Ewen Rose reports on
Underfloor Air Conditioning in North America
Hyping down claims for underfloor ac
Underfloor air conditioning still has a lot to prove, according to a leading consulting engineer. Ewen Rose reports on the difference between hype and reality.
Reality must be separated from the "hype" for underfloor air distribution systems, according to David Arnold of Troup, Bywaters + Anders, although he believes that significant benefits can be gained from the adoption of this approach in office buildings.
He told the CIBSE/ASHRAE Group that, although it has been around since the mid-60s, it was not until the 80s that underfloor cooling systems took off in Europe with the increasing use of raised access flooring in office developments.
"The technique evolved from computer suite air conditioning and is now applied in almost as many variations as traditional overhead or ceiling systems," said Mr Arnold who is a former president of CIBSE and currently a director of ASHRAE.
"The growth in popularity in the US lagged behind Europe, as raised access floors were less common there. However, this has changed and between 1995 and 2002 the number of new office buildings with raised floors that use underfloor air distribution has increased by 40% albeit that it still represents a small proportion of installed air conditioning systems."
Well-designed, installed, tested and commissioned systems can provide significant advantages over conventional ceiling systems for many applications. However, experience has shown it is not simply a case of applying the same design techniques, but with air outlets in the floor - the installation, testing and commissioning needs to be carefully planned and executed to make this approach work, according to Mr Arnold.
"To get the full benefit you need very good co-ordination between all the building trades throughout the design and construction process," he said. "The implications of using a raised access floor must be considered right from the outset. If the equipment needs to be maintained, then ease of access is key and if it is a pressurised system then the underfloor plenum must be properly sealed to prevent uncontrolled air leakage. If the plenum leaks the system will not work."
Division of responsibility with these systems is another potentially difficult area because it is linked to both the fabric of the building and the services, so testing and commissioning must be a combined effort, he explained. There is a real danger of disputes between the architect, builder and m&e teams over this issue. Managing the sequence of an installation is critical and often creates tension.
"Who gets in first?" said Mr Arnold. "The trouble is that the builder has overall responsibility for the structure and does he really know enough about what is needed to make sure an underfloor system works?
"Testing plenums once they are installed is easy, but too late - the building leakage rate needs to be addressed before the raised access floor is laid otherwise any underfloor system may only produce a small percentage of its capacity," said Mr Arnold.
Temperature pick-up in underfloor voids can also be a problem, he added, with the rule of thumb being a rise of 1 or 2degC per 10m between the air handling units/fan coils and the plenum. "So, the shorter the distance the better."
As a result of these extra difficulties, clients need to be convinced that they will reap the full benefits of these systems, according to Mr Arnold.
"The problem is that we never build the same building twice, so it is very hard to gain accurate comparisons between rival systems in operation. In theory, there could be energy savings, but there is no evidence of this and if there are plenum losses this will damage the energy performance of an underfloor system."
He also doubted the basis for claims about improved occupant comfort, health and productivity or for reduced lifecycle building costs due to lack of evidence.
"The biggest claim made for this approach - that it reduces the floor to floor height of new builds - could sell it on its own, however," said Mr Arnold. "I don't believe that it delivers greater flexibility in use because conventional air conditioning tends to be laid out on a modular basis with a fan coil every three meters so there is no need to move the ductwork if the use of an office floor changes."
More testing of underfloor air distribution systems in operation in large floor areas was required before the claims made by manufacturers of such systems could be backed up, Mr Arnold stated.CIBSE/ASHRAE Group chairman Tim Dwyer added this showed that even though modern air conditioning had been around for a century (and the Romans were doing it even earlier) "we are still arguing about the best way of getting cool air into an occupied space"
To download a PDF file of the presentation click here