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Community and Q&A

How best to install HVAC return duct on outside of house?

JGees | Posted in Mechanicals on

Basic facts: 3 level house built on a down slope hillside lot in 1994. HVAC zone one covers main and upper floors. Zone two heats lower floor bedrooms. Equipment room located on lower floor. Climate zone 3c—SF Bay Area home. Large SW facing windows heat up main floor (family room) and upper floor master suite.
I just had a 4 ton AC unit installed for zone one knowing it might under serve the 680 sq ft. upper floor master suite as there is no air return register upstairs and original system/ducting was designed for heating only. The AC installer said the existing return was undersized and made some modifications to slightly improve the return capacity during installation.
I am now considering installing an upper floor air return duct on the outside of the northwest side of my house, exiting the premise via an exterior wall in the master closet down two floors to penetrate the stucco wall into dead space under the house and run horizontal 25 ft to the air return in equipment room. The duct would be exposed to the sun apprx. 2 hours per day in the middle of summer.
The 3-level house “cascades” down the sloped lot so the floors do not stack up and there is not the ability for interior raceways or adding ducts. The master closest is on the opposite end of the floor from the sleeping area. I believe an air return located at the top of the closest wall is a better first step than adding another supply register in the sleeping area—which could be done but at considerably more cost and hassle.
What type of ducting? Can it be stuccoed over or will I need to build a raceway to hold the ducting? Is there a better way to camouflage the duct or a better solution all together?

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  1. GBA Editor
    Martin Holladay | | #1

    It's impossible to determine if there is a better solution without a site visit.

    Exterior ducts should be insulated to the same level as your thermal envelope. In Zone 3, walls need to be insulated to at least R-20 -- so that's a good minimum goal. If this were my house, I would build a chase, insulated on 3 sides with continuous R-20 rigid foam, large enough to contain the galvanized ductwork. All the usual requirements apply: duct seams need to be sealed with mastic, and the rigid foam needs to have taped seams and staggered joints.

    Once the chase was framed and insulated, I would sheathe it with OSB or plywood, and install siding to match the existing siding. If the top of the chase creates a horizontal shelf that projects out from the house, this horizontal shelf needs to be treated as a roof.

  2. Expert Member
    Dana Dorsett | | #2

    Are you sure you can't get there with jump ducts using the conditioned space as a return plenum? Building on exterior insulated chases with stucco siding seems like a complicated & expensive way to deal with this!

  3. Jon_R | | #3

    You have compared to the cost of adding a small mini-split?

  4. GBA Editor
    Martin Holladay | | #4

    I thought about that suggestion, but it's too late. Joe wrote, "I just had a 4 ton AC unit installed."

  5. Dana1 | | #5

    Even with the already-installed 4 tonner, if it's only the master bedroom suite that isn't being adequately served , it's probably cheaper to add a 1/2 ton or 3/4 ton mini-split to that zone than to build an insulated exterior duct chase.

  6. JGees | | #6

    Thanks for your thoughtful/useful responses. I'm particularly pleased with the "if this were my house" preface for building an exterior chase---I believe that's my ultimate direction. A mini-split was my first choice---but too much of the ceiling is sloped leaving virtually zero flat attic space on the top floor. It is 100% master suite so a "jumper duct" from there to 2nd floor ceiling wasn't a consideration for privacy reasons (the sleeping area is over the living and dining room, the bathroom is over the kitchen and the office/master closet over a second floor bathroom).

  7. Jon_R | | #7

    I didn't follow the connection between flat attic space and being able to install a mini-split.

  8. Expert Member
    Dana Dorsett | | #8

    A floor mount ductless cassette mini-split can work even in front of a knee-wall in a head-banger attic space. They even fit between the floor & window-sill of most windows. It's more expensive than the standard wall-blob, high-mount coils, but it's not huge cost adder.

    A single Fujitsu AGU9RLF / ARU9RLFF would almost surely cover it.

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