Can You Put an HVAC Supply Vent in an Attached Garage

No. Section M1601.6 of the 2018, 2021 and 2024 International Residential Code (IRC) says a furnace or air handler that supplies living spaces "shall not supply air to or return air from a garage," and R302.5.2 says ducts in the garage can't have openings into it. Code aside, by our estimate a 100 CFM branch off a small furnace on an 800 CFM blower puts about 6,400 BTU/h into a 55°F garage, about half of the 12,400 BTU/h our garage calculator gives for an insulated two-car garage at 5°F outside.

The question shows up two ways. A homeowner finds a register in the garage ceiling and asks whether to shut it, or a customer asks you to tee a run off the house trunk into the garage. The heat numbers here come from the example garages on the free garage heater size calculator, which is also where you size a heater that serves the garage alone.

What does the IRC say about supply vents in a garage?

Section M1601.6, titled "Independent garage HVAC systems," says furnaces and air-handling systems that supply air to living spaces "shall not supply air to or return air from a garage." The sentence reads the same in the 2018, 2021 and 2024 IRC. It doesn't separate heating from cooling, so a branch that only blows cold air in summer falls under it too.

2021 IRC § M1601.6

"Furnaces and air-handling systems that supply air to living spaces shall not supply air to or return air from a garage." - International Code Council, codes.iccsafe.org

The return side is covered twice. Section M1602.2 lists the rooms return air can't be taken from, and the garage is on that list in all three editions. Right under it, Exception 2 carves out the one case where a garage return is fine: "Dedicated forced-air systems serving only the garage shall not be prohibited from obtaining return air from the garage."

These are the model code texts. A state or city can amend them, so check the edition and amendments your inspector enforces.

Can house ductwork run through the garage?

Yes, if it's built for it. Section M1601.4.9 sends ducts in garages to R302.5.2, which allows ducts in the garage and through the walls or ceilings between the house and the garage when they're at least No. 26 gage sheet steel or other approved material, with no openings into the garage. So a trunk can pass through the garage as long as it has no register or other opening into it.

2021 IRC § R302.5.2

"Ducts in the garage and ducts penetrating the walls or ceilings separating the dwelling from the garage shall be constructed of a minimum No. 26 gage ... sheet steel or other approved material and shall not have openings into the garage." - International Code Council, codes.iccsafe.org

That's the layout in the source thread below, where the bedrooms sit over the garage. The supply duct to that side of the house runs along the garage ceiling, and the duct itself can stay. Whether flex or duct board counts as "other approved material" in that space is the building official's call, since the section names only sheet steel.

Should you close the garage vent or cap it?

To match R302.5.2, cap it. A register with its louvers shut still sits on an opening in a duct inside the garage, and that section says those ducts "shall not have openings into the garage." By our reading, pulling the register and closing the boot with sheet metal is what the text asks for.

In the source thread on r/hvacadvice, a duct from the furnace that serves one side of the house runs along the ceiling of an attached garage and has a vent into it. There's no return in the garage, the bedrooms sit above it, and the owner wanted to know whether to keep the vent shut or leave it open. The only answer in the thread said closing it wouldn't hurt.

Whether an old register has to come out is a separate question. The 2021 IRC, in R102.7, lets the legal occupancy of an existing building continue without change, except where this code, the International Property Maintenance Code or the International Fire Code covers it, or "as is deemed necessary by the building official" for safety. R102.7.1 says additions, alterations and repairs "shall conform to the requirements for a new structure." So cutting a new register into the garage today is an alteration held to M1601.6, and a register the house was built with is something to raise with the inspector when you're pulling a permit anyway.

How much heat would one branch put into the garage?

About 6,400 BTU/h in our worked example, and only while the burner runs. A 100 CFM branch at 114.4°F supply air into a 55°F garage carries 1.08 x 100 x 59.4 = 6,415 BTU/h. That's about 52% of the 12,434 BTU/h our garage heater calculator gives for its default insulated two-car garage at 5°F outside, and 20% of the same garage with no insulation, an uninsulated door and very leaky air sealing.

1. House and blower. The example house in ACCA's Manual D lesson plan moves 800 CFM through 8 supply runs. ACCA doesn't name a furnace, so we paired it with one that fits that airflow, Goodman's GMVC960403BNB. Goodman prints 38,400 BTU/h of high-fire output, a 20 to 50°F rise range and a lowest recommended high-heat airflow of 710 CFM at a 50°F rise.
2. Supply air. At 800 CFM the rise works out to 38,400 / (1.08 x 800) = 44.4°F. On a 70°F return, that's 114.4°F supply.
3. Branch airflow. 100 CFM is the 800 CFM split evenly over the 8 runs. ACCA's own heating airflows for those runs range from 63 to 123 CFM.
4. Temperature difference. 114.4 - 55 = 59.4°F between supply air and the garage.
5. Heat. Q = 1.08 x CFM x dT = 1.08 x 100 x 59.4 = 6,415 BTU/h.
6. Airflow to cover the garage. 12,434 / (1.08 x 59.4) = 194 CFM.

The loads below come from the calculator page's own example garages at a 5°F winter design temperature. Branch heat leaves out any loss from the duct before the register.

So the math cuts both ways. A branch can out-run a small one-car garage while the furnace fires, but the insulated two-car needs 194 CFM, more than any heating run in ACCA's example house, where the largest is 123 CFM. Across ACCA's 63 to 123 CFM runs, one branch puts about 4,000 to 7,900 BTU/h into the 55°F two-car. Either way, M1601.6 bars the branch, so cap it at the boot and size a separate heater from the heat loss column.

What does the code allow for heating a garage?

A system of its own. M1601.6 restricts systems that also supply living spaces, and Exception 2 under M1602.2 lets a dedicated forced-air system serving only the garage take its return air from the garage. As we read the section's title, "Independent garage HVAC systems," a garage-only furnace or air handler, a unit heater or a ductless head fits it.

Size that system from the garage's own heat loss. For the calculator's default two-car garage, held at 55°F with 5°F outside, the page lists about 12,400 BTU/h of heat output. That's 3.64 kW of electric heat, or about 15,500 BTU/h of gas input at 80% efficiency.

A gas or propane heater burns fuel inside the garage, and the calculator page points you to the manufacturer's instructions and local code for venting, combustion air and clearances. For the summer side, the garage AC size calculator runs the cooling load of the same garage in BTU/h and tons.

Sizing a garage-only heater with BuildSolver

Run the free garage heater size calculator with the garage's walls, ceiling, door, windows and the 99% winter design temperature for the location. It returns the heat output in BTU/h, the electric kW and the gas input at the heater's efficiency. It doesn't pick a heater, so compare its numbers with the rated output of the unit you plan to install.

1. Measure the inside length, width and wall height, and pick how many walls face outside. A garage with a long side on the house takes the "3, a long side against the house" option.
2. Enter the wall and ceiling insulation, the garage door size and type, the windows, the slab edge and the air leakage. With bedrooms over the garage, pick "Heated room above" for the ceiling. If the door has a tested U-factor from the DASMA program, enter it.
3. Enter the 99% winter design temperature for the location and the temperature you want to hold, such as 55°F for a workshop.
4. Compare the heat output with the heater's rated output, and for gas, compare the input line with the heater's input. Keep the house branch out of the plan, and cap any register that's already in the garage.

Use the result as a preliminary estimate for a quote. Where the permit calls for an ACCA-approved load calculation, see Manual J for permits.

Sources

• International Code Council, 2021 International Residential Code, Chapter 16, Sections M1601.4.9, M1601.6 and M1602.2: codes.iccsafe.org
• International Code Council, 2018 International Residential Code, Chapter 16, Sections M1601.6 and M1602.2: codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Chapter 16, Sections M1601.6 and M1602.2: codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Chapter 3, Section R302.5.2: codes.iccsafe.org
• International Code Council, 2018 International Residential Code, Chapter 3, Section R302.5.2: codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Chapter 3, Section R302.5.2: codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Chapter 1, Sections R102.7 and R102.7.1: codes.iccsafe.org
• Goodman, GMVC96 gas furnace product specifications SS-GMVC96, heating data and airflow data: goodmanmfg.com
• ACCA, Instructors' Lesson Plans for Manual J, Manual S and Manual D, EDU-1, September 2015, pages 34 and 35: higherlogicdownload.s3.amazonaws.com
• Allison Bailes, "The Physics of Heat and Air Flow," ACCA HVAC Blog, May 2, 2018: hvac-blog.acca.org
• Source question, r/hvacadvice, "Attached garage supply vent question": reddit.com

For estimation purposes only. The building official under the code and amendments your jurisdiction has adopted decides what's allowed in a garage, and the heater's installation instructions govern how it goes in.