Code cycles run on a 3-year clock, and the 2024 International Mechanical Code (IMC) is the current edition that's working its way through state and local adoption. If you're a working HVAC contractor in 2026, the 2024 IMC is either already in effect in your jurisdiction or it's coming. Knowing what changed before your first failed inspection is cheaper than learning it after.
This article covers the most relevant 2024 IMC changes for residential HVAC contractors. Codes vary by jurisdiction, so always verify with your local authority having jurisdiction (AHJ). The principles below are widely applicable, but specific requirements differ.
The big picture
The 2024 IMC was released by the International Code Council in August 2023, and adoption has been rolling out across states ever since. Currently, 47 states plus DC and US territories have adopted some version of the IMC at state or local level, though not all jurisdictions adopt the newest edition immediately. Some are still on 2018 or 2021. Check your local AHJ for which edition applies to your work.
The biggest single category of changes in the 2024 IMC reflects the EPA's HFC refrigerant phasedown. Beyond that, there are updates to ventilation requirements, exhaust terminal placement, and a notable formatting change that makes the new code easier to use day-to-day.
1. A2L refrigerants are now allowed
This is the change that affects almost every residential HVAC tech. The 2021 EPA rule mandated an 85% phasedown of HFC refrigerants over 15 years due to their high global warming potential (GWP). The 2024 IMC includes the code provisions that make using lower-GWP A2L refrigerants legal for residential AC and heat pump systems.
What it means in practice. R-410A is being phased out for new equipment. New residential systems sold in 2026 are increasingly using R-32 or R-454B (both A2L refrigerants). These have significantly lower GWP than R-410A but are mildly flammable, which is why the code had to be updated to allow them safely.
What you need to do.
- Get certified to handle A2L refrigerants. The EPA Section 608 certification process now includes A2L-specific training.
- Stock A2L-compatible recovery and recharge equipment. Old recovery machines designed for R-410A may not be safe with A2L refrigerants.
- Update your leak detection equipment. A2L refrigerants require different detection methods.
- Train your techs on the safety differences. A2L refrigerants are mildly flammable, which means open-flame brazing requires careful procedures and ignition sources have to be controlled in the install area.
Real-world impact: refrigerant work in 2026 is more involved and more expensive than it was 5 years ago. Customer pricing has to reflect the new equipment, training, and time required.
2. Clothes dryer exhaust terminal placement
Carryover from the 2021 IMC update but still tripping up techs in jurisdictions just adopting the newer codes.
The rule. Clothes dryer exhaust terminals must be at least 3 feet from any opening into a building (windows, doors, makeup air intakes, etc.).
Why it matters. Lint and combustion byproducts from gas dryers can re-enter the building through nearby openings. The 3-foot rule prevents that.
What fails. Older homes with the dryer vented out a wall right next to a window. Tight side-yard installs where the only available exterior wall puts the vent within 3 feet of a neighbor's window or a door. Crawlspace dryer vents that terminate too close to foundation vents.
What to do. Verify the 3-foot clearance during install. If you can't get clearance on a retrofit, work with the homeowner to find another route or to relocate the offending opening.
3. Ventilation rate updates
The 2024 IMC includes updated mechanical ventilation requirements that align more closely with the latest ASHRAE 62.1 / 62.2 standards.
The general rule for residential. Whole-house mechanical ventilation is required in tightly-built new construction. Rates are based on conditioned floor area and occupant count, with specific tables in IMC Section 403.
What's changed. Updated rates for animal care facilities and outpatient healthcare (mostly commercial impact). For residential, the most relevant change is a 30% reduction in minimum mechanical ventilation rates for whole-house balanced ventilation systems compared to earlier editions.
What it means. If you're installing energy recovery ventilators (ERVs) or heat recovery ventilators (HRVs) in tight new construction, the design rates may be lower than what you were used to in older code editions. Don't oversize for the sake of oversizing.
4. Plenum requirements and material restrictions
The 2024 IMC continues tightening the requirements for what's allowed in plenum spaces. Plenums are limited to one fire area, and there are continued restrictions on materials within plenums.
The rule. Pipe insulation, duct insulation, and any non-metallic piping in a plenum has to meet flame spread index 25/50 (flame spread no more than 25, smoke developed no more than 50). Foam plastic in plenums is heavily restricted.
Why this trips up HVAC contractors. When you're running condensate lines, refrigerant lines, or any other piping through a plenum-rated space, the materials matter. PVC condensate line that wasn't plenum-rated is a citation. Standard fiberglass duct wrap in a plenum return air space might also be a citation depending on its rating.
What to do. Verify materials are plenum-rated when running anything through a return air plenum. Most condensate line installs in plenums now require either flame-rated copper or specifically plenum-rated PVC.
5. Condensate discharge and identification markings
The 2021 IMC introduced more specific requirements for condensate disposal, and the 2024 IMC continues that trend.
The rule. Condensate must terminate at an approved location. Discharge restrictions have been added to prevent termination in places that could create slip hazards, structural damage, or sanitary issues. Identification markings on the condensate line are now required in some configurations to help with future service and inspections.
What fails. Condensate dumping into a sump pit that's not designed for it. Condensate terminating onto a roof or roof drain in a way that creates damage. Condensate from a high-efficiency furnace going into a drain that doesn't have proper neutralization.
What to do. Plan condensate routing during install. Verify the termination point is approved by your local AHJ. Mark the condensate line clearly where it disappears into a wall or chase.
6. Combustion air provisions
The 2024 IMC continues to refine combustion air requirements for fuel-burning appliances. Most residential furnaces and water heaters need combustion air either drawn from outside (direct vent) or from a sufficiently large interior space (atmospheric/natural draft).
The rule. Combustion air sizing depends on appliance BTU input, the type of room the appliance is in, and the ventilation in that room. Tight modern construction often can't supply adequate combustion air to atmospheric appliances without dedicated outdoor air ducts.
What's changing. Older homes being upgraded to high-efficiency equipment increasingly need direct-vent appliances rather than atmospheric ones, because the home's tightness has improved (new windows, weatherstripping, insulation) since the original install. This is why direct-vent and sealed-combustion equipment is becoming the default in retrofit work.
7. Format changes (the easy win)
The 2024 IMC includes what ICC calls "the most substantial format changes to the International Codes in 25 years." The text moved from two-column to single-column format, which makes the printed and digital codes much easier to read on tablets and phones. Section numbering was also harmonized across the I-Codes.
This isn't a code change per se, but if you've been using a 2018 or 2021 IMC and you upgrade to a 2024, the new format will save you time on every code reference. Worth the upgrade just for that.
What this means for your pricing
Code-compliant work in 2026 costs more than it did in 2018. Some specific cost drivers:
- A2L refrigerant equipment is more expensive than R-410A equivalents. Refrigerant itself costs more.
- A2L training and certification is an investment your shop has to make.
- Plenum-rated materials cost more than standard materials. If you've been using standard PVC for condensate, switching to plenum-rated adds a small per-foot cost.
- Direct-vent equipment for retrofit installs (where atmospheric is no longer adequate) costs more than the equipment it replaces.
- Updated ventilation requirements mean more ERV/HRV installs in new construction, which adds equipment and labor.
Your prices should reflect the actual current cost of code-compliant work. The shop down the street still installing R-410A out of old stock at last year's prices isn't your competition; they're going out of business.
The bottom line
The 2024 IMC reflects the industry's transition to lower-GWP refrigerants, tighter ventilation standards, and continued refinement of the code's safety and efficiency provisions. The biggest practical change for residential HVAC contractors is the move to A2L refrigerants, which requires new training, equipment, and a different cost basis for refrigerant work.
Verify which IMC edition your jurisdiction has adopted. Get your A2L certifications. Update your equipment. Train your techs. Price the work to reflect the real current cost of code-compliant installation. The contractors who handle this transition smoothly will own the market by 2027. The ones who don't will be playing catch-up.
Frequently Asked Questions
How do I know which IMC edition my jurisdiction has adopted?
Call your local building department and ask. Most state code adoption pages also list the current edition online. Don't assume the newest is in effect; some states are still on 2018.
What's the difference between A1 and A2L refrigerants?
A1 refrigerants (like R-410A) are non-flammable. A2L refrigerants (like R-32, R-454B) are mildly flammable. The "L" stands for "lower flammability" and these refrigerants require ignition source above a specific energy threshold to combust. Safe with proper handling, but the safety procedures are different from A1 refrigerants.
Do I need to retrofit existing R-410A systems?
No. Existing systems can continue running on R-410A, and replacement R-410A is still legal (though increasingly expensive). New equipment manufactured in 2026 and beyond is increasingly A2L. The phaseout is for new equipment, not existing.
What are the most common 2024 IMC violations on residential inspections so far?
The most common citations being reported are: condensate termination errors, combustion air inadequacy on retrofit furnace installs in tight homes, and refrigerant handling issues on A2L installs. Plenum material errors are also common as inspectors get more familiar with the updated requirements.
How often do I need to recertify on refrigerant handling?
EPA Section 608 certification is currently valid for life once obtained. However, most jurisdictions and many manufacturers now require additional A2L-specific training before you can purchase or handle the new refrigerants. That training is typically a one-time class with a certificate that's accepted by suppliers and AHJs.