Do You Need a Permit to Replace a Furnace or Air Conditioner?

Usually, yes. The International Residential Code (IRC), the model code that states and cities adopt with their own amendments, requires a permit to install or replace any gas or mechanical system it regulates, and exempts a short list: portable units, minor parts, piping inside the equipment and small self-contained systems. Federal consumer guidance treats a contractor who asks you to get the permits yourself as a warning sign, and your city or county building department sets the local details.

What the model code says

The permit rule is broad. Section R105.1 of the 2021 IRC says that any owner or owner’s authorized agent who intends to “install, enlarge, alter, repair, remove, convert or replace any electrical, gas, mechanical or plumbing system” regulated by the code must first apply to the building official and obtain the permit.

The exemptions are short. For mechanical work, Section R105.2 lists:

• Portable heating appliances, portable ventilation appliances and portable cooling units.
• Steam, hot-water or chilled-water piping within heating or cooling equipment regulated by the code.
• Replacement of any minor part that does not alter the approval of the equipment or make it unsafe.
• Portable evaporative coolers.
• Self-contained refrigeration systems with 10 pounds or less of refrigerant, or with motors of 1 horsepower or less.

The gas list is similar: portable heating, cooking or clothes drying appliances, and replacement of minor parts. The code adds that an exemption from a permit is not permission to do the work in violation of the code.

Those exemptions are why a plug-in portable air conditioner, or a self-contained unit within the refrigerant and motor limits above, normally goes in without a permit, while a central system does not.

Repair or replacement: where the line falls

A part is not the same as a system. The minor-part exemption covers replacing a component without changing what the equipment was approved as. Swapping an entire furnace, outdoor unit, heat pump or air handler goes well beyond that description; if in doubt, the building department decides.

Section R105.2.2 draws the same line from the other side. Ordinary repairs that need no application or notice do not include the addition, alteration, replacement or relocation of gas piping, or mechanical or other work affecting public health or general safety.

When the replacement cannot wait

A furnace that dies in January does not have to sit until a permit is issued. Section R105.2.1 of the model code lets equipment replacements and repairs that must be performed right away go ahead, with the permit application submitted within the next working business day.

States amend this. Oregon’s 2021 residential code, for example, gives five working business days for mechanical equipment replaced under that provision.

What the application asks for

The application covers basic facts about the job. Under Section R105.3, an application identifies and describes the work, gives the street address or other description of the property, states the use of the building and the valuation of the work, and is signed by the applicant or the applicant’s authorized agent. The building official can ask for construction documents and other information.

Once issued, the permit or a copy stays at the site until the work is complete (Section R105.7). Section R105.8 makes every person who performs the installation or repair responsible for complying with the code, not only the permit holder.

What the permit and inspection cover

The permit is how the code’s requirements get checked. Under Section R109.1, the building official inspects when the permit holder or agent gives notice, and either approves the work or states where it fails to comply. Section R109.1.6 requires a final inspection after the permitted work is complete. Several requirements tend to come up on an equipment replacement:

• Sizing. Section M1401.3 of the 2024 IRC requires heating and cooling equipment to be sized under ACCA Manual S from loads calculated under ACCA Manual J, or other approved methods.
• Carbon monoxide alarms. Under Section R311.2.2 of the 2024 IRC (Section R315.2.2 in the 2021 edition), alterations or repairs that require a permit trigger CO alarms placed as for a new home. Work on plumbing, and on mechanical systems that are not fuel fired, is exempt, so permitted work on a gas furnace or boiler brings the requirement with it. New Castle County, Delaware, for example, spells this out in its 2024 IRC policy. Philadelphia’s building department lists the fuel-fired trigger among the changes from the 2018 edition to the 2021 edition, so places still on older editions may not apply it.
• Venting. When an appliance is permanently disconnected from an existing chimney or vent, the 2024 International Fuel Gas Code (Section 501.15) requires the chimney or vent to be resized as necessary for the appliances that remain.
• Efficiency. In the Southeast and Southwest, a new split air conditioner must meet the regional standard in effect at the date of installation. The U.S. Department of Energy says it is illegal for a contractor to install one that does not.

Which edition of each code applies depends on what your state or city has adopted.

Who applies for the permit

The code allows either the owner or the owner’s authorized agent to apply. Consumer guidance points to the contractor: the Federal Trade Commission lists a request that you get the required building permits yourself among the signs of a home improvement scam. A contractor who expects to work without a permit, or asks you to take it out in your own name, is telling you something about how the job will be done.

When a professional is needed, and what to ask

Replacing a furnace, air conditioner or heat pump involves gas, refrigerant or electrical work, and the permit belongs to that job. Before work starts, ask:

• Will you obtain the permit, and can I see the permit number before the equipment is removed?
• When will the final inspection happen, and will I receive the signed-off permit afterward?
• Does this job trigger the local carbon monoxide alarm requirement, and who installs the alarms?
• Is the size based on a load calculation, and will the building department ask to see it?
• For a gas furnace: if a water heater or boiler stays on the old chimney, how will the chimney be resized for it, as Section 501.15 of the fuel gas code requires?
• In the Southeast or Southwest: does the proposed air conditioner combination meet the regional efficiency standard for this state?

Keep the permit, the inspection record and the equipment paperwork together. They are the record that the work was checked.