Return air: why undersized returns wreck a good system

Air goes round in a loop

A furnace or air handler is a fan in a box. It pulls air from the house through the return ducts, pushes it across the filter, the heat exchanger or coil, and out through the supply ducts to the rooms. The amount that goes out is exactly the amount that comes back. If the return side is too small, the fan cannot pull enough air, and the supply side gets less no matter how well it is designed.

Return ducts are easier to skimp on than supply ducts, because a house works with one big grille in a hallway and nobody sees the duct behind it. Many builder-grade systems have one return for the whole house, sized for a smaller system than the one later fitted. The result is the most common airflow problem in American houses.

What a starved return does

• Low airflow. A system designed for 1,200 CFM moving 900 CFM delivers well under its rated capacity, cooling and heating less while running longer.
• High static pressure. The blower works against more resistance, uses more electricity, and gets louder. Variable-speed blowers ramp up to compensate, using even more.
• A frozen coil or an overheating furnace. Too little air over an air conditioner’s coil lets it fall below freezing and ice up; too little over a furnace’s heat exchanger trips the high-limit switch and shortens its life.
• Whistling grilles. Air rushing through a small return grille is noisy, a clear sign the face velocity is too high.
• Pressure imbalance. Closed bedroom doors with no return path pressurize the bedrooms and depressurize the hall, pushing conditioned air out through the walls of one and pulling outdoor air in through the other.

Sizing the return grille

A grille is sized for the air it must pass at a quiet face velocity, allowing for the share of its face that is solid. Typical figures are 400 FPM for a plain return grille, 300 for a filter grille, and 60 to 80% free area. The return air grille size calculator does the arithmetic:

If the grille also holds the filter, the face velocity should be lower, around 300 FPM, and the grille larger. A 2-ton system split between two filter grilles needs two 12 × 24 in grilles. The duct size chart lists common grille sizes beside the ducts.

Sizing the return duct

Return ducts carry the same airflow as the supply and should be at least as large, often a size larger, because the return velocity limit is lower: about 600 FPM against 900 in a supply trunk. The duct velocity calculator checks an existing return:

The fix is a second return, not a faster fan. The duct size calculator sizes a return for the airflow at the return velocity limit, and the guide to duct sizing basics explains how friction rate and equivalent length combine.

Returns in every room or one in the middle

There are two common layouts. A central return, one or two large grilles in a hall or on each floor, is cheap and works when every room can push its air back easily, with the doors open or with gaps and jumper ducts. Returns in most rooms, or at least in each bedroom, cost more and work regardless of doors.

A bedroom with a closed door needs a return path. Without one, the supply air pressurizes the room and the airflow drops. A door undercut of an inch passes only a small amount; a transfer grille through the wall, a jumper duct through the ceiling or a dedicated return in the room passes the rest. Rooms with more than about 100 to 150 CFM of supply need more than a door gap. The CFM per room calculator gives each room’s supply airflow, which is what its return path must carry back.

Signs your return is the problem

A clear sign is a return grille that whistles or that pulls a sheet of paper hard against itself. Others are a dirty filter that sucks inwards, rooms that get much less air with the doors shut, and a technician measuring high total external static pressure, above about 0.8 in wc on a 0.5 in wc rated system. A system that seems oversized because it short-cycles may in fact be undersized on airflow, which the guide to oversized systems helps tell apart.

Filters and returns

The filter sits in the return path and takes some of the pressure. A dense high-MERV 1-inch filter in a small return grille is a common way to strangle a system. A larger grille, a deeper media filter, or a filter cabinet at the air handler restores the airflow, as the guide to MERV filters and airflow explains.