Can a return duct be too short?

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.

How big does a return duct have to be?

As for any duct, size is dependent on duct length, air flow, and allowable static pressure drop. For most systems, the minimum duct size (see Table 1) is sufficient for up to 25 feet (8 m) with no more than three 90° bends.

What happens if you don't have enough return air?

If there is not enough return air available, your HVAC system will not heat or cool properly. 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.

Why is sizing a return duct correctly important?

An undersized return duct system reduces the amount of air flow through the system and may cause reduced comfort and efficiency as well as poor supply airflow, duct sweating, improper refrigerant charge, and excessive air noise.

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.

Basic Return Duct Sizing Made Simple

What happens if ducts are too small?

Ducts that are undersized will increase the static pressure, which will result in excessive noise as air moves through the system. It is often loud enough to be distracting, so it will not be difficult to overlook and should be addressed, as undersized ducts overwork the heating unit.

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.

Is it better to undersize or oversize ductwork?

Using oversized ducts will erode the system's efficiency, while undersized ducts won't carry the necessary volume of air. Balanced airflow. To maintain neutral pressure in the HVAC system and throughout the house, the duct system must deliver and return a matching volume of air.

What happens if duct is too big?

Your system pushes air into the ducts, but the ducts are too large for it to build up any sustained amount of air pressure, and thus a “forced air” system doesn't have any force. As a result, you get poor air flow and no matter how hard your system seems to be working, you don't get to feel the benefit.

Should cold air returns be high or low?

To ensure efficiency during the cooling season, your home should have high registers. High return registers draw hot air that rises to the ceiling back into the system to repeat the cooling cycle.

How do I know if I have enough return air?

If you were to put a wet finger close to the vent that is generating airflow, you should be able to tell if there is a lot of airflow or not enough airflow. This is the simplest way to tell if you are getting enough return air. If you lick your finger and put it close to the return vents, you will feel the airflow.

How do you increase return air flow?

Can undersized ductwork cause low air flow?

Oversized & Undersized Ductwork

In the case of smaller ductwork, the airflow will be heavily restricted and will not be able to distribute evenly in your house, resulting in weak HVAC airflow. Unfortunately, you may have to replace your entire ductwork in case of wrong sizing.

Can you oversize a return duct?

For the most part, the only issue with oversized ducting is that it is a waste of money and space. However, HVAC blowers do require a certain amount of external static within the first three foot of the blower. ECM motors may not ramp up to speed if there is too little SP on the return side.

How big should my return air duct be for a furnace?

A typical return vent is 16 inches by 20 inches, according to Hunker. It includes the cold return air duct grille. If you consult the duct CFM sizing chart, you can quickly figure out that a 16×20 inch can handle about 1,500 CFM airflow.

Can I make a cold air return smaller?

Modify Cold Air Duct Sizes

Whenever possible, expand your existing ducts vertically by cutting out pieces of wallboard and installing a new cover and air filter to fit. If it's not possible to expand vertically, expand horizontally as much as possible to fit the existing ductwork.

Is it OK to reduce duct size?

By restricting the duct size it will cause more resistance and have less air flow because the air will take the path of least resistance. All the other ducts would have a higher flow with the 6" compared to a 8" duct because they are all in parallel. Such a reduction will increase noise.

Does duct length affect cfm?

Texas A&M did a study on the effect of flex duct not pulled tight and the results are astounding. In my article on this research, I showed from their results that a 6″ duct moving 110 cfm when pulled tight will move only about 70 cfm with 4% linear (longitudinal) compression and about 40 cfm or less at 15% compression.

Does reducing duct size increase pressure?

For sections that run parallel, always use the section with the higher pressure loss/gain to determine pressure losses/gains through a system. Adjust the branch with the lower pressure loss/gain by increasing the flow rate or decreasing the duct size to increase the pressure loss to that of the higher branch.

How do I know if my ductwork is undersized?

Undersized ducts and grilles restrict airflow and may be the cause of your comfort and temperature problems. If you notice loud airflow and strange noises such as whistling and popping, you may have improperly sized ducts or blocked vents and registers.

Why is duct size important?

Without properly sized ducts, you could compromise your return on investment. Without adequate air flow and well-designed ducts that match your system, you could end up with significantly reduced home comfort and much higher operating costs.

Do I need a return vent in every room?

While it is a myth that air return grilles are required in each and every room in the house, it is definitely necessary to have more than one of these grilles installed at strategic places in the house. The most important place to have these would be the bedroom.

How much return air do I need for a 4 ton?

(Example: 4 (ton unit) x 144 square inches = 576 squared. The square root of 576 is 24. Therefore, your return air duct and grill size will be 24 by 24 inches.)

Why is cold air coming out of my return vent?

So why is cold air coming out of your vents? It is most likely a leaky duct system—but you should have your home's heating efficiency checked out by an HVAC professional.