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Air Handler duct sizing

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Jon_blaney
Jon_blaney Member Posts: 317
I am planning to replace an oversized oil furnace with an air handler. As I plan the duct system I note that furnaces have fixed cfm output for a given capacity while air handlers have a variable capacity. The btu output is a function of fan speed and water temperature. Should you design the duct system for max output or something closer to you btu requirement? I am in a heat dominated climate.

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  • HVACNUT
    HVACNUT Member Posts: 5,858
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    The ductwork only needs to be sized to accommodate the CFM of the system its attached to. BTU's have nothing to do with it.
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    I asked the question because unlike a furnace, a particular fan coil can have a range of CFM outputs, different fan speeds. Lower cfm produced lower btu output if the water temperature is constant.
  • HVACNUT
    HVACNUT Member Posts: 5,858
    edited November 2019
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    > @Jon_blaney said:
    > I asked the question because unlike a furnace, a particular fan coil can have a range of CFM outputs, different fan speeds. Lower cfm produced lower btu output if the water temperature is constant.

    OK. Say max 20K BTU's at 180° through the coil at 1,200 CFM on High speed. The ductwork needs to be sized to handle 1,200 CFM at High speed. A lower fan speed actually has the capability to extract more BTU's from the coil.
    Ever notice in your car, with the heat on, the high speed fan temperature is cooler than low speed?
  • pecmsg
    pecmsg Member Posts: 4,881
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    Air handlers do not know what size coil is going on there so must cover a large range of sizes.

    The furnace is a fixed CFM + or - a small %.

    You do need to know how many CFM's are required per room to know what size ducts to run.

    Are you using metal or flex?
  • Jon_blaney
    Jon_blaney Member Posts: 317
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    OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.
  • HVACNUT
    HVACNUT Member Posts: 5,858
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    > @Jon_blaney said:
    > OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.

    Correct. Keep in mind, the main supply trunk duct needs to reduce in size as it goes down the line.
    For example, the supply starts out sized for 1,200 CFM. After takeoffs to say, two branch ducts, the main trunk must be transitioned down to keep up the static pressure. So if the first two branch ducts need 150 CFM each, then the trunk duct gets reduced just downstream of those branch ducts to accommodate 900 CFM, and so forth. And never tap a supply branch duct off the end cap at the end of the truck duct.
  • ChrisJ
    ChrisJ Member Posts: 15,767
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    HVACNUT said:

    > @Jon_blaney said:

    > OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.



    Correct. Keep in mind, the main supply trunk duct needs to reduce in size as it goes down the line.

    For example, the supply starts out sized for 1,200 CFM. After takeoffs to say, two branch ducts, the main trunk must be transitioned down to keep up the static pressure. So if the first two branch ducts need 150 CFM each, then the trunk duct gets reduced just downstream of those branch ducts to accommodate 900 CFM, and so forth. And never tap a supply branch duct off the end cap at the end of the truck duct.

    That's one way to do it but not the only way.
    When I did my ductwork I didn't go that way.

    I also sized everything for 0.05"WC drop @100'.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • pecmsg
    pecmsg Member Posts: 4,881
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    OK, Thanks. If I understand correctly, gross supply and return should equal the unit's designed max cfm. BTU output is them controlled by varying the water temp and fan speed.

    What you need is a accurate room by room load / loss (Manual "J") done, then a Manual "D" Duct Design.

    Do not try this yourself, it rarely works out well!