OK, I have heard from multiple HVAC contractors that a general rule of thumb in a single trunk return, single trunk supply duct system, the RETURN:SUPPLY ratio should be 1:1 or greater. In other words, the return should always be at least as large as the supply.

## How much bigger should return duct be?

Generally, if you are using a Duct Sizing Chart or calculator (such as the Trane Ductulator), size the re- turn ducts for 0.05 inches/100 ft (0.40 Pa/m) based on the expected airflow through that return air duct. The total sum of air from all the returns should be at least 250 CFM/nominal ton (33 l/s per-kW).

## Can return duct be too big?

No such thing as too much return. It will not take in any more air than it pushes out, thus no such thing as too big. Its hard to imagine a return that is too big ,but you want the return to have some draw to evacuate an area.

## Can a return air duct be too small?

If the return duct is too small, there are an inadequate number of return grilles or they are undersized, it can’t return enough air to recirculate back into the system. The average duct system has 25% less return air than required according to National Comfort Institute.

## Why is sizing a return duct correctly important?

They must be properly sized — Return air ducts must be large enough to carry a sufficient amount of expended air back to your furnace, air conditioner, or heat pump. If not enough air is brought back, your HVAC system will not be able to keep up with temperature demands.

## Should supply and return ducts be the same size?

OK, I have heard from multiple HVAC contractors that a general rule of thumb in a single trunk return, single trunk supply duct system, the RETURN:SUPPLY ratio should be 1:1 or greater. In other words, the return should always be at least as large as the supply.

## Can you have too much return air?

This isn’t the most efficient system. Having several return vents (ideally one in every room, but even two or three is better than just one) creates consistent air pressure. If you have one return vent, your home is fine. Keep the doors to each room open so air can properly circulate.

## What size return do I need for a 3 ton AC unit?

Recommended airflow is typically 400 cfm per ton of AC. A 3 ton AC would therefore need 1200 cfm airflow. With only 423 sq in of return your air velocity through the grille would be just over 400 ft/min, a good number as far as noise is concerned.