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Figure 2 - Effects of PositivePressurization

An important consideration in achievingpositive pressurization is that interiorbuilding partitions not adversely affect thedistribution of air.

The way buildings operate today isas a multitude of pressure vessels:interstitial spaces in an exterior wall;plenums; dropped ceilings; rooms thattypically have doors closed betweenthem. These different pressurerelationshipssome of which are very

well connected to the supply air, some

that are notinclude pressure-starvedareas and some pressure-excessiveareas.

Ventilation

Most building codes establishminimum ventilation requirementsin relation to occupancy or spacefunction. These requirements areusually based on ASHRAE Standard62-1989, Ventilation for AcceptableIndoor Air Quality. This standardspecifies the minimum acceptable

outdoor air requirements for occupiedspaces.

Providing conditioned outside air notonly helps pressurize a building butalso dilutes chemicals or particulatepollutants generated in the space.Outside air can also be induced inthe space by the HVAC system asventilation air. If the HVAC systemintroduces air into the space, thesystem must continuously dehumidifythe air. Of course, adequate

dehumidification should not besacrificed for adequate ventilation.

If the air is not continuously andadequately dehumidified, the moistureadded to the space might be greaterthan the HVAC systems ability toremove it. This moisture sourcenormally results in moisture-relatedmildew problems on the interiorsurfaces of the building (that is,interior finishes and the surface of

furnishings).Airflow Dehumidification

To provide proper dehumidification,an HVAC system must:

Fully dehumidify the air that flowsacross the cooling coil.

Provide sufficient run time toremove moisture from the interiorair despite the satisfaction ofinterior temperatures.

To fully dehumidify the airflow

across the coil, cooling coils must besized properly to meet the sensibleload (load associated with dry-bulb temperature) and latent load(moisture in air associated with wet-bulb temperature). This includes thecombination of both outside air andreturn air. This air must be broughtto a temperature that causes themoisture in the air to condense forlatent heat (or latent energy) removal.Simultaneously, the cooling coil isreducing the sensible temperature ofthe air to offset the sensible energygenerated in the space (lights, solar,people, equipment, etc.). A commonrange of temperature for the coolingof this air is between 50F and 55F.

At this temperature, most HVACsystem airflows will be at 100%relative humidity (RH) and willeffectively condense moisture fromthe air. Air provided to a space underthese conditions has the best chanceof maintaining interior conditions of

75F dry-bulb (Fdb) and 60% RH.

Dehumidification Run Time

If the system cannot provide sufficientdehumidification while it reacts totemperature control alone, it mustcontinue moisture removal withoutaffecting interior temperatures andoccupant comfort. This can beaccomplished by reheatinga form ofsimultaneously cooling and heating to

continue dehumidification while notovercooling the occupants.

Methods of reheating include director indirect gas-fired heating; hot-

water heating; hot-gas reheating forrefrigeration-based units; and for partsof the country that allow itelectric.

Devices added to the equipment, suchas wraparound coils, can also providea means of reheating. Wraparoundcoils simply transfer energy from theincoming cooling coil air stream tothe exiting cooling coil air stream.These coils are available in a passiverefrigeration-based unit or as a water-based system that uses pumps to movethe water through the system.

In conventional HVAC systems, twodehumidification methods are used.The first is a cooling-based systemcooling air below its dewpoint.Moisture condenses on the coolingsurface and is removed from the air.For example, a cooling-based systemcan cool an outside air stream from95Fdb (55% RH) to 77Fdb. At77Fdb the air is at 100 percentRH. If it is cooled below 77Fdb to55Fdb, 68 grains of moisture perpound of dry air are condensed out ofthe air and onto the cooling coil.

The second method involves the useof a desiccant that attracts moisture toits surface by introducing a low vaporpressure at the desiccant surface. Thevapor pressure of the moisture in the

air is higher, so moisture travels fromContinued on Page 14

Interaction Between the Building Envelope and the HVAC Systems

A/E Interaction Positive Building Pressurization Negative Building PressurizationCorrect Wall Mildew/moisture problems unlikely in wall systems Possible mildew/moisture problems in wall systemConstruction* and occupied space and occupied space

Incorrect Wall Possible mildew/moisture unlikely problems in Probable mildew/moisture problems in wall systemConstruction problems in wall system; occupied space and occupied space

* Wall construction refers to vapor retarders and air and rainwater barriers. Building envelope and HVAC system design must interact to reduce the

potential of moisture and mildew formation.

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Avoiding Moisture and Mold Problems in

Florida Buildings Continued

the air to the desiccant. The desiccantthen must be recharged through aheating process, allowing the moistureto be driven from the desiccant anddischarged to another location besidesthe cooling air stream.

One of the best strategies is a

combination of desiccant and coolingsystems, particularly for 100%outside air streams such as makeupair systems. Since air exits a cooling-based system at saturation, it onlymoves to a lower RH once it mixes

with the room air and heat is added toit. The desiccant, on the other hand,enters the space with very low RH,

and its RH increases to the rooms RHlevel once the two air streams reachequilibrium.

Behind the Book

In assembling his book, CommissioningBuildings in Hot, Humid Climates:Design and Construction Guidelines, co-author J. David Odom and LBDGhoped to literally fill in key gaps

missing in many reference manuals.It is not everything you need toknow about designing buildings inhot, humid climates, but a pathwayto crystallizing key issues that arenot picked up by other manuals like[the American Society of Heating,Refrigerating and Air-Conditioning

Engineers] ASHRAE Handbook:Fundamentals. Its second aspect,likened by Odom to a Windows-likecomputer operating system, is tofacilitate access and direct readers toother documents. The biggest gap thefirm feels it has identified is lack ofdetail in prescriptive vs. performancelanguage. ASHRAE Fundamentals, forexample, talks in terms of performancelanguage he says. It doesnt say youmust have constant pressurization, itsays infiltration must be minimized.

What does that mean? says Odom.To an architect, it means take out acaulk gun and fill up holes. Usingperformance language may work finein a more forgiving environment, butthere are some things that you need toget into very prescriptive languageDo this, dont do that.

About the Author: J. David Odom has been

a moisture and mold forensics specialist withCH2M HILL since 1978 during which timehe has managed some of the largest andmost complex moisture and mold problemsin the country including the recent $60Mconstruction defect claim at the HiltonHawaiian Village in Honolulu and the $20Mclaim at the Martin County (FL) Courthouse.

In 1998, he was named by IAQ Publicationsas IAQ Person of the Year and in 1999 he wasrecognized as one of the 50 Most InfluentialPeople in the IAQ field. He has published3 manuals and over 50 technical articles,including a recent monograph on moistureand mold for the National Council of

Architectural Registration Boards (NCARB).This article was reprinted by permission fromSealing the Cracks in IAQ in the September1999 issue ofConsulting-Specifying Engineer.

Commissioning Buildings in Hot, HumidClimates: Design and Construction Guidelines,

which has been successfully field-tested inover $2 billion in new construction since1990 and is available by reaching David at407-697-7227 or d.odom@lpdg.net.

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