Is there such a thing as too much ceiling plenum space?

Question:

We have all seen the job sites where there’s everything you can think of crammed into the ceiling plenum above a lay-in ceiling. In those cases you can hardly lift a tile out, not to mention access or work on anything. Obviously this is not ideal. Everyone would love to have another few inches, or even a few feet, of ceiling plenum space on most jobs, except the group paying for construction. It’s something that should always be reviewed during design and construction. This article isn’t focusing on that problem, but what about the other side of the coin? Have you given much thought to those more unusual spaces you run into once in a while where you might have a normal 8-10 foot ceiling height, but the floor structure above is up another 4 feet, or even more , above the top of the acoustical ceiling grid?

Solution:

Lots of plenum space sounds like heaven, right? Not so fast! Think about the specifications or general notes on the plans. Do they have some standard language such as “all ductwork shall be mounted as high as possible…”. I know I’ve seen similar language frequently. In most cases it’s a good idea. Even if they don’t have that note, most HVAC installers will install the ducts and equipment as high as possible because that’s what they do almost all the time.

So let’s put our owner’s maintenance technician hat on for a minute. You have a building like this and you get a BAS alarm or a complaint about an overly cool room. The room has a VAV box that’s not getting heat out of the reheat coil and you want to check the strainer, control valve, and actuator. A rather simple task, right? But you go take a look and find the VAV box and valves are 6 feet above the top of the grid. You are in a small room where there’s barely enough room to get a 8 foot step ladder set up with a 9 foot grid. How do you service that VAV box?

While they do make small lifts that will fit up through a 2’x4’ ceiling grid, not every owner has one in every building. In some cases, you can wiggle a taller step ladder up through the grid and make it work, which might require removing some ceiling grid sections, but do you have enough floor space to accommodate a larger A-frame step ladder? This may have turned a 1 hour task into a full day job. In some cases, there just isn’t a safe and practical way to service that equipment without moving furnishings and partially dismantling a ceiling. Not a practical solution.

The solution is to prevent the problem by mounting the VAV box, valve package, and other equipment requiring service within a couple feet of the top of the grid. That way service staff can use a ladder, or a lift, that is just below the ceiling grid level and still safely access the equipment without doing a balancing act on the top rung of a ladder which would make OHSA inspectors scream. So, keep that in mind as you look over the next set of plans and specs or visit that next jobsite with ceiling plenum space to spare. Add some special notes, or better yet, a section or elevation view of the ductwork in those unusually high ceiling areas to clearly convey where you want the ductwork and equipment hung in that big cavity. Sometimes there is too much of a good thing if you don’t plan to use it wisely.