The combination of a Raised Access Floor (RAF) and a well-designed Under Floor Air Distribution (UFAD) system, is one of the best environmental control systems for many types of commercial buildings. These are becoming more popular around the world and particularly so in India. It is a surprising fact that these systems were in use 30 years ago in Europe. For those unfamiliar with UFAD, many times, the use of UFAD is associated with computer rooms and accompanying once. Today that is not the case at all. Systems from a variety of major manufacturers are popularly used in all sorts of commercial buildings including: office buildings (particularly corporate campuses), university buildings, churches, casinos, elementary and high schools, libraries and government buildings.

These systems are in their 4th generation of improvement. Modern products are completely reliable, low cost and easy to apply. ASHRAE completed a widely read “UFAD Design Guide” in 2013. There have been thousands of projects installed in every corner of the planet. It is from this body of work that the author represents that many of the core requirements to implement this technology are really settled science, really no longer debatable. The cost relationships have been demonstrated and the user benefits documented.

New technology?          

New technologies: VRF, variable speed pumping, ECM motors, etc., have wide adoption and growing market share but are taking many years to reach their full potential. This often occurs because popular forces oppose change. Even when the new technology is adopted, some design influences want to resist the change and apply old habits and methods to implement the new systems.

If you were accustomed to building bridges with wood timbers, and then galvanised steel came along, would you use the same design with the new material? Certainly not. Something like that has been happening with the UFAD business, particularly in North America and other places, and to a lesser degree in India. Several old habits from overhead VAV systems are being used in UFAD designs to the significant detriment to system effectiveness and first cost. These include:

Using large ductwork under the raised Floor. This mistake has an interesting consequence. The cost of that large ductwork (instead of multiple small ducts that ft between the pedestals) saves the mechanical contractor Rs 210 per metre of duct. But the wider duct adds the need for “bridging” across that duct to support the raised floor every 600 mm. These bridges cost about Rs 2,700 for every 600 mm of duct length. So, the mechanical contractor saves Rs 210 per metre, but the raised floor contractor has an increase of Rs 4,500 per metre of duct. Since these two trades (access floor and sheet metal) are part of different specifications, different contractors and different designers; this mistake still happens today. The raised floor supplier won’t complain because his contract is larger, and he has little to no interest in ductwork! See figure 1.

The use of fan powered boxes in cooling only perimeter zones. Sometimes designers say they need fans because they need “a lot of capacity” in a certain location. Huge capacity is easy to accomplish without a fan. 840 CMH per running metre of perimeter window is easily accomplished with plenum pressure of only 12.5 pa. Using fan powered products has a higher first cost, requires high voltage power (installed by an electrician) and branch circuit overvoltage protection. See figure 2. Fans make noise, they vibrate, require maintenance and (according to the ASHRAE guide) use about 5 per cent more energy.

The use of deck to deck walls. This mistake is mostly made by acoustical consultants who may have no connection whatsoever with the General Contractor who is responsible for the RAF budget. When a few of these consultants are asked why they think the wall below the floor contributes to the better acoustics of the room, they seldom have an answer! It is a seriously costly mistake. It is advised that all walls except demising walls, core walls, wet walls and perimeter walls should be built on top of the raised floor. An independent third-party testing of such situation shows that the deck to deck wall, (particularly the part that is under the floor) do not add to acoustic isolation until you are building a wall above STC 51. That seldom happens in an ordinary project. The negative effect to the job of the wall protruding into the floor plenum is to dramatically increase the cost of delivering services to the isolated room. Voice, data, power, air all must penetrate two layers of sheet rock and require lots of extra sealing. Flexibility for adding services or reconfiguring walls is hindered for the life of the building. The other consequence is that the raised floor cost and construction time will dramatically rise. 25-50 per cent is probable. Much more field measuring and cutting are required. Once again, the access floor contractor gets a bigger contract, as do others. The only loser is the building owner trying to bring his project in under budget.

Figure 1
Figure 2

One of the worst mistakes is to install a perfectly good raised floor plenum and then not put the air under the floor. This one will be the focus of the balance of this article.

The author has been on several design and construction teams where the installed cost of the RAF is justified by reductions in other trade costs, particularly the reduction in ductwork cost. Then the owner receives all the benefits of a raised floor and UFAD for the price of a conventional system. The facility will feature better indoor air quality, lower operation cost, better worker health and productivity and greater flexibility to make changes through the life of the building. The RAF is the largest incremental cost to deploy an UFAD system. If the project already has a RAF in the design, it makes very little sense to pay a higher first cost for an overhead air distribution system.

Of course, there are many kinds of buildings and widely varying build quality that will change these estimates, but both columns will change similarly. The relationship will remain. Once the raised floor is considered as a part of the plan, the use of UFAD will save first cost and provide a significantly better HVAC system.

Additionally, a logical argument can be made for designers to consider the financial cost of the workers or occupants in the spaces they are designing. The LEED guidelines, and now the Well Building Standards, give good empirical evidence on the effect of the thermal environment on productivity. This value is many times greater than any first cost saving or energy cost expense related to the HVAC system itself.

In summary, it is recommended to consider using Underfloor Air UFAD as a way to reduce your first cost and improve the work environment for your clients.



Michael J. McQueeny
Jr., P.E., LEED AP,
Managing Partner and Chief Engineer,
Airfixture, LLC, Kansas, USA