A room has a supply register but no return path when its door is closed. What follows?

A forced-air system is a loop, and air delivered into a room has to leave it. With the door closed and no return path — no return grille, no transfer grille, no undercut at the door — the supply air pressurises the room until the pressure opposing the fan equals what the fan can deliver, at which point flow drops sharply. Two consequences follow. The room underheats or undercools despite an open register, which is the complaint the occupant reports. And the pressure imbalance forces air out through whatever gaps exist in the building envelope while drawing replacement air into the rest of the house through other gaps, which increases infiltration and, in houses with atmospheric appliances, can contribute to depressurisation elsewhere. The remedy is a return path, most simply an undercut door or a transfer grille.

With no return path the room pressurises against the supply, and the resulting back pressure reduces the airflow the register can deliver. The room then underperforms even though the register is open and the system is working.

Forced-air systems require a return path from every conditioned space. Where a dedicated return is absent, transfer grilles, jump ducts or door undercuts allow air to circulate back to the air handler and prevent room pressurisation when doors are closed.

Air In Needs Air Out. Close the door with no return and the room fills up until the fan gives up. Undercut the door or the register lies to you.

Test with the door closed. A register that measures well with the door open and poorly with it shut confirms the return path is the problem rather than the duct run.

• •Diagnosing equipment or duct failure when the door is the variable
• •Assuming a pressurised room draws air from the attic
• •Testing airflow only with the door open