We can address a few misconceptions about UFAD systems that have been unnecessarily persistent.
There is a general perception that since air is supplied at lower level near the floor, it leads to cold feet. This is not true, as UFAD air is supplied at 65 degrees Farenheit (18.3 degrees Celsius), which is not cold. However, if the conventional system air is supplied at 55 degrees Farenheit (12.8 degrees Celsius) at a lower level near the floor, it is cold and causes discomfort.
Calculations are the same as for conventional systems.
Heating and cooling load calculation methodology is different for UFAD and overhead systems. Stratification of heat above the occupied zone must be taken into account. Except for a few, most cooling load calculations software packages do not have a methodology to capture the effect of stratification, higher supply air temperature, higher return air temperature, splitting of air stream for optimum dehumidification, and mixing of air streams for higher temperature. The engineers have to factor all these variables accurately to arrive at the right system performance.
UFAD systems require more air.
The total air required for both UFAD and conventional systems is the same. The UFAD systems do not require larger duct shafts and fans.
Mechanical systems are the same for both.
UFAD systems require different fan systems and configuration that permit the air to bypass the coil, but that filter it and mix it with colder dehumidified air to produce 65 degrees Farenheit (18.3 degrees Celsius) air. This configuration of the fans is not common and is not a standard offering by most manufacturers of HVAC systems. The units have to be custom or semi-custom designed.
Large ductwork is required for UFAD.
Duct sizes are the same for both conventional and UFAD systems. Some early installations which were not designed correctly had high air leakage. To compensate the air leakage, additional fans and ducts were retrofitted. A correctly designed UFAD system does not require larger duct or fan.
Higher floor-to-floor height is required for UFAD.
There is a perception that the height of the underfloor air plenum adds additional height between slab to slab. This is not true. Additional floor-to-floor height is not required, and building height does not increase. The ceiling plenum height is smaller by the floor plenum height, making the overall slab-to-slab height the same for both UFAD and conventional overhead systems. In concrete structure or flat slab construction the floor-to-floor height can even be less than that for conventional systems.
When analyzed with accurate life-cycle costing, UFAD systems can often prove to be very cost-effective.
Generally the UFAD systems cost slightly more in first or capital costs than conventional overhead systems. One such study, done for the U.S. General Services Administration (GSA), shows that the increase in construction cost can vary from $2 to $6 per square foot ($22 to $66 per square meter), depending on the type of UFAD system and the size of the floor plate.
Another study shows that $1 of additional costs saves $5 over the building life cycle. A case study by Carnegie Mellon University shows that the additional first costs of a UFAD system (including raised access floor) was $0.27 per square foot ($3 per square meter). The savings at the first churn with the UFAD system was $4.66 per square foot ($51 per square meter).
Whether your bottom line is energy savings, operational adaptiveness, health and comfort, or cash ROI, there's a chance that in your next commercial building project, UFAD could be very cool, indeed.
Asif Syed, PE, LEED-AP, is an engineer and is currently a partner at AKF Engineers, where he is responsible for design and analysis of building mechanical systems.