The air ducts in your house are a more or less passive part of your HVAC system, but that doesn’t mean there’s nothing special about them. Although they have basically no moving parts, they have to be correctly planned and sized or the rest of the system won’t have a chance to do its job properly. So how do you size HVAC ducts right, and what happens if you don’t?


To get the right size of ductwork you first have to figure out how much heating and cooling you’ll need for your indoor space. This is called the load calculation, and it determines the size of the unit at the center of the whole system, with larger spaces obviously needing more powerful units.

The load calculation is measured in cubic feet per minute (CFM) and determined by estimating the size of the rooms in cubic feet and plugged into a standard called Manual J. Once technicians have the load calculation, they can use a chart to factor it against the output of the central unit to come up with the right duct size.

What if the HVAC duct size is wrong?

The best thing for the technician and for you is to get the calculations right the first time. But errors are always possible and some of those errors can be big enough to cause problems with your indoor air. Let’s look at the signs and symptoms of improperly sized ducts.

  • Too small – Undersized HVAC ducts will have higher pressure at the vents and will usually make a lot of noise. The high pressure puts strain on the central unit and can also lead to inconsistent room temperatures. If your HVAC system seems louder than it should be then you might want to call a technician.
  • Too big – Oversized ducts will give the heated or cooled air they carry more time to come to room temperature before they get through the vents and into the room, so you’ll end up with less heating or cooling where you want it, and the system being overworked to meet demand.
  • Insufficient returns – “Returns” means the air which the system sucks in from the rooms to cycle through and cool or heat. Bad duct sizing can lead to not enough air coming in the return route the system can’t deliver enough of the desired temperature and the rooms become badly ventilated and stuffy.

You can count on us

If you experience any of these problems and feel like your heating or air condition is running but not delivering the results it should get in touch with Twin Air in Manassas and we can take a look to see if improper duct sizing may be a contributing factor.  If any repair work is needed, you can count on us to get it right the first time! We have been serving the Northern Virginia area for fifteen years and bring many more combined years of experience to the work we do. You can call us at (703) 754-1062 or reach out on our website.

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