What Affects Furnace Replacement Cost in St. Louis

10 Factors That Shape Furnace Replacement Cost

1. Sizing for Your Home

Furnace capacity is measured in BTUs. Proper sizing starts with a load calculation (often called a Manual J) that accounts for square footage, insulation, windows, air leakage, and layout. An oversized furnace can cost more upfront, short-cycle, and wear out sooner. Sizing only from the old unit’s label can repeat an old mistake.

2. Efficiency Rating (AFUE)

AFUE shows how much of your fuel is converted to heat. Standard models are around 80% AFUE, while high-efficiency condensing models run 90% or higher. Higher-efficiency furnaces typically cost more to buy and install but may lower gas bills over time. Whether that pays back depends on your gas usage and how long you plan to stay.

3. Venting Changes

This is one of the most overlooked costs. An 80% furnace usually vents through a metal flue or chimney, while a high-efficiency furnace vents through PVC pipe and needs a condensate drain. Switching types means new venting. If a gas water heater shares the chimney with your old furnace, it may need a chimney liner or other correction to vent safely on its own.

4. Single-Stage, Two-Stage, or Modulating

A single-stage furnace runs at full output every time. Two-stage and modulating furnaces run at lower output most of the time, which often means more even temperatures and quieter operation. Variable-speed blowers add similar benefits. Each step up adds cost.

5. Ductwork Condition

A new furnace connected to leaky or undersized ducts may not deliver the comfort or efficiency you paid for. Some homes need duct sealing, repairs, or modifications. A good assessment checks airflow before recommending equipment.

6. Location and Access

A furnace in an open basement is usually the most straightforward install. Attics, crawlspaces, tight closets, and narrow stairways can add labor time.

7. Your Air Conditioner

Your furnace blower and your AC’s indoor coil work as a team. If your AC is near the end of its life, replacing both at the same time may make sense. An older coil may also not fit a new furnace cabinet. Read more in Should I Replace My Air Conditioner and Furnace at the Same Time?

8. Permits and Code Updates

Furnace replacement typically requires a mechanical permit and inspection from St. Louis County or your municipality. Gas lines, combustion air, or carbon monoxide detection may need updates to meet current code. A clear quote states whether permits are included.

9. Add-Ons and Indoor Air Quality

A new thermostat, a whole-home humidifier for dry winter air, or upgraded filtration all add to the total. See our indoor air quality options.

10. Brand, Warranty and Timing

Brand and model tier affect price, and labor warranties vary by installer. Manufacturer parts warranties often require registration. As an authorized Trane dealer, we can walk you through those terms. Timing matters too: a planned replacement in fall or spring gives you more time to compare options than an emergency replacement during a January cold snap.

How to Get an Accurate Furnace Quote

A reliable price starts with an in-home assessment, not a number over the phone. When you compare quotes, check that each one lists the equipment model and AFUE rating, venting and ductwork changes, permits, removal of the old unit, thermostat, and warranty terms. Comparing like-for-like often explains most of the price difference.

Also ask about current utility and manufacturer rebates (see our HVAC rebates page) and about financing options. At Thomas Hoffmann Air Conditioning & Heating, you get a written estimate before any work begins. Learn more about furnace replacement in St. Louis.