CPVC vs Copper in Hotels: Is there Still a Double Standard?

February 18th, 2009

Summary:  Is the risk for engineers specifying CPVC in hotels still higher than for specifying copper?

CPVC for domestic water appears to be an accepted product by the industry and is no longer judged by a different standard than copper when a failure occurs.

Over the past ten years I have investigated many cases of pipe failures in hotels.  Of those cases, there was always a different view of a failure of CPVC (or PEX) as compared to a similar failure of copper piping.  If there was a failure of CPVC, the insurance company immediately launched a massive investigation looking for someone to blame and another insurance company to subrogate.  But if a copper pipe failed, there appeared to be far less litigation involved.

As an example, one hotel in San Francisco about six months after completion had a lav fitting fail simply because it was not actually soldered.  (It is amazing how flux and a tight fit can hold water for a limited period of time.) Anyway, there was a quarter million dollars of water damage, but little fan fare.   But in a hotel in Seattle, the CPVC pipes failed due to poor isolation of the pipe from the fire stopping and a huge insurance investigation followed.

The copper failure in San Francisco was simply poor quality control, not the systematic failure of a piping system.  The CPVC failure in Seattle was a systematic failure of a product incompatibility. (The fire proofing dissolved the CPVC upon contact, and metal tape was used to separate the pipe from the caulking.  Any tear in the tape would result in consistent failure.

Any time an engineer specifies a product that is considered non-traditional, there is a greater risk of liability for a similar failure compared to a traditional product.  It appears, however, that CPVC has had sufficient time in service to expose and correct the deficiencies of the early product. I generally do not specify CPVC, but if a developer requests the product as a cost savings, I am OK specifying it.

Two Zone Hotel Recirc Systems

February 10th, 2009

Summary: Tall hotels require two or more pressure zones for domestic hot water.  Here are some approaches to the hot water recirc systems.

Hotels over 15 floors generally have two or more pressure zones for the domestic hot water.  These pressure zones are controlled with Pressure Reducing Valves (PRVs).  Assuming there is one hot water boiler and storage tank system, the PRVs separate the hot water storage tank from the hot water piping zone.  If a conventional recirc system is installed, the recirc pump must pump through the PRVs.  If the pressure drop through the PRV is 60 psi, then the recirc pump must be selected with a pump head to include the 60 psi plus the pressure loss through the system which is generally about 5 to 10 psi.  The required pump must then have a total head of about 70 psi.  The result is a pump which uses significant horse power to generate the required flow. 

An alternate approach is to avoid a recirc loop that includes the PRV.  Rather, provide a recirc pump for each pressure zone and return the recirc water back to the header down stream of the PRV.  Of course, this creates a recirc loop that does not pull new hot water from the storage tank.  So how do we keep the loop from gradually going cold during the night?  The answer is to provide a separate source of heat in the loop. 

One source of heat is an electric or gas hot water heater.  Refer to Hot Water Recirc Booster Heaters Simplify Hotel Commissioning for sizing this auxilary heater.

Another approach is to utilize a heat exchanger to transfer heat from the low pressure hot water loop to the high pressure hot water loop.  The heat exchanger acts as a pressure isolator and allows the heat of one loop to move to the other loop without pumping across the pressure drop of the PRV.  The drawing below shows how this is done. 

 

 

Note that the upper level is the low pressure zone.  This makes sense because the natural head loss due to elevation eliminates the need for a PRV.  The lower floors are served by a zone downstream of a PRV.  The heat exchanger primary takes it’s heat from the main upper zone riser which is always at 120 degrees due the recirc action of its recirc pump.  The secondary of the heat exchanger is then the source of hot water for the hot water zone served by the PRV.  The water temperature of the secondary of the heat exchanger will be slightly less than 120 degrees due to the approach temperature of the heat exchanger, but it will be satisfactory for purposes of keeping the loop warm.  Of course, a practical adjustment of the water temperatures would be to have the low pressure loop initial temperature set at 125 degrees.  Any initial temperature up to 127 degrees is generally considered safe since the water temperature drops before reaching the guestrooms anyway.

Here is an enlarged diagram for piping the heat exchanger.  Note the cross flow to assure efficient heat transfer. 

Hotel Mitsubishi City-Multi VRV Installed at Sheraton Carlsbad Resort & Spa

February 7th, 2009

Summary: Here is a photo tour of a successful VRV installation in an operational hotel.

VRV HVAC systems are making their way onto the American scene.  Although the VRV technology is common in Europe and Japan, it is a newcomer to America.  As such, there are few installations for engineers to observe.  At the Sheraton Carlsbad Resort & Spa, California,  the Mitsubishi City-Multi VRV system has been successfully installed.  I visited the site and was impressed by the equipment and performance.  Most notably, the guestroom unit is almost completely silent.  This photo tour was made possible by Bruce Zelenka who enthusiastically allowed me to see all the pieces of the system from the roof to the guestrooms.

Bruce Zelenka was instrumental in getting the Mitsubishi City-Multi VRV system installed in the Sheraton Carlsbad Resort & Spa.

 

The condensers are modular and can be placed like soldiers shoulder to shoulder.  Here they are about six inches apart, but if space is at a premium, they can be shoved completely together.

A custom curb is used to create a platform to support the condensers.

The refrigerant piping is light weight and can be routed above the roof membrane on off-the-shelf supports.  Here is an example of the piping stacked two layers high.

Here is an overall view of the piping neatly racked across the roof.  The electrical is extended through roof jacks from disconnect switches mounted on the wall of the parapet.  Alternatively, the disconnects could be located at the roof penetration, but this is a cleaner installation.

The piping transition from the roof to a shaft down the building is shown here.  Typically, refrigerant piping penetrates a roof with a roof jack, but with this large number of pipes, it is more efficient to create a roof hatch that handles a bundle of pipes.  Also, the risk of a roof leak is very low with this detail.

The BC Controller is what Mitsubishi calls the unit which manifolds the refrigerant lines to the guestroom units.  The best analogy to describe it is an electrical branch panel.  Only one pair of refrigerant lines extend to the roof like a panel feeder, and each guestroom unit is separately served by refrigerant lines like branch circuits.

Looking up at the ceiling of the corridor, the refrigerant piping can be seen routed horizontally.

 

In a guestroom the fan coils are mounted in ceiling spaces near the corridor.  This is no different than a four-pipe fan coil installation.

In this installation, the air filter is mounted behind the return grille to simplify filter replacement.

This is the standard Mitsubishi thermostat.  This thermostat is under review by Marriott and Hilton for acceptance in their hotel brands.

Mitsubishi has a fantastic design, but there is still no magic to deal with condensate.  Here the condensate from the fan coil unit in the ceiling is piped to the bathroom lav trap.

Mitsubishi offers a variety of fan coil unit styles.  Here is a four-way cassette unit suitable for a kitchen or work area.  This unit is installed without a ceiling, but as the trim would indicate, it is intended for a ceiling installation.

Mitsubishi has a special condensate trap that does not require a vertical loop.  This simplifies installation in ceiling cavities with limited clearance.

This wall mounted style of fan coil is an economical alternative to a built-in type for a guestroom.  Although this installation made no attempt to conceal the electrical power or the condensate drain, these units can be installed in guestrooms with a clean look not too different than a PTAC.  However with this unit, it is mounted high on the wall and does not require any floor space near the outside wall.  I have seen these units successfully applied to a college dormitory.

Minimum Hotel Bathroom Plumbing Clearance

January 21st, 2009

Summary:  Just how little space is required to accommodate bathroom plumbing? 

Hotel developers and architects are always complaining that engineers ask for too much space above a bathroom ceiling for plumbing.  Well, maybe they have a case.  Typically, 10 inches clear for toilet and bathtub fittings in the easy answer.  We have found a jobsite where the plumber has proven it can be done in 6 inches clear.  The picture below is from the San Diego Gas Lamp Residence Inn being plumbed by Sherwood Mechanical Contractors.  As you can see, the clear space is only the length of the pen, which measures 6 inches.  Obviously, there were no long runs with slope.  Also, this was partly made possible by full 3D shop drawings which provided excellent coordination between trades.  There is no space for random pipe crossings.  Notice that one of the sheet rock supports was trimmed to fit the trap.

Hotel Design Due Diligence – Mechanical, Electrical, Plumbing

January 4th, 2009

Summary: The first step to designing a hotel, or any project for that matter, is a thorough due diligence.  The due diligence checklist below is a proven tool I recommend for hotel design.  The same document is available in Word format by downloading a file using this link:

 Click here to download Word document! 

If you have improvements to this checklist, please comment. 
 

ARCHITECT

 

Company:

Address:

 

Tel:

Fax:

Contacts:

 

 

 

PROJECT LOCATION

 

Address:

 

 

 

Directions:

 

 

 

APPLICABLE CODES

 

BUILDING:

MECHANICAL:

PLUMBING:

ELECTRICAL:

FIRE:

ENERGY:

Notes/Comments:

 

 

 

 

BUILDING PLAN REVIEW

 

Name:

Address:

 

Contacts:

 

Tel:

Fax:

Email:

 

Building Importance Factor:

Seismic Zone:

Equipment Screening Expectations:


MECHANICAL PLAN REVIEW

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Are there any special local amendments to the codes?

Who submits plans for review?

How is U-value for tapered roof insulation computed?

Design conditions: ____/____dry bulb/wet bulb cooling,

_____ design heating temp

            ________ extreme heating temp

 

 

 

 

PLUMBING PLAN REVIEW

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Are there any special local amendments to the codes?

Who submits plans for review?

What are plan review expectations?:

Plastic pipe allowed below slab?

CPVC allowed for distribution?

Cold water pipe insulation recommended?

Are isometric riser diagrams required?

Show fixture counts on risers only OK?

Water service back flow preventer location:

Sovent plumbing system allowed?

Other plan review data required?

 

 

 


ELECTRICAL PLAN REVIEW

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

Are there any local amendments to the codes?

Who submits plans for review?

Verify plan review expectations

NMC allowed for 3 story building?

MC allowed?

Aluminum OK for 100 amps and larger feeders?

Red fire alarm cable allowed?

Interpretation of dwelling unit vs hotel unit?

Receptacle layout greater than 12’ spacing OK per NEC 210.60(B)?

Air quality permit required for diesel emergency generators?

Documentation for fuse and breaker coordination for elevators?

 

 

 

 

 

 

FIRE MARSHAL REPRESENTATIVE

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Who submits plans to Fire Department?

Sprinkler pump required?

Fire pump required?

Emergency power for fire pump?

Hazard Classification:

Flow required per sf:

Head spacing requirements:

Attic coverage:

Coverage above A/C unit in closet?

Residual pressure required at furthest head:

Changes to pressure expected in the future due to area growth?

Air quality permit required for engine driven fire pumps?

Egress sign color (red)(green)

Egress signs high and low required? Exactly where?

Emergency power elevator?

Emergency power for other systems?

Effects of building height:

Hoods: Which fans run during fire? Hood? MUA?

Hood/MUA fans need emergency power? (only for smoke removal?)

Detectors required in which spaces?

Fire fighters’ communications system?

 

 

 

 

 

NATURAL GAS COMPANY

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Initial Application to Start Utility Engineering:

Gas available?

Size of service pipe:

Standard pressures available for service:

Rules/Publications for service:

 

 

 

 

ELECTRICAL UTILITY

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Initial Application to Start Utility Engineering:

Overhead or Underground Service?

Standard service voltages available?

Transformer: (Pad mounted) (UG)(Pole mounted):

Clearances around transformer:

Pad or vault required?

Rules/Publications for service:

Application forms for temporary service:

Contact:

Document requirements:

 

Application forms for permanent service:

Contact:

Document requirements:

 

 

 

 

CABLE TELEVISION UTILITY

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Who will be contracting for the service? Owner/Contractor/Engineer?

Rules/Publications for service:

Expectations for interface on this project:

Demarc Location:

Raceway requirements:

Longest run allowed:

Backboard:

Electrical power:

Air Conditioning requirements:

 

 

 

 

TELEPHONE / INTERNET UTILITY

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Who will be contracting for the service? Owner/Contractor/Engineer?

Rules/Publications for service:

Expectations for interface on this project:

Demarc Location:

Raceway requirements:

Longest run allowed:

Backboard:

Electrical power:

Air Conditioning requirements:

 

 

 

 

HEALTH DEPARTMENT

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Who submits plans to Health Department?

Kitchen Rule/Publications:

Pool Chemical Treatment:

Spa Chemical Treatment:

Garbage Disposers allowed?

Sizing criteria for grease interceptors:

 

 

 

 

ADA PLAN REVIEW

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Accessible room doorbell: Is light and horn required for bathroom?

 

 

 

 

CIVIL ENGINEER

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

WATER AND SEWER PROVIDER:

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

SEWER

Location of connection:

Pipe size:

Invert at point of connection:

Invert of nearest upstream manholes flood rim (back water valves):

 

WATER SERVICE PRESSURE

Static Pressure:

Residual pressure:

Basis of pressure:

Future pressure expectations:

 

FIRE SPRINKLER WATER

Location of connection:

Pipe size:

Invert at point of connection:

Flow demand:

Back flow device:

Booster Pump?

 

DOMESTIC WATER

Location of connection:

Pipe size:

Water Hardness (provide water softener if > 7 grains/gal):

Invert at point of connection:

Flow demand:

Meter location:

Back flow device:

Booster Pump Required?

 

IRRIGATION WATER

Location of connection:

Pipe size:

Flow demand:

Meter location:

Back flow device:

 

 

 

 

SOILS ENGINEER (GEOTECH)

 

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Settlement Expected:

Soil pH (corrosive)

Plastic pipe allowed below grade?

Cathodic protection required?

 

 

 

 

STRUCTURAL ENGINEER

Name:

Address:

 

Contact:

 

Tel:

Fax:

Email:

 

 

Seismic Zone:

Zone Factor:

Foundation footing locations:

Shear wall locations:

Limitations on penetrations:

 

Hot Water Recirc Booster Heaters Simplify Hotel Commissioning

January 3rd, 2009

Summary: A small water heater in the recirc line is a simple way to avoid low flow recirc temperature drift when the storage water temperature is greater than 120 degrees.

Hot water mixing valves function effectively when hot water is in heavy use and cold water is mixing with the hot water.  However, when there is no net usage of hot water, such as at night in a hotel, the recirculation line can become progressively warm until it reaches the temperature of the hot water storage tank.  This temperature is often at 160 degrees, and the first user of hot water in the morning gets a slug of extremely hot water.  Of course, the common answer is to configure the mixing valves properly and balance the low flow condition.  The flaw in this concept is that many plumbers are not experienced in performing this work and it falls on the shoulders of the engineer to come to the hotel during the first few weeks of operation and perform the balance.

But there is a fool-proof alternative that requires no guesswork.  The method is to place a small hot water heater in the recirc line to compensate for the temperature loss in circulation.  Since the heater has its own internal thermostat, the only commissioning effort required is to set the water heater thermostat to 120 degrees.

But what BTUH rating is needed for the hot water heater?  The answer is not easy because it depends on the heat loss rate of the hot water piping in the path that is being recirculated. One could compute the heat loss of the piping, but that is an arduous task.  The approach I use is to start with the basic assumption that the recirc flow rate will be designed to achieve a return temperature with maximum temperature drop of 15 degrees.  This matches real experience where we supply 120 to 125 degree water with a return temperature of about 110 degrees.   Since we all have experience in selecting the flow rate in the risers to achieve a delta T of about 10 to 15 degrees,  this approach has a practical starting point as compared to computing the heat loss of a piping system.  So multiply the total circulation flow rate by 15 degrees and by 500.  For example, if the hotel has 20 risers with 1/2 gpm per riser, the total flow is 10 gpm.  This results in 15 degrees x 10 gpm x 500 = 75,000 BTUH.  That is a small water heater if it is gas, or a 20kw heater if it is electric.

Aside: It is interesting to think about the implications of this heat loss as it relates to energy waste.  This heat loss is not only a direct loss of heating energy, but it also requires constant mechanical cooling to remove this wasted heat.   So this is a double argument for better hot water system insulation.

Hotel Dual Electrical Services for LEED Credits

January 2nd, 2009

Summary: Requesting two service transformers with two voltages from the utility service is energy efficient and cost effective.

Large hotels generally have a 480/277 volt service provided by the utility and step-down transformers for 208/120 volt loads.  Since a hotel is largely 120 volt loads, the size of the step down transformers is about half of the capacity of the service.  This results in dual transformation of a large amount of power.  The energy efficient approach is to request two service transformers from the utility:  One transformer at 480/277 volts, the other at 208/120 volts.  The lower voltage transformer will eliminate all the losses from stepdown transformers in the hotel while keeping the utility losses the same.  For a large hotel, this is generally allowed by the utility.  The following is an example of the analysis applicable for computing energy savings from dual service transformers.

Based on an actual hotel of 333 rooms, the following LEED energy analysis was performed:

Assumptions:

Hotel Room Count:  330 guestrooms

120/208 volt load per NEC:  948 kva (Say 1000 kva)

Heat losses from distributed dry-type transformers  throughout the building:  2.0% of NEC load

Cost of electricity:  $0.08 per kwh

Load Factor:  Verified load factor data for hotels was not available at the time of this study.  However, load factor data for other facility types was available, and the data indicates a load factor of 30% to 50% is probable.  Since the load factor is an important part of this analysis, a continuing effort is in progress to obtain better load factor data specific to hotels.  However, we know that the magnetizing losses are a constant regardless of load and represent about 1% of the transformer losses based on nameplate data.  Therefore the I squared R losses will vary with load factor, but not in some fraction of a proportion.  For this study, the load factor is assumed to be included with the transformer loss number of 2%.  Peak losses for small transformers are commonly stated at 3%, which is the sizing criteria for cooling equipment.

Cooling Efficiency: Transformers located within the hotel require mechanical cooling to remove the heat generated by the transformers.  Since utility transformers are allowed to be cooled with ventilation air, there is an additional energy cost for dual transformation that must include the cost of mechanical cooling of the transformers.  This study is based on 1 kw per ton of cooling.

Computation of Savings:

Transformer energy savings per year in dollars =  1000kva x 0.02 efficiency x 8760 hrs/yr x $0.08 /kwh  =   $14,000 savings

Associated Cooling Savings =  1000kva x 0.02 efficiency x 3413BTUH/kw / 12000 BTUH per ton x 1 kw/ton x 8760 hrs/yr = $4,000 savings

Total Savings = transformer savings + cooling savings = $14,000 + $4,000 = $18,000 per year.

Trouble Shooting Hotel Hot Water Recirc Systems

December 30th, 2008

Summary: This article is about trouble shooting hot water recirc systems for hotels.  For many different reasons,  hotel hot water recirc systems can be a major challenge to commission at the opening of a hotel.  My goal here is catalog all the problems and associated solutions to hot water recirc systems that I or anyone else knows.  I encourage readers to share their examples.   This will be an article that continues to grow over time.

Related Article: Hotel Hot Water Recirculation Systems

Problem:  Cold water entering the hot water system:  If you are observing cold water entering the hot water system, it is likely that cold water pressure is higher than the hot water pressure.   When this pressure difference exists,  there will be a natural tendency for cold water to migrate through mixing valves into the hot water system.  This problem often occurs when a water softener is installed serving only the hot water.  This problem is easily fixed with a pressure regulating valve for each of the hot and cold water systems.  Locate the hot water pressure regulator on the outlet side of the water softener.  Then set the pressure of the hot water regulator a few pounds higher than the cold water.  This will assure any migration of water through the mixing valves will be toward the cold water, which is a forgivable situation in most cases.

Problem:  Plumber installs more risers than originally designed.  Sometimes a plumber will decide that it is easier to install a separate riser for each stack of fixtures instead of combining risers to serve several fixtures via horizontal branch connections.  This change is OK, but it creates more risers than included in the original design.  With more risers to serve, the recirc system must be upsized to maintain the original design flow in each riser.  Generally this will involve upsizing the recirc pump and the return piping. 

Problem: Recirc lines leaking after several years of operation:  Recirc lines need to be sized for continuous flow or they will erode in a few years and develop pin-hole leaks.   The sizing criteria for recirc lines should be a maximum velocity of about 3 feet per second.  This is much slower than the criteria used for domestic water lines, because water lines operate very intermittently. 

Problem: Oversized Recirc Pump:  We all know it is better to oversize a recirc pump than undersize it.  But this is only true if balancing valves are installed.  If the oversized pump is allowed to operate without balancing, the circ lines will erode from excessive water flow velocity.  I recommend a conservatively sized recirc pump (i.e. oversized), and the installation of a balancing valve at the outlet side of the pump.  Of course, someone must actually perform the balancing, which often is the greatest challenge of all.

Problem: Hot water entering the cold water system:  There are many ways this can happen, but one example I encountered was a high rise hotel with two zones of domestic water.  Since PRVs were involved to create the two pressure zones, the recirc loop was required to circulate through a PRV.  This required the pump to have a very high head to overcome the PRV pressure drop.  The result was a back-pressure that forced hot water into the cold water system system in the mechanical room.  This problem only occurred under low flow conditions, since at that time there was no other release for the hot water system.  The solution was to install a check valve in the cold water pipe serving the boiler.  This eliminated any reverse flow caused by the recirc pump. 

Aside:  Although we solved the problem, I was never fully satisfied I understood the physics.  Here is why.  The mystery was how could the recirc pump force hot water back against the cold water source?   If you “count” molocules of water coming and going, there is no way that a hot water recirc pump can create a net increase in system water that would force hot water back into the cold water.  My only guess is that there really was no perfect separation of the hot and cold water systems due to the mixing valves throughout the hotel.  And what was really happening was that the high pressure recirc pump was recirculating hot water through the mixing valves with the resulting “appearance” of hot water being forced back into the cold water.  Regardless, the check valve solved the problem.  Howver, I felt very unsatisfied not really understanding why the fix worked.  I would be interested in hearing about similar cases.

Hotel Guestroom Fault: Limiting to Under 10,000 Amps

December 18th, 2008

Summary: Save money on costly high AIC rated breakers by adding length to panel feeders.

Hotels with 208/120 volt services can have very high fault currents near the service entrance.  It is tempting to locate some of the branch panels in the electrical room to serve nearby loads, including the guestrooms in that vicinity.  However, this can result in excessive fault current exposure to the guestroom circuits.

One simple way to reduce the fault at nearby guestrooms is to route the feeders down and back the corridor with sufficient extra circuit length to bring the fault current below 10,000 amps.  The impedance of small circuits is quite high and has the effect of reducing fault current effectively with relatively short runs. 

As an example, suppose you have a 100 amp guestroom panel in the room adjacent the service electrical room.  The feeder size is probably #1 copper conductors and the length of the feeder to the panel is 25 feet including conductors within the switchgear.  Assume the fault at the switchboard is 65,000 amps.  The fault at the panel would be 24,000 amps symmetrical.  By simply routing the feeder 30 feet down and back the corridor to extend the feeder length to 75 feet the fault is reduced to about 9,000 amp symmetrical.  This added feeder length is only required for the first few rooms until the feeder length exceeds 75 feet anyway.

 

Below is the Design Master fault printout with the guestroom panel feeder only 25 feet long based on shortest route from switchboard to panel.  Fault is above 10,000 amps and the lowest cost panels would not be adequate to handle this fault.

 

Below is the same system in Design Master but showing the fault below 10,000 amps with the feeder length extended to 75 feet.

 

Hotel Elevator Hoistway Venting

December 15th, 2008

Summary:The IBC requires the top of all elevators to be vented to the outside. The size of this vent is important because in many cases it is routed through the elevator machine room.

 
The IBC requires the top of all elevator hoistways to be vented to the outside.  Since almost all hotels have  elevators, this is a feature the architect will ask the mechanical engineer for help sizing.  Therefore, this article is intended to be a quick reference.  The purpose of the vent is to allow smoke removal from the shaft in the event of a fire.   As an aside, I have also heard the argument that the vent provides relief from the piston effect of the elevator, but I have not seen this reason confirmed in writing anywhere.  Of course, if the piston effect reason was actually true, then there should be a corresponding relief opening at the bottom of the shaft, and the code does not address a vent at the bottom.
 
The vent size is determined by IBC Section 3004.3, which requires the area of the vents be not less than 3.5% of the area of the hoistway, but not less than 3 square feet for each elevator car. Dumbwaiters hoistways are also included in this requirement with a minimum of 0.5 square feet per dumbwaiter.
For hydraulic type elevators, the top of the elevator shaft is generally at or above the roof and providing this vent is simple. However, for elevators with machine rooms at the top, access to the outside is often through the machine room. This is accomplished done by routing a duct through the machine room in a rated enclosure. Routing of the duct and shaft involves close coordination with the elevator machine equipment layout and the associated working clearances.
The code allows up to 2/3 of the area of the opening covered with annealed glass. In mild climates, this is not generally required. However, in extreme cold climates, the glass can reduce the heat loss from cold air coming into the hoistway.
The table below shows typical vent sizes required for elevator shafts with one, two, and three cars. Obviously, there is no single size for an elevator. However, the table below can be useful during schematic design to quickly assess vent sizing requirements. Note that for two and three car banks with a common hoistway, the sizing criteria is dominated by 3 sf per car rule rather than the 3.5% rule. Therefore, estimating for multiple car hoistways is quite easy.
Table:
Shaft Size (Ft2) Number of Cars Vent Size Free Area (Ft2) Louver size (in) with 50% free area (nominal)
80 1 3 30X30
90 1 3 30X30
100 1 3.5 36×30
110 1 3.85 36×36
120 2 6 48×36
130 2 6 48×36
140 2 6 48×36
150 2 6 48×36
160 2 6 48×36
170 3 9 48×60 (close to 48×48)
180 3 9 48×60
190 3 9 48×60
200 3 9 48×60

 

 

Code:
  • 3004.1 Vents Required

Hoistways of elevators and dumbwaiters penetrating more than three stories shall be provided with a means for venting smoke and hot gases to the outer air in case of fire.

  • 3004.2

Vents shall be located at the top of the hoistway and shall open either directly to the outer air or through noncombustible ducts to the outer air. Noncombustible ducts shall be permitted to pass through the elevator machine room, providing that portions of the ducts located outside the hoistway or machine room are enclosed by construction having not less than the fire protection rating required for the hoistway. Holes in the machine room floors for the passage of ropes, cables, or other moving elevator equipment shall be limited so as not to provide greater than 2 inches (51mm) of clearance on all sides.

  • 3004.3 Area of vents.

Except as provided for in section 3004.3.1, the area of the vents shall not be less than 3.5% of the area of the hoistway not less than 3 square feet(0.28 m2) for each elevator car and not less than 3.5% not less than 0.5 square feet (.047 m2) for each dumbwaiter car in the hoistway, whichever is greater. Of the total required vent area, not less than one-third shall be permanently open. Closed portions of the required vent area shall consist of openings glazed with annealed glass not greater than 0.125 in (3.2 mm) in thickness.