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HVAC Air Duct sizing

orange_cat | Posted in General Questions on

I had a a mechanical engineer oversize the system. We caught it half-way and now the system is smaller (Heating load dropped from 80BTU to less than 60; his cooling load was calculated as 3.6tons). We are installing a 48BTU cooling/54 BTU heating 4 ton Mistusbishi heat pump.

The ducts to bedrooms are 8″ diameter (the main duct from air handler is 12×12). The installer states they need to be 5″ diameter.

I read  a little on pressure loss and so on, but how worried should I be (redo the ductwork level worried or….?).

The air supply is combined with the ERV.
In the winter the idea is to use the air supply for ventilation and supplemental heat but rely on in-floor radiant as main heat source.

What are your opinions?

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  1. Expert Member
    Akos | | #1

    There is nothing wrong with oversized ducting. Except for taking up space and costing a bit more there are very little downsides. About the only one is they carry noise.

    Those SEZ units have very low static capability, so I would be careful with reducing ducting too much. Unless it has a lot of bend 8" hard pipe does sound a bit too big.

    I would not trust an installer to pick the right size though in this case as it is easy to restrict the flow and have poor delivery using the low static unit. As a rule of thumb, keep the velocity for feeds on the low static unit to under 500FPM, your man J should have a heat loss/cooling load for that room so you can back calculate roughly the duct size.

    If the room has a lot of glazing, you might need that 8" feed if it is west facing to keep it from overheating in the summer though.

  2. maine_tyler | | #2

    If ducts are to be oversized, don't all ducts need to be proportionally oversized so that distribution is per the load calc? In other words, if just one register was 'oversized' wouldn't it get too much CFM relative to other rooms and therefore too many BTUs? Or are there other controls for ensuring proper BTU delivery per room?

    1. jberks | | #3

      Duct size and register (diffuser) output are different things.

      Diffusers play a role in how much flow gets outputted. Ideally they're controllable so you can balance the system.

      1. maine_tyler | | #4

        Surely ducts play a role. In fact, is it not the combination of all components which dictate the resistance and therefore the flow for a given branch?

        I take your point about damping with a control on a register (or elsewhere) but if ducts don't matter, why do they need to be sized to any specific size at all?

  3. walta100 | | #5

    Tyler if you are stopped at a red light and the lanes are 10 feet wide or 14 feet wide the same number of cars will get thru each green cycle. The flow rate is set by the restriction of the light /register.


    1. maine_tyler | | #6

      I have a heating system with zero throttle controls anywhere. The size, length, and bends determine flow. That must be unusual?

      If there is a constriction, it significantly reduces flow, but the flow rate is determined by the total system dynamics. Series resistors sum.

      I guess my point is that (I thought) duct sizing was supposed to get the room CFM requirements close and dampers were meant to make small adjustments.

  4. walta100 | | #7

    Sadly, not that unusual.

    When contractors are forced to bid for the lowest possible cost systems then yes, they will select the smallest possible duct that will deliver the calculated amount of air, eliminate the dampers and select registers without flow controls. In short, they save you a few dollars but the air moving thru smaller ducts flows at higher speeds making more noise and the fast flowing air blows on people making then uncomfortable.

    Value engineering


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