Good & Plenum

03 Jun

One of the central elements of the redesign on the first floor is the bulkhead. It defines the dining/kitchen peninsula space and delineates the kitchen area from the good room. As noted in earlier posts, the bulkhead also serves a functional purpose as the east-west highway for infrastructure elements.

This post addresses the most intriguing feature of the bulkhead’s role in house infrastructure – serving as a plenum for return air to the AC.

The following series of photos documents the progression of the bulkhead from the early stages of framing to date:

In the photo below, the duct to the left is for AC supply; the duct on the right is for AC return in the rear of the house (there’s another at the top of the stairs). The plenum will serve to route air to this return duct.

The bad supply lines . . .

. . . and the good:

A plenum is a separate space provided for air circulation for heating, ventilation, and air conditioning typically provided in the space between the structural ceiling and a drop-down ceiling. In our case, the plenum will be an encapsulated space within the bulkhead to allow for flow to the return air vent, so we don’t have to put an unsightly, big-ass register in the ceiling (it would need to be 10″ x 10″). (The return air flow in this area will complement the return air register at the top of the stairs.)

Our plenum must be functional, but discrete, since it will be centrally located and we don’t want it to look like what it is. If properly designed, the plenum also will help to fulfill one of the design objectives of the bulkhead: define the dining space below by incorporating a raised area in the bottom of the bulkhead that provides visual interest and a home for pendant lights. This raised area would be part of the bulkhead regardless of HVAC needs and could be executed very simply in order to meet the design objective, but the return air flow function of the plenum gives us the opportunity to address both form and function goals with a single, elegant solution.

Here’s the plan that Natalie developed to meet the need, based on a design concept from the owner of the GC (who’s also an architect) one morning during a site visit:

The first step in executing on these drawings was to extend the return air duct and add a 90-degree turn, so it opened on the bottom. The next step was to surround the duct opening to create what would be the top of the plenum. Here’s how it looks at this point:

However, something’s clearly amiss. The pendant light junction boxes aren’t distributed equally, as a result of the location of the vent:

It turns out that the guys were, um, looking at the plans upside down when they started to built this (in the top-down view on the right side of the drawing above, the short end on top (west side) was mistakenly considered to be on the bottom (east side)). The vent actually fits perfectly between the second and third junction boxes if they’re evently distributed.

The problem was easily corrected – here’s the correct configuration (evidenced by the hanging electrical wires), after this has been addressed and after drywall has been installed:

The bulkhead is becoming less intrusive and more an organic part of the floor after the drywall has been primed and the wood trim has been added at this stage:

Now for the functional design details. The plenum needs to allow 100 sqare inches of air flow to allow the return to perform properly, but the “floor” of the plenum (which will be the top of the raised area of the bulkhead) needs to be attached to the already installed plenum ceiling, and we don’t want the return air functionality to be evident from the dining peninsula, so we can’t have holes in this floor. The solution is to have air flow around the sides to enter the plenum. To attach the floor of the plenum to the ceiling and still permit this flow, crenelated trim is used:

Here’s a better view to show how air will flow around the sides and through the holes to enter the return air vent:

To hide all of this, the sides of the bulkhead are trimmed out to 1″ below the beadboard, and the trim is turned on the bottom to extend 6″ into the center.

As a result, you can no longer see the sides that have to be left open for air flow, and the whole unit now appears exactly as it should from a design perspective. The dining peninsula will be directly below this raised area, which now conveys the visual interest needed for this element.

In the photo below, cellulous sound insulation has been shoved into every nook and cranny of the bulkhead around and on top of the plenum and the ends of the bulkhead have been sealed up with trim. Installation of crown molding also has begun.

As noted in the kitchen design post, cabinetry extending 24″ out from the party wall will occupy the area under the right side of the bulkhead in this photo, which will come within 6″ of the raised area. The dining peninsula will extend to 6″ to the left of the raised area, and the remaining space will serve as the passageway between the good room and the kitchen area.

Here’s a parting shot of the completed bulkhead after the contentious crown molding has been installed:


4 responses to “Good & Plenum

  1. Scott

    June 3, 2012 at 10:53 am

    Now I know what a plenum is….an engineering marvel!

  2. Gerald Connell

    June 3, 2012 at 1:10 pm

    ple·num (pl”ÆnÃm, plenÆÃm), n., pl. ple·nums, ple·na (pl”ÆnÃ, plenÆÃ).

    3. a space, usually above a ceiling or below a floor, that can serve as a receiving chamber for air that has been heated or cooled to be distributed to inhabited areas.
    [Webster’s Unabridged Dictionary]

  3. Gina Ziegler

    June 3, 2012 at 10:13 pm

    Great photos Kev!… You can really see how it is all coming together!… Looks great and thanks for the good and plenum!



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