St. Louis City is independent from St. Louis County (separate since 1876). City addresses: Building Division, 1200 Market St, Room 426. Missouri has no statewide building code. 70% of permits issued same-day; Hot Spot Desk Mon & Wed 9am–3pm for immediate review. Online portal launched March 2026.
St. Louis HVAC permit rules
St. Louis Building Division requires a mechanical permit for all HVAC system replacements and installations under the 2018 International Mechanical Code. The mechanical permit covers the full installation scope: furnace or boiler installation, gas connection and pressure test (Spire manages the meter), combustion venting, and refrigerant connections for split systems. A separate electrical permit is required for wiring changes. Missouri-licensed HVAC contractors perform all permitted work. Hot Spot Desk (Mon & Wed) for same-day residential HVAC permit review.
St. Louis's Zone 4A balanced climate — approximately 4,500 heating degree days and 1,500 cooling degree days — makes both heating efficiency and cooling efficiency economically meaningful investments. Unlike Madison (heating dominates) or Chandler (cooling dominates), St. Louis homeowners benefit from upgrading both the furnace and the AC when replacing aging equipment.
One unique St. Louis HVAC feature: forced hot water boiler heating systems are far more common here than in any other city in this guide. The city's pre-1950 brick flats and row houses were originally heated with coal-fired, later gas-fired, cast-iron boilers with cast-iron radiators. Many of these systems remain in service. Boiler replacement requires boiler-specialist contractors with experience in hydronic systems — distinct from general HVAC technicians who primarily work on forced-air systems.
Three St. Louis HVAC scenarios
| HVAC Work | Permit? | Est. Fee | St. Louis Note |
|---|---|---|---|
| Gas furnace replacement | Mechanical + electrical | ~$200–$400 | 92% AFUE min (DOE North); Spire manages meter |
| AC condensing unit replacement | Mechanical + electrical | ~$200–$350 | 13 SEER2 min (DOE North); Ameren Missouri |
| Hot water boiler replacement | Mechanical permit | ~$200–$400 | Common in pre-1950 brick flats; boiler specialist |
| Heat pump installation | Multiple permits | ~$300–$550 | Zone 4A excellent climate; IRA incentives |
| Filter, cleaning (maintenance) | No permit | $0 | Pre-heating tune-up: October |
St. Louis's balanced HVAC climate
Of all ten cities in this guide, St. Louis has the most genuinely balanced HVAC demand. The 92% AFUE furnace minimum and meaningful 1,500 cooling degree days mean that efficiency upgrades on both the heating and cooling sides of the HVAC system yield proportional long-term savings. A move from 80% to 96%+ AFUE furnace saves meaningfully over 4,500 annual heating degree days. An AC upgrade from SEER2 13 to SEER2 18 saves meaningfully over 1,500 annual cooling degree days.
What the inspector checks
Gas connection and pressure test; combustion venting clearances; condensate drainage; refrigerant connections. Boiler: pressure relief valve, expansion tank, water quality. Electrical: equipment disconnect and wiring. Call 314-622-3315 for scheduling.
What HVAC replacement costs in St. Louis
Gas furnace + AC: $5,500–$10,000. Hot water boiler: $5,000–$10,000. Heat pump system: $7,500–$15,000 before incentives. Permit fees: $150–$550.
Does replacing a furnace require a permit in St. Louis?
Yes — mechanical permit required. Gas connection pressure-tested by inspector. Missouri-licensed HVAC contractor required. Hot Spot Desk (Mon & Wed 9am–3pm, 314-622-3315) can process standard furnace replacement permits same-day.
What is a hot water boiler heating system?
A gas-fired unit heating water circulated through pipes to radiators or baseboard heaters — common in St. Louis's pre-1950 brick residential buildings. Provides comfortable even radiant heat suited to high-mass brick construction. Replacement requires boiler-specialist contractors with hydronic system experience distinct from forced-air HVAC.
What AFUE is required for furnace replacements in St. Louis?
DOE North region minimum: 92% AFUE for new residential gas furnaces in St. Louis's climate zone. 96%+ AFUE condensing furnaces are the mainstream tier and provide meaningful savings in St. Louis's 4,500+ heating degree day season.
Is a heat pump a good choice for St. Louis?
Yes — Zone 4A is excellent for cold-climate heat pumps. Cold enough for meaningful heating efficiency vs. electric resistance. Cold snaps below 0°F are occasional. Federal IRA incentives significantly offset higher cost vs. gas furnace. Mechanical permit required.
Do Spire and Ameren need to be contacted for HVAC work?
For standard same-size equipment replacements, typically no utility contact needed. Spire manages gas meter. Ameren manages electric meter. For service entrance upgrades (new heat pump requiring larger electric circuit), Ameren coordinates the meter base. For new gas service, contact Spire at spire.com first.
Is St. Louis City the same as St. Louis County for HVAC permits?
No — independent city since 1876. Building Division at City Hall Room 426 (314-622-3315) serves city addresses. County municipalities have separate permit offices.
General guidance based on publicly available municipal sources as of April 2026. City of St. Louis uses 2018 IRC. Verify at 314-622-3315. For a personalized report, use our permit research tool.
Phone: 314-622-3315 (Hot Spot Desk: Mon–Wed 9am–3pm)
Hours: M–Th 8am–4:30pm, F 8am–4pm
Online: stlouis-mo.gov/building/permits
Codes: 2018 IRC · 2017 NEC · 2021 UPC · 2018 IECC
Cultural Resources: 314-657-3865 · Spire: spire.com · Ameren MO: ameren.com
Missouri One Call: 800-344-7483