the city inspects the finished furnace afterward.
• Webster Groves, about 40 minutes: permits go only to city-registered contractors, with $40 plan review and $40 per inspection.
• Ballwin, about 55 minutes: installers need an active St. Louis County license plus a Ballwin contractor license.
• Wentzville, about 1 hour 5 minutes: population grew from 7,058 in 2000 to 29,070 in 2010, and city contractor licenses renew every February 28.