Building Pressurization Control - Trane Q&A

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Engineers Newsletter Live Satellite BroadcastCommercial Building Pressurization — April 17, 2002

Questions and Answers

Copy of Building Pressurization ENL Q A final.doc

http://www.trane.com/commercial/library/BuildingPressurizationENLQandA.pdf

APPLICATIONGrocery store with an air door, store has 20,000 cfm of exhaust, nearly 30,000 cfm of makeup air, 20% positive air pressure to satisfy door air requirements. Makeup air isbeing introduced into the space through fixed outdoor-air dampers on rooftop HVAC

units. When several culinary hood exhaust fans get turned off at night, the building is soover-pressurized that it is difficult to close some manually-operated doors. What is the

best way to maintain the 20% positive pressure in the space?While it might or might not be the “best” method, one way to assure positive pressure isto coordinate the operation of the rooftop units (which provide intake air) with the

operation of the exhaust fans. That is, turn off one or more rooftop units as one or moreexhaust fans turn off. Another possible approach is to install a barometric gravity

damper connected to the space to relief excess intake air when exhaust fans are off.

Discuss barometric damper location on a leeward side of the building and the effect or

eddy currents on barometric dampers with wind speeds in excess of 50 mph.Refer to ASHRAE 2001 Fundamentals, Chapter 16. The effective negative staticpressure on the leeward side can vary widely, depending on wind speed and direction.

Since wind speed and direction vary widely, too, no single location for the leeward sidebarometric damper is always “best.” That’s why we recommend using barometric

dampers on more than one building façade.Using Equation 18, 2001 ASHRAE Fundamentals, Chapter 26, the effective leeward-side static pressure due to a 50 mph wind may be negative 0.94 in. w.g. Assuming that

proper relief airflow across the leeward-side relief damper requires only 0.20 in. w.g.differential, the building must be negative 0.74 in. w.g. positive to achieve the same

relief airflow as would be achieved in still air with 0.20 in. w.g. positive pressure. Sincebuilding pressure is almost certain to be less negative than 0.74 in. w.g., differentialpressure will be greater than 0.20 in. w.g. and relief airflow will exceed the required

airflow. In general, the building will be more negative than it would have been in still air.

How would you control a semi-high rise building (12 floors) with high door openingfrequency, coupled with connecting corridors to two other buildings?

A twelve-story building should be designed with entry vestibules and with corridor

“airlocks” to separate the building from adjacent attached buildings. The designer

should consider mounting indoor sensors on the entry-level floor in an area somewhatremoved from the doors.

Are there any guidelines for maximum building size to pressurize? What about a very

large building that cannot be compartmentalized, such as a warehouse or factory withlarge overhead doors?

Exfiltration airflow is determined by building construction and differential pressureacross the envelope. Even a small differential pressure across a leaky envelope can







result in significant exfiltration regardless of building size. On the other hand, a very

large differential pressure across a very tight envelope results in very little exfiltration,again regardless of building size. With these points in mind, any size building can bepressurized provided the envelope is tight enough and provided that the outdoor air intake and supply fan can deliver airflow in excess of local exhaust, exfiltration, andreturn airflow.

In a single story building with a potentially toxic environment application, we mustpressurize the main room to +0.1” w.g. and other support rooms to +0.05” w.g. We

currently use a fan-powered filtration unit in series with the building units. Thoughts oncontrol?

Each subspace needs to be controlled relative to the main space. This requires the useof a separate pressure control system per sub-zone. Control the relief or incoming rate.(Generally the relief rate.) Because of the toxic issues, the subspace pressure control

loop needs to be fairly quick to respond. But, be aware of stability. In a small spacevolume, the simple opening of a door can significantly change pressure. This change

needs to be dampened out. The supply/exhaust systems must have sufficient pressurecapacity (0.05 in. w.g. plus duct and diffuser losses) and sufficient airflow capacity tohandle exfiltration through cracks and especially, access doors.

In a “large box” retail application, with multiple rooftop units (RTUs) each containingbarometric relief dampers, I find that the supply air (SA) fan must be run continuously

instead of automatically to prevent heated air from escaping “through the roof.” Thiscauses excessive negative pressure. To offset this, the SA fans are run continuously.

This is perceived as wasting energy to keep the relief damper closed when we are not

using the economizer. Your thoughts?! Apparently you have a “stack and hot air” effect in the building when the supply fans are

off. You could increase the pressure required to open the barometric dampers. Whilereducing exfiltration, this could increase thermal stratification making the space hotter.

Instead, you can leave the fans running—as you have been doing or for energysavings— add supply fan inverters to let them act as space “mixers” when in the “off”mode.

Please discuss compartmentalization as it relates to building pressure control.

A compartment is an enclosed area separated by physical boundaries. Example: A floor with seals at elevators and separate outside air intake and relief dampers and

indoor/outdoor pressure sensor. Refer to “Building science for a cold climate” by NealHutcheon and Gus Handegord, National Research Council, Canada, 1995.

What issues must be considered for controlling a zone within a zone? (Example:biology lab)Measure space to space differential pressure, airlocks/door interaction with space

pressure, use variable supply to match variable lab hood exhaust.







How effective are air curtain fans in controlling pressure in large buildings with manyoverhead doors?

Air curtains have minimal effect on building pressure. Their primary benefit is thethermal barrier created. Pressure-wise, air curtain fans have no impact on buildingpressure if the airstream is directed straight across to the opposite side of the door. If

the airstream is aimed inward or outward, they will help pressurize or depressurize thebuilding.

If you are using a variable speed drive for supply air and no central return fan with theconstant flow of incoming outside air, how can you control building pressure setpoint?

You are referring to a VAV system that has some means of ensuring that the enteringoutdoor (intake) airflow remains constant as the supply airflow varies. With the intakeairflow quantity fixed, the only remaining way to control building pressure to a desired

setpoint is to vary the relief (exhaust) airflow. One option is to use a central relief fanthat is modulated based on the measured indoor-to-outdoor pressure difference. This is

the system configuration outlined on page 7 of the Engineers Newsletter thataccompanied this broadcast (www.trane.com/commercial/library/newsletters.asp).

What changes in modulation of the central relief fan can be effected by ambientbarometric pressure changes that occur rapidly?When directly controlling building pressure, the capacity of the central relief fan is

modulated in response to the measured difference between indoor and outdoor staticpressures. This control loop is typically setup to respond rather slowly in order to

provide more stable system control. Generally, outdoor (ambient) pressure changes

relatively slowly, even slower that the speed of the building pressure control loop.However, if for some reason the building is expected to experience rapid changes in

outdoor pressure, the loop can be tuned to respond faster.

CONTROLSIs it better to control building pressurization by OA or EA?

It is better to control building pressure by modulating relief airflow. Exhaust airflow isnormally determined by contaminant removal requirements. Outdoor intake airflow isnormally determined by minimum ventilation requirements or mixed air temperature

control requirements during economizer modes.

How do you sequence control of multiple exhaust fans?Multiple exhaust fans, which may be used in laboratory setting, would normally besequenced based on changing exhaust airflow needs, not building pressure. Multiple

relief fans that serve a common building area, on the other hand, should be modulatedtogether from the same building pressure signal. Separate control loops could easily

result in “fighting” between relief fans; some fans at full capacity, others at zerocapacity.







What issues must be considered for zone control for proper building pressurization?Zones can be set up if the spaces are compartmentalized. So, first the space must becompartmentalized. Then it needs to have some type of pressurization system appliedto the zone.

Please explain optimized damper control further.Optimized damper control assumes that three separate damper actuators independentlycontrol the intake, return and relief dampers. The intake damper modulates to maintain

outdoor airflow and assure proper ventilation, regardless of mixing box pressure. Thereturn damper modulates to maintain the intake damper nearly wide-open, reducing

mixing box pressure to the minimum required for proper intake airflow. The relief damper modulates to maintain building pressure. Finally, the return fan modulates tomaintain either the relief or return damper nearly wide-open, reducing return-air plenum

pressure to the minimum required for proper relief or return airflow.

I have on numerous occasions controlled building static simply by utilizing a buildingstatic pressure sensor and modulating (VFD) the return fan. This has always workedwell in a VAV system with a central supply and return fan setup. You didn’t address this

in your broadcast. Do you see any problems with this type of control? I haven’t.The operation of your specific system configuration passes one very important test: itworks satisfactorily for you. We didn’t discuss it because it doesn’t work equally well for

all intake/supply fan combinations. If outdoor air intake flow is sensed and maintained(using TRAQ dampers for instance), the return fan can be modulated to successfully

maintain building pressure because the intake damper compensates for changes in

return-air and mixed-air plenum pressures. On the other hand, if the outdoor damper isset to a minimum position, modulating the return fan based on building pressure results

in variations in both return-air and mixed-air plenum pressure: if return fan speedincreases, relief airflow increases and intake airflow decreases, causing improper

ventilation and unstable operation. The approach we discussed maintains buildingpressure by modulating the relief damper and maintains return-air plenum pressure bymodulating the return fan, thereby minimizing the effect that return fan speed has on

mixed-air plenum pressure and intake airflow.

Have you ever tried to control building pressure by only using supply air, return air,exhaust air, and outside air volume measurements and a fixed calculated difference

between exhaust and OA to provide building pressure? (i.e., no pressure readings, onlyair volumes CFM)This seems to describe an airflow tracking technique similar to the one discussed in the

broadcast. This indirect building-pressure control technique requires very accurateairflow sensing to avoid wide pressure variations. Consider a building with 3000 cfmintake airflow and 1000 cfm relief airflow at design, assuming a constant local exhaust

and exfiltration airflow of 2000 cfm. If sensor accuracy is 10% of full scale, actual intake







airflow ranges from 2700 cfm to 3300 cfm and actual relief airflow ranges from 900 cfm

to 1100 cfm. Actual differential airflow is set to 2,000 cfm but is actually maintained in arange from 1600 cfm to 2400 cfm. Indirectly controlled building pressure may be morenegative or more positive than expected, even when the control system is operatingproperly. Without differential reset control, airflow tracking assumes that local exhaustand exfiltration are relatively constant, even though exfiltration actually varies

significantly in most climates due to changes in outdoor temperature and wind.

Can you control building pressure using supply and return fan air flow measuring

stations and maintaining a fixed differential between the two flows? This eliminatesissues with locations of building pressure sensors.

This is the airflow tracking technique that we discussed during the broadcast. Indirectbuilding-pressure control using airflow tracking requires very accurate airflow sensing toavoid wide pressure variations. Consider a building with VAV system with design supply

airflow fo 10,000 cfm and design return airflow of 8,000 cfm, a 2 ,000 cfm differential for pressurization. At 50% supply airflow, the supply sensor “sees” 5,000 cfm and the return

sensor “sees” 3,000 cfm. However, if sensor accuracy is 10% of full scale, actual supplyairflow ranges from 4,000 cfm to 6,000 cfm and actual return airflow ranges from 2,200cfm to 3,800 cfm. Actual differential airflow is set to 2,000 cfm but is actually maintained

in a range from 200 cfm to 3,800 cfm. Building pressure may be very negative or verypositive even when the control system is operating properly. Without differential resetcontrol, airflow tracking assumes that local exhaust and exfiltration are relatively

constant, even though exfiltration actually varies significantly in most climates due tochanges in outdoor temperature and wind.

In the drawing above (see fax), is it necessary to control the relief damper or control thesupply and return dampers?

Without more detail, our recommendation would be as follows: control the OA damper to maintain ventilation or economizer airflow, link the recirculated return damper to the

OA damper (if “optimized damper” control is not used), modulate the relief damper tomaintain building pressure, and modulate the return fan to maintain return air plenumpressure.

DESIGNHow large an opening is required for the barometric dampers to properly work?Depends on the maximum allowed space positive pressure. The size of the damper

sets the damper pressure drop. Typically this is .2”. The other work proper factor is thedamper location. It determines the barometric offset. Example: A damper in theoccupied space has an offset of 0” so a .2” pressure drop damper would have a .2”

maximum space overpressure.

Does the moisture control problem go away with all glass or EIFS (Exterior Insulationand Finish Systems) buildings?







No. Infiltration of humid outdoor air is only one component of the indoor moisture load.

As long as the envelope has seams, holes, or cracks, air will be able to infiltrate if building pressure is allowed to become negative enough. Both glass and EIFS may beable to reduce infiltration, but they cannot totally eliminate it.

How does the use of occupancy sensors or CO2 sensors (for reducing outdoor air

levels) affect the selection of the building pressurization system?When occupancy or CO2 sensors are used to reduce to intake (outdoor) airflow duringtimes of less occupancy, direct control of building pressure becomes even more critical

because intake airflow is no longer constant. The concern is that as intake airflow isreduced, it drops below the local exhaust airflow and the building can become negative.

Because the intake airflow is controlled to deliver a certain quantity of outdoor air, theonly remaining means for controlling building pressure is to vary the relief airflow.

Should outdoor enthalpy be used to control building pressurization?Directly controlling building pressure requires both an indoor and an outdoor static

pressure sensor. An outdoor enthalpy sensor may be used for activating the airsideeconomizer, particularly in VAV systems.

What is a reasonable percentage of excess supply air to design for to assure proper building pressurization?Building pressure is primarily a function of intake airflow and relief (exhaust) airflow, not

supply airflow. As we showed with the model during the broadcast, when the outdoor-air damper was closed (100% recirculated air) building pressure was neutral.

To achieve positive building pressure, intake airflow must exceed relief (exhaust)airflow. The amount of excess intake airflow required primarily depends on 1) the

desired indoor-to-outdoor pressure difference, 2) the surface area of the buildingenvelope, and 3) how leaky the envelope is. These factors are difficult to relate to

supply airflow, so there are no generally-used rules-of-thumb based on a percentage of supply airflow.

The “Managing Building Moisture” applications engineering manual published by Trane(SYS-AM-15, page 14, Figure 8) includes a method for estimating the amount of excess

intake airflow required to allow for leakage through the buildings exterior walls.

EQUIPMENTBarometric dampers located in equipment doesn’t seem to work very well (air handlers,rooftop units). What do you suggest?

Locate the barometric dampers in the space or use powered central relief, either on/off or modulated based on building pressure.







In actual application, couldn’t wind directed at the relief damper affect its ability to

relieve air, potentially causing the building to over-pressurize?For powered relief the effect of wind is minor because the fan velocity is an order of magnitude higher then the wind pressure. Therefore, no problems. Wind however caneffect barometric damper performance. This limits the barometric damper systemperformance.

What would be the thinking about selection of the supply fan and/or central relief fan HPfor a rooftop unit when considering a 40 mph wind, or say a 20 mph wind blowing

directly into relief fan outlet?Using Equation 18, 2001 ASHRAE Fundamentals, Chapter 26, the effective windward-

side static pressure due to a 20 mph wind is about 0.39 in. w.g. Assuming that proper relief airflow across the windward-side relief damper requires about 0.20 in. w.g., thereturn air plenum must at least 0.59 in. w.g. positive to achieve the same relief airflow

as would be achieved in still air with 0.12 in. w.g. positive pressure. Higher relief fanpressure at the same airflow requires more horsepower.

The illustrations concerned central fan systems. What considerations for pressurizationare there for “modular” units, such as fan-coils, water-source heat pumps? Does it

require a separate building pressurization system?The importance of building pressure control is the same. However, many systems thatuse terminal units (fan-coils, water-source heat pumps, etc.) do not bring in outdoor air

through the terminal units. Instead, ventilation and building pressure control is handledby a separate, dedicated ventilation system. In this configuration, a central relief fan

could be used for direct control. On the other hand, when outdoor air is introduced

locally, as it is in unit ventilators, a central relief fan is often required to assure proper intake airflow as well as proper building pressure.

What is the best way to ensure proper balancing of a dorm or classroom using PTACs?

Do you recommend barometric dampers in most cases?In a PTAC, intake (outdoor) airflow is usually constant (i.e. no economizer). If naturalrelief and local exhaust are not sufficient, then local barometric relief would be one

option for relieving some of the excess air from within the space.In some cases, a relief fan must be used to assure proper intake airflow.

With my Trane Voyager packaged units (constant volume) with Trane Tracker zoning

controls, what would be the preferred relief?You are referring to a Trane VariTrac system, which consists of a constant-volumerooftop in a changeover-bypass VAV system. When a VariTrac system includes an

airside economizer, the preferred method for providing relief is to configure the Voyager rooftop unit with a relief (powered exhaust) fan, and turn it on and off based on theposition of the outdoor-air damper. (The Trane controls default to energizing the central

relief fan whenever the outdoor-air damper is more than 25 percent open.) You must







provide some method of central relief in this system to avoid pressurizing the return

plenum with bypassed supply air.

PRESSURE SETPOINTS Any ideas to dampen out turbulence and pressure spikes?Check with the pressure sensor or controller manufacturer for dampening methods. It’s

not uncommon to use a long coil of tube to filter the sensed signal. Alternatively, somesensors include electronic signal filtering to minimize signal “noise.”

Are there any negative effects of moderate positive pressure in cold, dry weather?If indoor dewpoint is above outdoor temperature, exfiltration of indoor air may cause

condensation to form within the building envelope. Depending upon the envelopeconstruction and materials, this condensation may or may not result in damage or mold

growth. Most experts agree that buildings in cold climates should be neutral or slightlynegative. This is especially important in buildings with masonry facades.

How are occupied/unoccupied periods handled? Is the system on during unoccupiedperiods, or is it off to conserve energy? Does the building lose humidity control duringunoccupied periods?

In addition to ventilation, the HVAC system provides makeup air for local exhaustsystems (such as in restrooms and kitchens). In some buildings, either through

oversight or design, the ventilation system is turned off during unoccupied periods, whilelocal exhaust systems continue to operate. This creates a negative pressure within thebuilding, resulting in infiltration. If this creates a problem, either shut down all local

exhaust systems (if possible) or continue to deliver a small amount of makeup air tomaintain positive building pressure. Systems that control relative humidity during

occupied hours lose humidity control if they are turned off during unoccupied hours. Of course, systems that control humidity indirectly many see a wider range of relativehumidity during unoccupied hours, especially if the ventilation system continues to

operate.

Is it better to operate an extremely leaky brick building with no wall insulation slightlynegative in the winter and offset the infiltrated air with heat (higher energy costs) andrisk freezing the exfiltrated moisture in the wall?

A concern with brick facades in cold climates is to prevent moisture from getting into themortar and exposing it to recurring freeze and thaw cycles. In cold climates, many

designers address this issue by controlling building pressure to be neutral or even slightnegative during the winter. This avoids indoor moisture from being driven into thebuilding envelope.

In heating mode, if building pressure is negative, won’t ambient dry bulb temperatures

below interior dewpoint cause condensation where infiltration airstreams mix withinterior air?







During the heating season, the outdoor dew point is typically very low, so there is little

risk that infiltration will result in condensation within the building envelope. However, toomuch infiltration may result in cold surfaces within the occupied space (e.g. window anddoor frames) or the ceiling plenum. Combined with a high enough indoor dewpoint,moisture could condense on these cold surfaces. In most building types, however, theindoor dew point is fairly low during the heating season in cold climates. In warm or

moderate climates, where it does not get very cold during the heating season, it may bebetter to maintain slightly positive building pressure year-round.

Please discuss settings within the operating modes (cooling, heating, economizer).When directly controlling building pressure, most designers place the indoor pressure

sensor on the ground floor and set it to maintain the pressure between 0.05 and 0.10inches positive at all times. This may be enough pressure to prevent infiltration andprovide adequate pressure control, especially for smaller buildings in warm climates.

But, in multi-story buildings, the best setpoint range really depends on the impact of stack effect and wind on the pressure difference at the top floor.

Additionally, controlling to the same pressure year-round may not be the best solutionfor all buildings in all climates. Sometimes it might be better to change the setpoint

range as operating mode changes. For example, when it’s cold and dry outdoors, aneutral or slightly negative building pressure may be desirable to prevent exfiltration of moist indoor air, especially for tall buildings where winter-time stack effect can cause

significant positive pressures at the upper floor.

How does fixed outside air for ventilation requirement work with your strategies for

building pressurization with a return fan?For all the control methods discussed for a return fan, we assumed a constant quantity

of outdoor air for ventilation. This is fairly easy to achieve in a constant-volume system,but is more complicated in a VAV system. We recommend direct measurement and

control of outdoor airflow in VAV systems. This could be accomplished with a flowstation and modulating damper or a combination flow- measuring damper (which is whatwe showed on our schematics for VAV systems). Because we want to maintain

constant outdoor airflow is the reason why we must control the static pressure in thereturn plenum. During the broadcast, we suggested controlling building pressure by

modulating the relief damper and controlling the return fan to maintain a pressuresetpoint in the return plenum of the air handler.

As typical with most factories, more exhaust is added and added without addingmakeup air. Is there anything measurable (safety, health, environmental) as far as

justifying makeup air units?It’s not very easy to quantify the problems with running a factory under a severenegative pressure. If the owner does not perceive any problems, it is a difficult sell. As

we stated in the broadcast, the effects of overly negative building pressure are:







1) Infiltration of cold outdoor air during the winter, which leads to discomfort and higher

heating energy consumption.2) Infiltration of humid outdoor air during the summer, which can lead to condensation

on cold indoor surfaces and potential microbial growth. In a factory, this could be asafety issue with slippery floors or with electrical equipment. This also increasescooling energy consumption.

3) As indoor pressure becomes more negative, you need more exhaust fan power toexhaust a given quantity of air, so fan energy consumption increases.

With some work, you could estimate the amount of infiltration and perform an energy

analysis to quantity its impact on cooling and heating energy use. The same could bedone for exhaust fan energy use.

SENSORS/SENSOR LOCATION

Speak to location of static pressure sensor in return plenum, since it is critical.If possible, locate the return-air plenum static sensor in a relatively “stagnant” place,possibly near the inlet to the plenum. Alternatively, an outdoor pressure pickup, like the

one we showed in the broadcast, could be mounted in the plenum or an averagingprobe might used to minimize the effects of turbulence.

When there are two units on a 1-2 story building, is it better to have one sensing pointfor both units?

If both units serve both floors use one building pressure system with one indoor sensor to control the relief rate of both units together. If one unit per serves each floor use, twobuilding pressure systems, one per unit.

Wind can greatly affect the outside pressure reading often located on the roof. I have

read suggestions to place this OA reference on a parking lot pole. Today, securityequipment is often place on such poles and conduit is already there to use. What areyour thoughts on this? Also, what are your thoughts on using 4 reference points in the 4

corners of a building roof?Wind changes the effective static pressure on the building envelope and wind velocity-

pressure can result in erratic static pressure readings. Assuming that the outdoor sensor blocks wind gusts (the Dwyer A-306 does a good job), location of the sensor is

job-specific. If the roof static pressure is consistent regardless of wind velocity and

direction, a single sensor is adequate. If the roof static pressure changes significantlywith wind (due to obstructions on the roof, for instance), a parking-lot sensor or

averaged “four-corner” sensing configuration may provide a better reading of localambient (barometric) pressure.

Is there any concerns of sunlight affecting the plastic construction of the outdoor sensor? [Dwyer A-306 shown in broadcast.] Does the material deteriorate due to UV

rays?







It is designed for outdoor use. It does not rust or collect water. If it should fail with time,

it is very inexpensive to replace.

Please comment on the effect of long tubing runs between sensing probes andtransducers. Does this affect response time of the equipment?Long runs of tubing tend to filter or “snub” rapid pressure fluctuations, often a

requirement for stable operation.

Please discuss stack effect in high-rise buildings over twenty floors tall with a common

air handling system and the proper location of indoor static pressure sensor or sensors.See the Building Pressurization Manual for more details on this subject. Trane

recommends building pressure control zones or “compartments” no greater then 10stories. Many projects include a separate pressure control zone for each floor. Eachpressure control zone includes an air handler with relief airflow capabilities

Is there validity to installing pressure sensors on multiple floors and averaging the

signals, especially in high-rise buildings served by central air handling systems?We see little advantage in averaging building pressure sensors “vertically”. Use aseparate indoor pressure sensor per floor if each floor or set of floors is separate

pressure control zone with it’s own relief damper.

Is it better to have one space static sensing point or multiple sensing points?

GeneralIy, one space sensor is enough. Multiple indoor sensors are sometimes used onlarge indoor spaces effected by wind (factories, for instance) or where one relief fan is

handling several building pressure control zones.

MISCELLANEOUSIs there a way we can show poor building construction for air tightness, such as thermalimaging?

Blower door tests will pull a negative or positive on the building zone and can be used toestablish envelope integrity.

Do you recommend air curtains to help stabilize a building that has a high level of entryand exit? Do they help stabilize building pressure?

Air curtains can be used but are “application sensitive” and need to be running all thetime. In generally we recommend revolving doors for stable building pressure control.

Is an elevator door a sufficient air lock to provide floor-by-floor compartmentalization? An elevator door may seal sufficiently to improve floor-to-floor compartmentalization.

Check with elevator manufacturers for specifications related to door sealing. The outer door must seal between space and shaft when closed, to minimize space-to-shaft

airflow when the elevator car is not present, and the elevator doors themselves must







seal between space and shaft when open, to minimize space-to-shaft airflow when

elevator car is present with its doors open.

In multiple story buildings with a return-air plenum, explain the effect of full height wallswith regards to return air opening sizes and maintaining building or space pressure witha VAV system.

Above a dropped ceiling, airflow velocities are typically below 1500 fpm. If return air must flow from one ceiling plenum to another, the opening connecting the two plenumsshould be large enough to keep both pressure drop and flow velocity low. Small

plenum-to-plenum openings can require very negative plenum pressure near the air handler return opening.

How do you compartmentalize a building vertically? By zoning the air handling systemvertically? How about vertical elevator shafts and stairways?

To compartmentalize, seal between floors, seal corridors or use doors to isolateelevator shafts, seal stairwells, and use an air handler with relief dampers for every floor

or every building pressure control zone. Refer to “Building science for a cold climate” byNeal Hutcheon and Gus Handegord, National Research Council, Canada, 1995, for more information.

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Building Pressurization Control - Trane Q&A