📊 Exhaust Coverage
🏠 Donor Room Remaining
Calculation Breakdown
Full Calculation Table
| Parameter | Value | Result |
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How to Use the Transfer Air Calculator
Enter the exhaust airflow of the room that needs transfer air, such as a bathroom fan or range hood.
Enter how much spare supply air the donor room, such as a hallway or bedroom, actually has to give up.
Pick a door undercut, transfer grille, jump duct or louvered door, check its free area, and enter how many paths are installed.
Choose the pressure difference the path may operate at, then click Check Transfer Air to see whether the path and the donor surplus cover the exhaust.
How transfer air avoids the need for makeup air
Every room that exhausts air needs that air replaced from somewhere. Natural Resources Canada explains that exhaust devices pump indoor air outside and can depressurize a house, which is why the replacement path matters. The cheapest source is air already in the house, moved from a room with more supply than it needs into the room doing the exhausting. That is transfer air, and it works through a door undercut, a transfer grille, a jump duct or a louvered door.
Two things have to line up
First, the transfer path has to be big enough to pass the required airflow at a small pressure difference. Second, the donor room has to have that much spare supply air, meaning it receives more from its own supply register than it exhausts on its own. A large grille connected to a donor room with no surplus moves nothing, because there is no air available to transfer. The tool takes the smaller of the path capacity and the donor surplus as the air that can actually move.
How a path is sized
Flow through an opening follows the orifice relationship Q = Cd × A × sqrt(2 × ΔP / ρ). In imperial terms the Building Science Corporation uses Q = 1.07 × A × √ΔP, with Q in CFM, A in square inches of free area and ΔP in pascals. At 3 Pa that is about 0.54 in² per CFM, so 50 CFM needs about 27 in². The tool uses a discharge coefficient of 0.6, which matches that table. In the default example, a 25 mm by 800 mm undercut has 0.02 m² and passes about 27 L/s (57 CFM) at 3 Pa, so it covers a 25 L/s fan at about 2.6 Pa. Pressure limits come from the Building America Solution Center, run by Pacific Northwest National Laboratory, which gives the ENERGY STAR limits of ±3 Pa, or ±5 Pa for rooms with 150 CFM or more, with bedroom doors closed.
Undercuts, grilles and jump ducts
A door undercut is the simplest path but not the best one. The same guidance says undercuts are commonly used but not a recommended approach, because a wood door moves with the seasons and added carpet can block the gap, and in most applications an undercut would need to be 1.5 in (38 mm) or more. Transfer grilles in the door or wall, and jump ducts, are the better options, and offsetting the room-side and hall-side grilles keeps sound and light down. A fully louvered door has the largest free area but gives up privacy. The same closed-door pressure logic applies to supply and return paths, which the bedroom comfort calculator looks at from the comfort side.
What happens when transfer air falls short
If the path and the donor surplus cannot cover the exhaust, the shortfall is pulled through the envelope or from other rooms through gaps never meant for it. How leaky the house is decides how much that costs in pressure, which a blower door test calculator can help estimate. For exhaust loads too large for transfer air alone, dedicated makeup air is the more reliable fix, sized with the makeup air calculator. The balanced ventilation calculator checks the whole-house result.
Related tools
Find the exhaust rate for the receiving room with the bathroom fan calculator or the kitchen exhaust calculator, and check the room-level pressure relationship with the pressurization calculator.
Frequently Asked Questions
Transfer air is air already inside the house that moves from a donor room with a supply surplus, such as a hallway or bedroom, into a room that exhausts more than it receives, such as a bathroom or kitchen. It moves through a door undercut, grille or jump duct. It works for smaller exhaust loads when a nearby donor room can spare the air, and it avoids bringing in dedicated outdoor makeup air.
The path must pass the flow at a small pressure difference, so size it with the orifice relationship Q = 1.07 × A × √ΔP, with Q in CFM, A the free area in square inches and ΔP in pascals. At 3 Pa that is about 0.54 in² of free area per CFM, so 50 CFM needs about 27 in². A grille's free area is only part of its face, often about 75 percent. The Building Science Corporation sizing table works from this relationship, and this tool uses the same physics with a discharge coefficient of 0.6.
ENERGY STAR for new single-family homes asks for no more than ±3 Pa (0.012 in. w.c.) between a closed bedroom and the rest of the house, or ±5 Pa for rooms with 150 CFM or more of design airflow. Those limits come from bedroom return pathways, but the same pressure keeps doors easy to open and limits uncontrolled air leakage when a bathroom or kitchen fan runs. The tool lets you choose 2, 3 or 5 Pa.
Then the shortfall comes from somewhere else, usually leaks in the envelope, or from other rooms through gaps never meant for it. Add a larger or second transfer path, find another donor room, or bring in dedicated makeup air. The makeup air calculator sizes that option, and the balanced ventilation calculator checks the whole-house balance.