Replacing a furnace in St. Louis? Describe the system.

What this work involves

Furnace replacement in St. Louis, MO

A furnace replacement looks like one appliance swapped for another, and on the invoice it often is. What separates a good job from a bad one is everything around the box: whether the new unit was sized from the house or from the old nameplate, what happens to the venting, whether the cooling coil sitting on top of the furnace still matches, and whether the gas, electrical and duct connections are redone or merely reattached.

Sizing is the first thing to ask about. The furnace being removed was sized for the house as it was when it went in — before the attic got more insulation, before the windows were replaced, sometimes before an addition. Copying its capacity repeats any mistake made back then. The industry method is a room-by-room heat-loss calculation (ACCA Manual J). Not every contractor runs one for a straight swap, but you are entitled to ask how the size was chosen and to get an answer that is not "same as before".

Venting is the second. A high-efficiency condensing furnace exhausts cool, wet flue gas through plastic pipe and drains condensate, so it usually stops using the old masonry chimney. If a gas water heater is left venting into that chimney on its own, the chimney can be far too large for it to draw properly, and the remedy is typically a liner. Ask any company quoting a condensing unit what it intends to do about the water heater. If the answer is nothing, ask why.

Is this your problem?

Common faults and what they usually mean

• The furnace is old enough that parts are getting hard to findAge alone is not a reason to replace, but a unit where every repair starts with a search for a discontinued board is telling you its repair costs will keep climbing.
• Rooms at the far end of the house never warm upOften ducts, not the furnace. A new furnace on undersized or leaky ductwork gives the same cold bedroom with a newer label. Ask for the ducts to be looked at before the quote is written.
• Short cycling: on for a few minutes, off, on againA classic sign of an oversized furnace, which also wears faster and heats unevenly. Worth knowing before buying the same size again.
• A company has condemned the heat exchangerThat finding ends a furnace. Ask to see the crack or the inspection images and get it documented, then get a second replacement quote — you are buying a furnace either way, so the comparison is free money.
• The air conditioner is about as old as the furnaceThe indoor cooling coil usually sits on the furnace. Replacing one and not the other can mean re-fitting an old coil to a new cabinet; ask for both prices so you can decide with the numbers in front of you.

Local conditions

What makes St. Louis different

Two things, and the second one catches people out. The first is the climate: St. Louis carries the heaviest heating load of the six cities with a service guide here, with dozens of freezing nights a year, and still has a proper cooling season on top. A heating system here is a primary appliance, and the cooling coil and blower that share its cabinet work hard in summer as well. That is why a furnace quote here should always say what happens to the coil and the blower, not just the burner section.

The second is who licenses the work. Missouri has no statewide licence for heating contractors, so licensing is local. The City of St. Louis and St. Louis County are separate governments with separate mechanical licences: the City issues mechanical permits — which cover furnace installation — only to contractors it has licensed, and the County keeps its own list of licensed mechanical contractors. A company can hold one and not the other. Which one matters depends on which side of the city limit the house sits.

So the useful question is not "are you licensed?" but "are you licensed where my house is, and will you pull the permit?" A contractor who offers to skip the permit to save you money is offering to skip the inspection that checks the gas connection and the venting — the two parts of a furnace installation that can hurt someone. If you live in a municipality inside the County, ask your city hall who issues the permit; do not assume.

Local facts, with sources

St. Louis, MO in numbers you can check

• Heating load4,433 heating degree days a year (base 65°F) — the heaviest heating load of the six cities with a service guide here. Source: NOAA NCEI U.S. Climate Normals 1991–2020
• Freezing nights85 nights a year at or below 32°F. Source: NOAA NCEI U.S. Climate Normals 1991–2020
• Hot days a year47 days at or above 90°F, and 1,713 cooling degree days — the system has a real summer job too. Source: NOAA NCEI U.S. Climate Normals 1991–2020
• State licenceMissouri issues no statewide HVAC or mechanical contractor licence; the state’s list of licensed professions includes electrical contractors and no heating trade. Source: Missouri Division of Professional Registration — professions list
• St. Louis CountyThe County licenses mechanical contractors and publishes a list of those licensed to work in the County. Source: St. Louis County Mechanical Licensing
• City of St. LouisMechanical permits, which cover furnaces, are issued only to mechanical contractors licensed by the City’s Building Division. Source: City of St. Louis — Apply for a Mechanical Permit

Climate figures are 1991–2020 averages at NOAA station St Louis Lambert Intl AP, the nearest major weather station; a given neighbourhood can run warmer or cooler. Licence sources were read on 27–28 October 2026.

What happens next

If you send a request, this is the whole process

1. 01

   You describe the problem

   Tell us what stopped working, how urgent it is, and where you need help. Sending a request is free.

2. 02

   We check for a match

   We check whether a company is eligible to receive your request in your area. There may be no available match.

3. 03

   A company may contact you

   If a matched company pays to unlock the request, it can access your contact details and decide whether to call or email. Contact and appointment availability are not guaranteed.

Repair or replace

Which one is actually the right call

Get at least two written quotes, and make them comparable: same capacity or an explanation of why not, same treatment of the coil, the venting and the water heater, and the permit named as included. The cheapest quote is often the one that left one of those out. Cooling equipment efficiency has been rated in SEER2 since 1 January 2023, so if the quote includes an air conditioner, compare SEER2 to SEER2.

After hours

If the old furnace fails before the new one is in

Questions

Straight answers

Do I need a permit to replace a furnace in St. Louis?

In the City of St. Louis, furnace installation is covered by a mechanical permit, and the City issues those only to mechanical contractors it has licensed. In St. Louis County and its municipalities the permit rules are local — ask the contractor which permit they will pull and from whom, and get that written into the quote.

How do I check a contractor’s licence here?

Missouri has no state licence to look up, so check locally: the County publishes a list of the mechanical contractors it licenses, and the City’s Building Division can tell you whether a contractor holds a City licence. Ask the contractor for their licence number for your jurisdiction and check it before you sign.

Are the companies on this site licensed?

We do not vet companies, and we do not check insurance. The one exception is a listing that says “Licence checked”: our staff looked that licence up by hand in the public record it names, on the date shown. That is one lookup, not vetting, and a licence can lapse after it. For everyone else, the licence question is yours to ask, and the answers above tell you where to check it.

Should I replace the air conditioner at the same time?

Not automatically. If the air conditioner is young and its coil fits the new furnace, keep it. If it is close to the end of its life, doing both together saves a second visit, a second permit and an awkward mismatch between a new furnace and an old coil. Ask for the quote both ways.

Is a high-efficiency furnace worth it here?

In a city with this much heating each winter, efficiency is worth more than it would be further south, because the furnace burns more fuel each year. The catch is the venting and the condensate drain — make sure the quote covers both, and what becomes of a water heater left on the old chimney.

How long does a replacement take?

A straightforward swap is commonly a single day of work once the unit is on hand. Venting changes, a new coil, duct alterations and the permit inspection can stretch that. Ask for the schedule, including when the inspection happens, before the old furnace is removed.

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