Return Air Grill Sizing

Kinds of Grills

There are three different ways air is brought into the closed systems present in your home.

Vent grills are coverings for vent openings where air is forced into or out of a room. This air is then circulated into either the heating or cooling unit.

Heat registers are vent covers that cover the hole in the wall usually near the floor or in the wall where the duct goes into the room. It may have adjustable dampers that will control air flow.

An air return is a vent grill that is often located in the hallway or the ceiling. It extracts air from the room and sends it back through the closed air system.

Return Air Grill

Air that is in your HVAC closed system is either coming into the system or going out. The return vents are connected to the return ducts which means the air is moving from the room to the inside of the system. When the air handler is functioning, it will suck the air from the rooms, into the return ducts and back to the heating and cooling system.

A closed HVAC system will be pushing air into a space causing an increase in air pressure in that room. The air must have a place to either vent itself or go back through the system. The return vents and ducts serve to equalize this pressure and allow for the continued circulation of air. The cover is usually adjustable and will allow the air to be either increased or decreased based on personal preference.

The return air grill will also usually have a filter inside it to trap particulate matter and other contaminants in the atmosphere of your dwelling. A return air grill will also look similar to the supply side grill and can only be differentiated by the flow of the air.

Keep the areas near and around the return air and supply side vents clear of furniture, boxes, and other items to ensure adequate air flow.

Air Flow Measurement

The efficiency of an HVAC system may be determined by the measurement of air flow within the system. This air flow measurement is a determiner of the amount of air per unit of time coursing through a vent. This air flow is measured in terms of volume or mass. An air flow meter is used and will effectively determine the amount of air flow through a space. The air flow rate is expressed in terms of cubic feet per minute, meters per second and sometimes as feet per minute. This is further expressed as cubic feet per minute(CFM), meters per second(m/sec), feet per second(ft./sec) or feet per minute(ft./min.

 Symptoms of an Incorrectly Sized Return Grill

Sizing of the Return Grill

HVAC technicians are held accountable for the guidelines and standards as set forth by the Air Conditioning Contractors of America(ACCA), codes as created by the State of Alabama Heating, Air Conditioning and Refrigeration Contractors Act, and the Code of Alabama. The size of a grill, the mechanical movement of air, and the mathematics that govern the establishment of correct return air vents are determined by these organizations. Installation standards and demands are adhered to in order to produce a standard of performance across the career field and ensure the professional installation of HVAC systems.

So, to solve the problem of the correct size return air grill an example will lead you to the solution. Let’s say you live in a 1400 sq ft space and the ceilings are 7 ft high. The total air volume of this space is 9800sq ft.

If we think of changing the air in a space at least 6 times in one hour the HVAC system will need to move 9800 x 6 = 58,800 cubic feet of air per hour. 58,800/60 min. equals 980CFM.

In order to determine the size of the tonnage you have, simply divide 980 by 400 CFM per ton. Doing this for this space will yield a tonnage of 2.45. Always round up and we have a tonnage value of 2.5.

A return air grill should be sized for an air speed of 300-500 FPM(feet per minute). 400 FPM is the median air speed for this calculation.

We are after the ability to move 980CFM so you now divide this number by 400 FPM yielding 2.45 or 2.5. Convert this measurement to inches. Do this by multiplying 2.5 x 144 = 360 square inches. Take the square root of 360 which is 18.97366596.

You will want to find a filter close to this size of essentially, 19 x 20in.

There are other factors that go into this calculation such as the amount of leakage from the envelope of the home. Another factor needed for this process is the technical information from the air handler. You will have to determine the maximum external static pressure drop or the intake resistance to air that still allows the air handler to deliver 980 CFM.

You will have to purchase filters that do not exceed this drop. Purchase a filter with the highest MERV rating that has a pressure drop rated for your size filter that is below the value of the air handler static pressure.

It is recommended that the calculation and assessment of your situation be done by a trained HVAC technician.