If the return duct is too small or there are an inadequate number of return grilles, your HVAC system won't be able to return enough air to recirculate into the system. Again, it's like if you were breathing in through a straw while jogging.
Undersized return ducts can cause up to 20% of the conditioned air in your home to escape. This occurs due to air pressure and not a hole in a duct. The excess pressure is caused by exhaust air staying in a room longer. The room may become uncomfortable and stuffy.
Undersized ductwork restricts the volume of return air, causing the HVAC system to struggle. Properly sized return air ducts are crucial for maintaining adequate airflow and balanced system performance. Measure duct sizes and compare them to system requirements.
The Size Of Your Return Ducts
In most cases, if you are installing ducts in each room where there is a supply, you will want to go with the same size as your supply vents. If you are installing just one or two return vents, you will want to go with a 12-inch return for 2-ton units and lower. 14- inches for 3 and up.
An undersized HVAC system has to work harder and run longer to maintain the desired temperature, which leads to higher energy bills. Frequent cycling and extended run times can also significantly increase energy usage. Consider upgrading to a properly sized HVAC system.
Neither oversizing nor undersizing your AC unit is ideal. Both scenarios can lead to issues with efficiency, comfort, and longevity of the system.
One of the most used methods is the HVAC duct sizing rule of thumb. In fact, even professional AC contractors might use this method to size the AC ducts in your home. According to this rule, you need to install an air conditioner of one-tonne capacity for every 500 square feet of the floor area of your residence.
An approximate rule of thumb to use when engineering data is not available is to multiply the filter grille area in square inches by 2 CFM for each square inch. This should keep the face velocity of the filter grille below 400 FPM.
Typically a 16X20 would be fairly standard for most applications on a 3 ton unit. However if you want superior air flow I would highly suggest putting in a 20X20 as that gives you far better airflow hands down.
The air now has to re-pressurize to regain its turbulent flow so it can continue to push its way down the duct. It requires about 24′ or two feet to do this. This is the 'Two-Foot Rule?. Ideally, you should allow at least 24′ between takeoffs and at the end of any duct run.
Dirty filters, leaky ducts, and poor insulation can cause cold return vents. Leaky ducts often cause cold air to blow even after cranking the heat up. 20-30% of air, especially hot air, is lost because of leaky ducts. Cracked, holey ductwork needs sealing or replacement.
By installing more return vents throughout your home, they can trap the air and remove it, improving air circulation. A drawback with this method is that your ductwork needs enough space to support additional vents. Smaller homes that can't add more vents need to pursue other options.
Some quick math - a 1 ton of nominal 400 cfm airflow at 300 FPM (Max velocity for a filter-grill) = 1.333 square feet. 1.33 square feet times 144 (square inches in a square foot) is 192 square inches. So yes, 200 square inches per ton is a good number to base minimum size based on velocity.
Restricted Airflow. Hot and cold spots are a telltale sign of air-distribution problems. With undersized ducts, you may notice that the vents nearest your HVAC unit blast air while the pressure drops substantially in other parts of your home.
Due in large part to the short cycling issues mentioned above, components on an oversized heating and/or air conditioning systems are more likely to fail sooner than those on a properly sized system. As a result, you may end up having to replace your HVAC system sooner than expected.
Just as oversized ducting can wreak havoc on your heating and cooling systems, so can undersized ductwork. When the ducting is too small for the specific HVAC system that you have, air flowing through it will start to back up. The air pressure within the ductwork will become higher.
125 sq inches of Return air per ton is a good standard to go by. 125 sq inches of Return air per ton is a good standard to go by. Actually a 20" X 25" is onmly good for a 2-ton not a 4-ton with a conventional return air filter grill.
For example, when considering what size air conditioner is needed for a 2,000 square foot home, you would likely choose a 3 or 3.5 ton unit, depending on the climate where you live.
Inconsistent air temperatures throughout your home could be a sign of not enough return air. You may also notice poor indoor air quality that feels stuffy. You also may notice an increase in your utility bill since your HVAC system might be working harder due to the lack of return air.
In most applications, the door undercut would need to be 1.5 inches or greater to allow enough air to pass. There are some through-the-door alternatives that can be effective.
While return vents don't always require filters, adding them provides additional filtration, improving indoor air quality and keeping dirt and debris out of your system.
However, most HVAC contractors use a rule of thumb to determine the number of air vents needed. For example, for a standard residential HVAC unit, you'll generally need one supply vent and one return vent for every 100 to 150 square feet of living space.
12" flex at the recommended residential velocity of 600 Ft/min gives almost exactly 500 cfm. Double that and you're at 1000 cfm which is exactly what a 2.5 ton unit needs (400 sq' per ton). The can size at 14x24 is plenty as well.
You generally need at least 1 CFM per square foot, meaning that room's duct should have a 144 CFM capacity. Although a 4” x 8” duct is ideal for a 12' x 12' room, the actual duct size you need varies based on different aspects of your home (size/number of windows, insulation type, etc.).