We get this question often when comparing the cost of a simple diffuser with integral backpan versus a diffuser face with a separate plenum. Why have a plenum? Why not just an integrated backpan? It would be cheaper, more compact and already assembled.
Plenum boxes have multiple features which are much more beneficial in the long term than the initial savings of other options.
1. Reliable Performance
The main function of the plenum box is equalizing the air pressure in order to even the air distribution.
Many air distribution problems are the result of the air entering the diffuser through a flexible duct with a kink or an angle. With a standard 3 cone diffuser or louvered diffuser, a lot more air comes out of one side of the diffuser when this happens.
The result is uneven air distribution in the room, due to some diffusers supplying much higher air velocities with longer projections in random directions. People experiencing these high velocity air drafts are likely to feel serious discomfort. Other areas have a lack of air flow, and it is in these areas where people usually ask to turn up the heating or air conditioning.
This starts the unending chain of occupants’ complaints, costly maintenance and energy waste, not to mention the higher sound levels caused by variations in velocity pressure. In such cases, the theoretical NC can be increased by as much as 15 db.
The use of a pressurized plenum box, when combined with a high induction diffuser face like AXO, NEX, OTO or PLAY, solves these problems. A 360 degree even air diffusion can be achieved independently of the direction of air coming into the plenum. These diffusers with plenums ensure that the system will perform with catalog projections and NC.
2. Limited Space in the Ceiling
In response to the issues raised in the first point, one may answer that this problem is caused by poor installation and that it could and should be prevented by proper installation. Unfortunately — or fortunately — this is rarely the case. Many buildings don’t have enough space in the ceiling to respect the ASHRAE Standard 70’s requirement of having a length of three diameters of straight duct entering the diffuser.
PERFAIR plenums for square, round and rectangular diffusers are offered with either a top or side duct connection. Only 14” of ceiling space is needed for a proper installation when using a plenum with side connection.
As stated above, you still get a perfectly reliable performance from the diffuser.
For ceilings with very limited space, a new product is available: the PERFAIR-XS Compact Plenum With Side Connection. PERFAIR-XS optimizes air diffusion in ceilings with at least a 6″ clearance. When combined with AXO, NEX, OTO or PLAY, PERFAIR-XS will greatly reduce the negative effects of limited ceiling space: pressure loss, increased energy use and uneven air diffusion.
3. Noise Absorption
Last but not least, acoustical liner can be added to plenum boxes in order to help attenuate the sound coming from the duct and equipment. Although this is commonly used with linear diffusers, standard diffusers with integral backpan can’t provide that option. Yet these diffusers are often installed right above employee workspaces in offices and other commercial spaces. Wouldn’t it make sense to add acoustical insulation in these diffusers as well?
Not only does the plenum prevent extra diffuser-generated noise up to 15db, it can also be insulated in order to attenuate equipment generated noise.
Improved energy efficiency, reduced noise and greater comfort despite installation limitations are the reasons we use plenum boxes with our high performance diffusers. We also believe that the additional investment is wise when considering the benefits and the resulting short and long term savings on operational costs.