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Forgotten HVAC Feature: Static Regain

Wednesday, September 1st, 2010

You have used Design Master HVAC to size ductwork using the constant pressure drop and constant velocity methods, but have you used the static regain calculation in our software?

Static regain is the third sizing method for ductwork included in Design Master HVAC. It is most often used in the high pressure ductwork between the main AHU and the VAV boxes.

The calculation works by keeping the static pressure in the ductwork constant throughout the system. The air velocity is decreased so that the velocity pressure drop matches the total pressure drop in the system. Sizing ductwork using the static regain method results in small ducts and a system that is nearly balanced when first installed.

5 Responses to “Forgotten HVAC Feature: Static Regain”

  1. Tom Klein says:

    Static regain method for duct sizing can be effective in certain applications. Sizing is counter-intuitive, however. As air is distributed off the main duct, the duct stays the same size, or actually increases in size. On occasion, this will aid to deliver more air to the end of overly long or contorted ductwork runs.

    If used, be ready to explain to Architects and Owners if they question why the ductwork gets larger as air is unloaded off.

  2. David Robison (Design Master Software) says:

    That’s a good point Tom, and something that can be confusing when using static regain. Thanks for sharing!

  3. Edward Schmidt says:

    @Tom I have been a mechanical engineering consultant for four year and been designing ducts using the static regain method. I have never designed or even seen a duct that gets larger at the end of a run.

    I have done 10 case studies of comparing friction method ducting to static regain. The friction method ducting is contorted due to the VAV box requirements. Static regain is a far neater layout and in every case has resulted in less duct work so far.

    Where did you get your information on duct getting larger as air is offloaded? Bizarre.

  4. David Robison (Design Master Software) says:

    @Edward Imagine a long run of duct with no branches. As you move along the duct, the total pressure goes down. The velocity in the duct is constant so the velocity pressure is constant. The static pressure goes down as the total pressure goes down.

    Now put a transition in the middle of the duct. What should happen there?

    Static pressure wants to keep the static pressure constant. It has dropped during the course of the run. If you increase the size of the duct at that point, the velocity goes down, the velocity pressure goes down, and the static pressure goes back up to what it was before.

  5. Alfred Kuettner says:

    I have used static regain with a software program called DD4M. I am not sure if it is still available. Normally, the size will decrease after a sizable tap to the terminal box(es). With a smaller amount of air, the trunk duct may stay the same. In sizing the duct, there may be a case where the trunk duct would get larger. If that is the case, normally, I would go back and increase the duct to that size at the previous transition. This is so that ductwork does not go from smaller to larger. This should be doable. The benefits of static regain are being able to have all runs closer in pressure loss and using less sheet metal.

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