From Hot & Cold, to Comfortable: Making Under Floor Air Distribution A Cool Choice in Hot Climates

Under-floor air distribution (UFAD) has been unmasked as primarily a mild climatic system.¹ When integrated in structures located in a hot and/or wet climatic such as those found in Phoenix, AZ, UFAD gives way to increased costs and design issues, particularly in multi-level structures.

The first UFAD system was designed and built in Phoenix in early 2000. Fortunately for the facility owner, it was used in a single-story structure with no windows, other than a few at the lobby entrance. This straightforward building plan made it possible to install a simple system featuring a constant-volume approach, with spinner-type diffusers and an air-handling unit as the sole method of zone control. Due to the essentially windowless plan minimizing exposure to Phoenix's extreme conditions, this system performed well and achieved comfortable heating and cooling environment throughout the completed facility.

In the years that followed, UFAD systems made their way into other more complex structures, including those featuring numerous windows and multiple levels with increased exposure to Arizona’s extreme temperature shifts.

The challenges associated with windowed, multi-story UFAD projects in Arizona are similar to those a Kansas-based professional engineer (PE) faces when trying to get reciprocity in California, New York or Florida: additional education and on-the-job experience are required to do the job successfully. But, with increased knowledge and experience, success is possible. This article is intended to assist architecture and engineering firms, as well as owners, in increasing their success and reducing the costs of their UFAD systems in facilities throughout the Southwest, including Arizona, New Mexico and Texas.

All Things Not Considered
When introduced to the market in early 2000, raised-floor vendors and manufacturers promoted UFAD systems for their flexibility, underscoring the sustainability of these systems as a benefit to the LEED certification process. The hot-button association with LEED caught the immediate attention of architecture and engineering firms, which wasted no time in integrating UFAD systems into multi-level projects in Arizona.

Unfortunately, few firms considered how UFAD's performance might be affected by multi-level structures in Phoenix's wet/hot climatic. This ultimately resulted in unhappy facility owners who, along with their staff, complained about inconsistent, uncomfortable temperature zones across office environments during the work day.

According to the ASHRAE UFAD design Guide³ , which was first published in 2005, UFAD was developed and tested for mild climatic conditions found in Berkeley and San Diego, CA. Not surprisingly, Phoenix exceeds most of indicated values found in the UFAD Design Guide. Therefore, the inconsistent, uncomfortable temperature zones causing complaints were the results of ineffective UFAD system designs and minor construction issues in light of the desert location.

Consider the following facts:

• Phoenix has the most days of sunshine in the U.S. (86% per year) and the highest outside design conditions in the U.S., with 110F daytime temperature, and a record high of 122F.
• Phoenix has the widest daily average temperature range: 27F. The city’s hottest months – July and August – are also the most humid.
• Phoenix has one of the nation’s highest Mean Daly solar Radiation (Lang Leys), thus creating one of the hottest developed plumes in a multiple-story structure.

To remedy such issues in Arizona projects, architecture and engineering firms must take a modified approach to UFAD system design and construction. The ASHRAE GREENGUIDE - The Design, Construction and Operation of Sustainable Buildings², page 56, in the paragraph SITE ORIENTATION, sub-paragraph two, states: "Buildings that minimize east and west exposures, especially where a lot of glass is used, are generally more energy-efficient because of the huge solar heat gains associated with east-and-west facing elevations during the cooling months."

This is true not only in Phoenix, but in the Southwest in general, because sunlit exposures pose significant challenges to UFAD systems, especially during the summer and winter months in buildings with east/west exposures.

Sundt Construction has been involved in 706,000 sq.ft. of UFAD across six projects in the Phoenix area. In three of these completed projects (totaling 231,000 sq.ft.), our team has been able to achieve consistently comfortable cooling and heating throughout the facilities. Using the knowledge acquired from these projects, Sundt is currently designing and constructing another 125,000 sq.ft. of UFAD systems across two projects. Our experience has given us the ability to support our local engineers, drawing on the field experience obtained in producing three properly functioning systems (and a year of research and lessons learned from corrections made to designs).

Accordingly, for the benefit of other architects, designs and owners, here is a closer look at design issues, complaints and solutions Sundt gleaned from previous projects:

• Cold interior spaces caused by too much CFM per sq.ft. at too cold a temperature (60F or 61 F) instead of the recommended UFAD temperature (63F or 64 F). Consequently, unit heaters must be furnished for the interior zones.
• Exterior-zone cooling loads that have not been adequately calculated force operators to lower the supply air temperature from the recommended 63F to as low as 60F. This attempt to achieve comfortable exterior zones then results in uncomfortable interior zones.
• Uneven temperatures during the working day caused by control systems without the capacity to automatically regulate temperatures per building conditions and Arizona weather. An OSA schedule is required to overcome this problem.
• Cold building environments in the winter mornings and warm mornings or afternoons in the summer. This requires proper monitoring, controlling and fine-tuning of the morning warm-up, cool-down and set-back temperatures.

The UFAD in Arizona requires a complex control system that can result in design issues, some causes of which include:

• Architects failing to specify thermal barriers at the exterior walls (skin interior) to stop the heat plume from traveling vertically from floor to floor, thereby adversely affecting the exterior cooling-zone capacity.
• Architects and engineers failing to read, understand and abide by the methodology outlined in the UFAD design guide³; and not adjusting the design and build processes in light of the fact the UFAD system is intended for mild climatic, consequently demanding modification for implementation in the extreme wet/hot climatic of Arizona and the Southwest at large.
• Teams failing to establish load-calculations procedures after determining the basic block-heating and cooling loads. Remaining design procedures must be modified to provide accurate UFAD design conditions, namely:
  • keeping the farthest zone less than 60 feet from the supply shaft (50 feet is better);
  • calculating the thermal decay (degrees added to the shaft SA temperature);
  • avoiding diffusion of the supply air from the supply ducts to maintain desired thermal decay using velocity;
  • providing an interior diffuser with at least a 75% shut-off capability to help control interior zone temperature;
  • sizing the interior zones at 0.6 CFM(ground floor) and 0.8 CFM (elevated floors) per square feet;
  • providing plenum operating pressure of 0.05" to 0.08"wg, so that the constant-velocity-round diffusers can function properly;
  • calculating the exterior zone's correct amount of CFM to handle sun-lit skin in Arizona;
  • and installing air handlers with the maximum capacity to provide a minimum of 1.5 CFM per square feet of UFAD area.

Estimating load calculations for a UFAD system adds a modified three-step process beyond the standard heating and cooling block load. This is touched upon briefly in the ASHRAE JOURNAL Issue, Fred Bauman, P.E. "Cooling airflow design calculations for UFAD – October 2007.¹"

In summary, Sundt’s project portfolio includes past work on six projects, four of which are completed and two of which are under construction. This portfolio includes the first two-story UFAD structure in Arizona and current work on two additional multi-level projects (one three-story structure and one 10-story facility). Such experience gives Sundt the edge and allows the team to deliver effective design/build of UFAD systems customized for Arizona’s hot/cold climatic – creating projects that offer consistent, adequate cooling and heating environments for owners and employees immediately upon occupancy.

About the Author: Thomas R. Garrison is an employee/owner of Sundt Construction, Phoenix, AZ. Sundt is a 120 year-old full service construction company.

References:

1 – 'Cooling Airflow Design Calculations for UFAD', Fred Bauman, P.E. and Tom Webster, P.E., ASHRAE Journal, October 2007, pages 36-44.

2 – ASHRAE Green Guide – 'The Design, Construction and Operation of Sustainable buildings'

3 – Fred Bauman - 2003, "Underfloor Air Distribution (UFAD) Design Guide". Altanta: ASHRAE.

4 – ANSI/ASHRAE/IESNA Standard 90.1 – 2004, 'Energy Standard for Buildings Except Low-Rise Resident Buildings'.