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Some Thoughts on UFADs
by Soheil Rastan
first posted 2004.02.09

Disclosure: the issues raised in this commentary do not, in any way, reflect PWGSC's stand or opinion on UFADs. These are but personal opinions of an individual to be shared with ASHRAE's Ottawa Chapter colleagues and generate some sort of dialogue.

At the outset, I would like to emphasize that the Healthy Building Technology unit at PWGSC is neither with nor against the use of UFADs in PWGSC buildings. There is no meta-analysis, which I am aware of, that did perform an empirical IAQ review on UFADs vis-à-vis non-UFAD systems. I have only few scattered reviews here and there.

For reference, by "IAQ" I mean indoor air quality with respect to total dust, particles and VOCs, not temperature and air mixing. The latter is more or less called IEQ (Indoor Environment Quality), though there are no standard definitions to these nomenclatures. In fact there is one more; called PAQ (perceived air quality), which makes reference to subjective perception of building occupants through sensory assessments. For example, providing means where occupants adjust and control their immediate microen(vironment greatly adds to the PAQ even if both IAQ and IEQ were poor. To limit the use of these jargons, I added them up as the 3Qs of indoor air. Indoor environmental control systems need to address the whole (i.e., QQQ) and not the part (i.e., IEQ). UFAD provides a good IEQ and addresses few PAQ issues since occupants associate with their own "private" diffusers. But the issue of IAQ may be of concern. To this end I have brainstormed my own brain! in an attempt to seek some answers.

The following "nine" points are but a reflection of my personal thoughts borrowed from reviews of scattered documents and provoked by my own "in-duct" sampling and air monitoring experiences at a few office buildings such as Alcan's headquarters in Montreal, ROM in Toronto, McGill University and several DND facilities in Ontario and Quebec that had standard ceiling-based HVAC systems.

  1. Using non-ducted air supply UFAD may raise some IAQ concerns. We did not yet solve the problem of air ducts and the practicality of cleaning them. However, ducted systems provide confined microenvironments to work with. These air ducts could at least be "somehow" reached, controlled, fished-through and vacuumed. Now, with a UFAD system we are introducing - "in the name of energy and sustainability" - a pool of floor plenums, the cleaning of which can only be performed by a number of highly trained and specialized Swiss-Webster mice!
  2. Plenum systems may be good leverage idea for return air. However, I have to think twice before acknowledging plenums as an effective system for supply air. This concern is for both indoor air quality as well as HVAC velocity-pressure control quality. Plenums (ceilings or floors), as we all know, are used not only for air but also for electrical and communication wirings, plumbing, auxiliary fans, evaporators, etc. The premise of return air passing by these exposed items and carrying whatever these wires and fans emit has been accepted (though reluctantly) by acknowledging the fact that such air will eventually pass through the filtration of the HVAC system (though with very limited removal efficiency) before it is delivered as a supply air. In such a case, the plenum air is at least being filtered after "being-in-contact" with all the plenum-based gears. If we decide to go with a UFAD system, we may need to add filters on each floor supply diffuser. This entails that we recalculate pressure drops in the design of UFAD fan capacities. The latter may well eliminate some of the fan-based energy savings that are being claimed by UFAD advocates. Regardless of savings or losses (in $), one needs to secure a post-plenum filtered air delivery to occupants, not a pre-plenum filtered air! Thus, the use of DOF (Diffuser Outlet Filtration: lately promoted by several manufacturers in response to chemical and biological threats to buildings-at-risk) needs to be added into the design of a UFAD system. In this case, I have no problem with ductless UFADs + DOFs provided we solve the cleaning problem in the plenums.
  3. Supplying air from floor plenums (or even from the easier-to-clean ducted UFADs; since there are UFAD systems with full and partial under floor supply ducts) does not eliminate the need for ceiling plenums or ceiling based return ducts. Claims on savings on space or ceiling height by using UFADs need to be revisited. Even with UFADs (inline with floor Displacement systems), a minimum desired ceiling height may be needed to leverage the velocity pressure and the thermal buoyant force needed to raise the stagnation spectrum above the breathing zone of building occupants. Regardless of construction savings claimed by reducing ceiling heights, the relative high ceilings that we currently maintain in office environments are one of the very few simple and positive design criteria towards improved QQQ. Let us preserve this one! At this juncture of thoughts, it may be of interest to mention that it was around the 15th century when people in UK began to realize that air inside buildings might have transmitted disease among people in crowded low-ceiling rooms of the time. This influenced King Charles I of England (1600 AD) to decree that no building should be built with a ceiling height of less than 10 ft (3 m) to increase indoor air volume and effectively control the quality of indoor air.
  4. Some cost saving in UFAD vis-à-vis non-UFAD systems are imported from savings on churn costs (i.e. the cost of reconfiguring the workspace). Such savings are mainly due to the reduced time in wire fishing and communication cables. However, with today's office-based wire-embedded workstations that only need a single access point to energize an entire row of workstations, a revisit of the factorial analysis performed in claiming such cost savings is needed.
  5. According to data from Berkeley University, if two floor panels (in a UFAD ductless system) are removed for service or repair, the amount of air delivered to floor outlets at respective zones are reduced by half. One only needs to observe the number of ceiling tiles left uncovered since eternity while walking between our own offices in Portage. This indicates that non-ducted supply air UFADs are very sensitive to O&M practices, especially when the plenum is the supply end. Leaving few ceiling tiles uncovered at the return end is not as critical. In addition, in ceiling plenums, maintenance personnel cannot leave their greasy tools and oil sprays inside, thanks to gravity! But in floor plenums, this may well happen. I am just beginning to imagine the complaints of occupants when WD-40 is being sprayed inside a UFAD for maintenance of an HVAC plumbing system. We need to bear in mind that these UFADs are under positive pressure that supplies air to occupants. Ceiling plenums in traditional HVAC systems, on the other hand, are under negative pressure, which remove air (hence any WD-40!) away from occupants.
  6. Ceiling level services are still required for sprinklers, toilet drainage etc; hence space needs to be allocated regardless of the system used. Unless, however, we start designing floors with zones at different heights! Now we are talking reverse-savings!
  7. According to the Center for Building Performance and Diagnostics at Carnegie Mellon University, inability to fully dehumidify and control relative humidity because of the higher supply air temperatures in UFADs; potential for condensation and moisture accumulation in the raised floor plenum systems; and the likelihood for dust and debris accumulation in the plenum are the three major IAQ-UFAD performance concerns listed.
  8. In some reviews the issue of IAQ advantages of UFAD over non-UFAD systems were based on uniform temperature and air distribution. These determinants were leveraged as IAQ advantages of the UFADs (c.f. IAQ vis-a-vis IEQ). Moreover, in some UFAD projects, personal environmental control systems (i.e., workstations with integral fan-unit, air delivery and filtration system) were connected to the UFAD system used and the advantage of the perceived air quality (PAQ) was "indirectly" credited to the UFAD instead of it being credited to the personal environmental control system used. If the same personal environmental control system was connected to a standard ceiling duct HVAC system, one would have received similar PAQ responses. Thus, the benefits of UFADs over non-UFADs are convoluted. In-depth reading of each article is needed including footnotes and prevailing assumptions.
  9. We have yet to answer the original question (let us even assume that the ~20% savings are all attributed to the UFAD): What type of HVAC system is easier and simpler to a) clean; b) clean; and c) clean, regardless of the savings in energy costs, let alone remodeling costs? Energy cost is a small portion (~1/250th) of the average yearly personnel salaries per area occupied.

As per our mandate as building practitioners, the aim is neither to criticize any innovation nor to simply conclude investigation by a mere critical review. The aim is to attempt to provide innovative, adaptive and smart solutions to promising ideas that may fall short in some corners.

One answer may be to implement "ducted" UFADs instead of ductless UFAD and use Diffuser Outlet Filtration (DOF) on each outlet. The latter may entail some adjustment to the pressure at respective zones. Such ducted UFAD+DOF system not only supports the "assumed" and claimed advantages of UFADs (easier access to plenum gears and systems, temperature uniformity and air mixing) but also: (a) eliminates the plenum-housekeeping dilemma by using a fully ducted version, hence providing accessibility to standard duct cleaning services; and (b) eliminates plenum supply depressurization problems, and (c) facilitates the use of healthier, long lasting and easier to clean floor materials such as ceramic tiles instead of the fleecy dirty-by-use carpets (i.e., as sinks); since floor-to-floor sound transmission issues may be easier to control by the raised floor system of the UFAD.

In Summary:

  • Standard UFADs as currently described, bring in contaminated air (i.e., from the gears in the hard-to-reach and difficult-to-clean floor plenums) to a naked diffuser at the floor. This post-plenum-unfiltered-air can cause the wannabe settling particles to become airborne towards the breathing zone of building occupants. This unfiltered and partially polluted air first hits the breathing zone and then moves on to the return grille. This is exactly the opposite of what piston effect PBZ (Personal Breathing Zone) is all about.
  • Supply air from traditional ceiling air ducts may also be contaminated but these ducts are "at least": 1) easier-to-reach and more possible-to-clean if compared with a ductless UFAD; and 2) does not disturb the already settled particles while delivering air.
  • As per plenums: I am not yet aware of a contractor that is offering, "air plenum cleaning services; floor-based or ceiling based". I could only imagine the mess this activity generates!
  • Plenums are to be kept for return air and ducts for supply air, regardless of the system used. In fact I would rather prefer an HVAC system, UFAD or not, that delivers both its supply and return air through "round" and smooth ducts for ease of cleaning.
  • Energy claims using UFADs need to be revisited, especially now that we are in the age of sustainability (by sustainability I mean "holistic sustainability" that includes the health and well being of building occupants).
  • Only a ducted UFAD with DOFs and ceramic tiles may qualify to be a green and smart design indeed. In other words, if we are to change HVAC architecture, then we might well change it all the way, not half way!
  • As per energy savings: Energy cost is ~1/250th of salaries per floor area per year. If we save some 20% (0.0008% of personnel salary) on energy by a UFAD, while disturbing, hovering and displacing particles to the PBZ of occupants and acting against the "wannabe settling particles", this is a practice that may need an in-depth justification.
  • Contaminants should move away from the breathing zone not into it! Ceiling-based ventilation brings particles down from ducts to PBZ, but the direction is: 1) top-down pushing way from the PBZ (in principal); and 2) gravity is working with displacement not against it. Displacement ventilation is a term borrowed from the industrial ventilation practice. The concept had to do with keeping the personal berating zone of workers at positive pressure (by fresh air) compared to the surrounding area so that no particle or gas finds its way into personal clouds. Now this term is used in office ventilation as reference to some UFADs. I do not see any positive displacement concept in the latter. Thus energy savings vis-à-vis healthy indoors are once again head-to-head. Personally (I can be wrong) I am not a matchmaker of these two together. They proved to be, once and again, incompatible. After all, why jeopardize the elephant ($1250) for an ant ($1)!
  • Ductless UFADs are potentially great dirt reservoirs. One only needs to check under a living room sofa and see the mess! However, "ducted" UFADs are no doubt easier to clean and maintain than those hidden jungles called ceiling ducts, although one needs to fish along the ducts (floor based or ceiling based) since access is limited to few spots only. The ease of cleaning is valid but the cost and tools involved may be the same if compared with ceiling based ducts. Whether the duct cleaner stretches up or bends down is semantic. Believe me, duct-cleaning contractors may charge you the same, unless you convince them otherwise!

I have attached an abstract of a recent article that reviews some basic parameters on this issue (see below).

Indoor Air
Volume 13 Issue 2 Page 200 - June 2003
Air flow and particle control with different ventilation systems in a classroom S. Holmberg, Q. Chen

Most ventilation and air conditioning systems are designed without much concern about how settling particles behave in ventilation air flows. For displacement ventilation systems, designers normally assume that all pollutants follow the buoyant air flow into an upper zone, where they are evacuated. This is, however, not always true. Previous studies show that high concentrations of settling respirable particles can be found in the breathing zone, and that the exposure rates can be a health hazard to occupa(nts. The emphasis here is on how ventilation systems should be designed to minimize respirable airborne particles in the breathing zone. The supply and exhaust conditions of the ventilation air flow are shown to play an important role in the control of air quality. Computer simulation programs of computational fluid dynamics (CFD) type are used. Particle concentrations, thermal conditions and modified ventilation system solutions are reported.

Practical Implications
Gravitational settling of airborne particles influences the design of ventilation systems. An important question is whether one should design ventilation systems so that particles settle to the floor (or other surfaces) and then clean the floor carefully, or whether one should design the ventilation systems to keep contaminants airborne and then evacuate them by the exhaust air. Probably a combination of these two techniques is the best choice. For this purpose we need better ventilation design guidelines.( This paper compares concentrations of 10 um (aerodynamic diameter) particles in a classroom with different ventilation methods.

- end of article -

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