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I'm installing a new 3-ton heat pump and am trying to plan the crawlspace installation of the indoor unit (1200cfm air handler with heat strips) and its connection to the existing ductwork.

These numbered scale drawings show the location of the existing ductwork and various possible locations for the new air handler.

Three elements are common to all drawings and are unchangeable: "obstacle" is a giant foundation pier (for a cinderblock wall nestling a wood stove); a stackhead (lower-right) holds the return air filter in a central location in the living space, and I'll put an 18" elbow at the bottom of it; and access to the supply trunk is via a rectangular-section elbow (upper-left) that comes out from the bottom of the supply trunk. The thick lines represent flex duct.

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It's really going to be impossible to provide the straight run of 2-3x the duct diameter (probably 18" for my 1200cfm-max air handler) at both ends of the air handler. So these various drawings represent different ways of compromising on this. A key issue is whether it's worth adding airflow resistance to the supply side, to reduce airflow resistance at the return side, IF the total airflow resistance is lowered; put another way, are supply-side and return-side airflow resistance (and thus static pressure) are "created equal" ?

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  • I know of vertical and horizontal instal, but not of 45 dgr
    – Traveler
    Commented May 14 at 22:03
  • HVAC guys can fabricate most anything out of sheet metal, I believe. That's the case with the only 45 in my sketches (not counting flex duct). Commented May 14 at 22:22

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