Furnace vent clearance from window openings can vary depending on how the furnace is installed. A high-efficiency furnace is often vented through the side of the house with PVC pipe. Sometimes you’ll see two pipes—an exhaust and a combustion-air intake. Other installations may have only the exhaust terminating outside.
So how close can that furnace vent be to a window?
The answer depends first on what type of installation you’re looking at.
Under the 2021 International Residential Code (IRC), the clearance requirements for a through-the-wall vent terminal can be different depending on whether the installation is direct vent or nondirect vent.
Before measuring anything, identify the installation.
Two Pipes or One Pipe?
A typical direct-vent condensing furnace may have two PVC pipes terminating outdoors.
One supplies combustion air directly from outside.
The other discharges the flue gases outside.
But don’t assume every high-efficiency furnace must have two pipes.
Some furnaces are permitted by their listing and manufacturer’s installation instructions to operate in a nondirect-vent configuration. In that arrangement, the furnace can discharge its flue gases outdoors while obtaining combustion air from the space in which the furnace is installed.
That’s why simply counting pipes isn’t enough.
For a deeper explanation of the difference, see What Does “Direct Vent” Actually Mean for a Furnace?
Furnace Vent Clearance From Window Requirements
This is where 2021 IRC Section G2427.8 (503.8) comes in.
G2427.8 addresses venting system terminal clearances and directs through-the-wall direct-vent and nondirect-vent terminals to Figure G2427.8 and Table G2427.8.
For an operable window or door, look at Row B of Table G2427.8.
For a direct-vent terminal, the minimum U.S. clearance is:
- 6 inches for appliances with an input of 10,000 Btu/hr or less.
- 9 inches for appliances over 10,000 Btu/hr and up to 50,000 Btu/hr.
- 12 inches for appliances over 50,000 Btu/hr and up to 150,000 Btu/hr.
For appliances over 150,000 Btu/hr, Row B directs the installation to the appliance manufacturer’s instructions and requires a clearance not less than the clearance specified for nondirect-vent terminals in Row B.
For a nondirect-vent terminal, the U.S. clearance is:
- 4 feet below or to the side of the opening.
- 1 foot above the opening.
These are the minimum clearances in the 2021 IRC model code. The actual installation must also comply with the furnace listing and manufacturer’s installation instructions, along with the code cycle and any amendments adopted by the local jurisdiction. Always verify those requirements before locating or approving the vent termination.
That’s why identifying the installation matters.
A clearance that complies when a furnace is installed in a permitted direct-vent configuration may not comply when a furnace that allows both configurations is installed nondirect vent.
For the complete figure and clearance table, see the official ICC source:
2021 IRC G2427.8 — Venting System Terminal Clearances
What About the Exhaust and Combustion-Air Intake?
On a direct-vent furnace, the exhaust terminal and combustion-air intake are commonly located near each other.
How far apart do those two pipes need to be?
Don’t use Table G2427.8 to establish that separation.
G2427.8 contains an exception stating that the clearances in Table G2427.8 do not apply to the combustion-air intake of a direct-vent appliance.
The required relationship between that appliance’s own vent terminal and combustion-air intake is determined by the appliance manufacturer’s installation instructions.
This should not be read as a blanket exemption for combustion-air intakes.
For example, Row J of Table G2427.8 addresses a vent terminal’s clearance to a nonmechanical air-supply inlet to the building or to the combustion-air inlet of any other appliance.
That’s a different condition.
The Manufacturer’s Instructions Still Matter
The IRC doesn’t stop with the clearance table.
Section M1307.1 requires appliance installations to conform to the conditions of their listing and label and the manufacturer’s instructions.
For listed direct-vent appliances, Section G2427.2.1 (503.2.3) specifically requires installation in accordance with the manufacturer’s instructions.
So when you’re looking at those two PVC pipes outside the house, don’t assume there is one universal separation between the intake and exhaust.
Identify the furnace and check its installation instructions.
Applying This in the Field
If you’re checking a furnace vent near a window, start with four questions:
- Is the furnace installed direct vent or nondirect vent?
- Is the nearby window or door operable?
- If it is direct vent, what is the appliance input rating?
- What do the manufacturer’s installation instructions require for the vent and combustion-air intake configuration?
Once you know those answers, you can apply the correct requirement instead of assuming every PVC furnace vent gets the same clearance.
Don’t Forget Other Exterior Terminations
Furnace vents aren’t the only exterior terminations that need to be located correctly.
Dryer exhaust systems have their own termination, clearance, duct and installation requirements.
See my article:
Dryer Vent Code Requirements Under the 2021 IRC
Applicable 2021 IRC Sections
G2427.8 (503.8) — Venting System Terminal Clearances
Establishes that through-the-wall direct-vent and nondirect-vent terminal clearances must comply with Figure G2427.8 and Table G2427.8.
Table G2427.8 — Venting System Terminal Clearances
Provides the applicable clearance requirements, including clearances to operable windows and doors.
M1307.1 — General
Requires appliance installations to conform to the conditions of their listing and label and the manufacturer’s instructions.
G2427.2.1 (503.2.3) — Direct-Vent Appliances
Requires listed direct-vent appliances to be installed in accordance with the manufacturer’s instructions and requires through-the-wall vent terminations for listed direct-vent appliances to comply with Section G2427.8.
Field Reminder
Before measuring a furnace vent clearance, identify whether you’re looking at a direct-vent or nondirect-vent installation.
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