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TECHNICAL FEATURE
This article was published in ASHRAE Journal, June 2015. Copyright 2015 ASHRAE. Reprinted here by
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ashrae.org.
The Sistine Chapel and
Saint Peter’s Basilica with
the new HVAC system under
construction.
The Sistine Chapel
© 2014 MUSEI VATICANI
New HVAC System for Cultural Preservation
BY MICHEL GRABON; JACKIE ANDERSON, MEMBER ASHRAE; PETER BUSHNELL, MEMBER ASHRAE; ARITZ CALVO; WILLIAM CHADWICK, MEMBER ASHRAE
This article provides a review of the new HVAC and environmental control system designed
to preserve the majestic frescoes of the Sistine Chapel. The frescoes that decorate the
Chapel’s walls and ceiling faced increasing environmental challenges as a result of a
dramatic rise in visitors. When the original HVAC system was installed over 20 years ago,
it was designed to accommodate 700 visitors at a time; today there are up to 2,000. Critical
factors in the design of HVAC systems for museums and buildings with historic artifacts
include air temperature, air humidity, air circulation, airborne pollutants and sound level.
In addition, it is important to maintain stable conditions to avoid cyclic effects and gradients that accelerate
aging of the artifacts. The Chapel’s new system addresses
these factors and new requirements based on studies
directed by the Vatican Museums, while providing a
level of redundancy that previously did not exist. Air
quality management within the Chapel now includes
adjusting volume flow to dilute CO2 concentrations
while maintaining still air at the frescoes. The system
uses special control schemes with video imaging to
track visitor levels and respond intelligently to maintain
precise conditions. This article reviews the history of
the frescoes, describes the new challenges and system
requirements, and details the modeling, analysis, and
design process used to develop this unique HVAC system
that went into service in October 2014.
Background
The Sistine Chapel was built in the time of Pope Sixtus
IV (1471 – 1484) as a religious chapel and site of conclaves
for electing new popes. The Chapel interior measures
134 ft (40.9 m) long by 44 ft (13.4 m) wide—the dimensions given in the Bible for the Temple of Solomon. The
vaulted ceiling is 68 ft (20.7 m) high, with 12 large windows arranged 10 m above the floor for light and ventilation. The Chapel walls are masonry construction ranging from 7 ft to 10 ft (2 m to 3 m) thick. The Chapel was
originally decorated with religious scenes by artists from
Michel Grabon is fellow and director, Carrier AdvanTE3C Solutions. Jackie Anderson is senior engineer, Carrier Air Management Systems Technology. Peter Bushnell is fellow, Air Management
Systems Technology. Aritz Calvo is controls engineer and William Chadwick is a staff engineer at Carrier AdvanTE3C Solutions. All are with United Technologies Building & Industrial Systems.
This article is published in ASHRAE Journal through a license granted by Carrier Corporation. Carrier retains copyright of this article’s content.
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TECHNICAL FEATURE FIGURE 1 System steady-state model showing the main system components and architecture.
Chapel
Air-Handling Unit (AHU)
Dry Cooler
Chiller
Skid
Airstream
AHU
1 VFD Fan
2 Cooling Coil
3 Reheating Coil
4 Electrical Heater
5 Steam Humidifier
Refrigerant Stream
Water Stream
Dry Cooler
6 Adiabatic Ramp
7 Dry Cooler VSD Fans
8 Dry Cooler Coils
Skid
9 Heat Recovery Heat Exchanger
10 Heating Coil Pump
11 Cooling Coil/Evaporator Loop Pump
12 Condenser/Dry Cooler Loop Pump
the late fifteenth century, including Rosselli, Perugino
and Botticelli. Shortly after its completion, the vaulted
ceiling developed serious problems due to settling of
the building. The original paintings on the ceiling and
upper walls were heavily damaged and the structure of
the building had to be repaired.
In 1508, Pope Julius II commissioned Michelangelo
di Lodovico Buonarroti Simoni, commonly known as
Michelangelo, to repaint the upper walls and the ceiling. Michelangelo completed the work between 1509
and 1512, using the fresco technique where water-based
pigments were applied on freshly laid plaster, resulting
in a vibrant and durable painting that is integral with
the wall. Michelangelo was later commissioned by Pope
Paul III to paint the wall behind the altar, which became
the site of the Last Judgment scene, painted during the
period 1536 – 1541.1 Michelangelo’s frescoes transformed
the Chapel into a masterpiece for the ages.
The Sistine Chapel frescoes have endured through the
centuries, despite exposure to dust, smoke and pollutants from candles, heaters, outdoor air and people. They
Chiller
13 Flooded S&T Evaporator
14 VSD Screw Compressor
15 S&T Condenser
16 Electronic Expansion Valve
were also treated with washes and varnishes, which
temporarily improved their appearance, but gradually
darkened. All of these effects greatly diminished their
original beauty. In 1960, the Vatican began an ambitious
34-year restoration project that removed the layers of
dirt and varnish and unveiled the frescoes in their original state.1,2 When the restoration was finally completed
in 1994, the results were spectacular.
In the 1980s the Vatican commissioned a survey of
the interior climate of the Chapel. The findings showed
highly variable interior temperature and humidity,
and significant temperature gradients. It was clear that
an environmental control system was needed. This
led to the launch of a major project, and in 1993 the
Chapel’s first HVAC system was completed. The system,
described in ASHRAE Journal in 1996, was designed
for the thermal load of approximately 700 visitors at a
time, with precise control of air temperature, relative
humidity, air velocities and pollutants.3 Designed to be
invisible and inaudible to occupants, this system performed very well, meeting its design intent. However,
J U N E 2015 ashrae.org A S H R A E J O U R N A L
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TECHNICAL FEATURE in just over 20 years—from 1990 to 2011—the number of
visitors to the Chapel increased dramatically, from 2
million per year to 5 million.4,5 In 2010 new concerns
about the Chapel environment were raised when a
“whitening” phenomenon was observed on several
areas of the frescoes.5
New Requirements for the Chapel Microclimate
The Vatican Museums formed another commission
in 2011 to study the microclimate in the Chapel. The
study found that the original HVAC system was not able
to handle the loads produced by the dramatic increase
in visitors. Based on these studies, a new set of requirements was formed and a new project was initiated to
design, develop, install and commission the next generation HVAC system for the Sistine Chapel. Critical
environmental and performance requirements for the
design of the new system and the Chapel space were
established as follows:
1. Increased airflow while maintaining still air at the
fresco surfaces;
2.Controlled air temperature and surface temperatures at the frescoes;
3.Controlled air relative humidity;
4.Controlled pollutant and CO2 concentrations;
5.Controlled dust and particle concentrations; and
6.Minimal noise level.
Air circulation is a critical factor to ensure uniform
conditions and avoid stagnant regions within the Chapel.
The new design requires greater airflow for increased
cooling capacity, dilution of CO2, and improved air mixing within the space. In addition, maintaining low velocities near the fresco surfaces is vital, as it reduces the rate
of particle deposition on the walls, avoids potential for
surface erosion, and limits CO2 exposure over the frescoes. Near-wall air velocity must not exceed 98.4 fpm
(0.5 m/s) anywhere in the Chapel.
To reduce thermal cycling, space dry-bulb air temperature setpoints remain the same as in the original system:
68°F (20°C) in winter and 77°F (25°C) in summer with a
smooth transition between seasons. It is also required to
maintain a wall surface temperature that is close to the
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adjacent space temperature (within ±1.8°F [±1°C]) to minimize dust deposits on the frescoes. A third temperature
constraint is to keep the wall surface temperature 18°F
(10°C) above the dew-point temperature at all times to
avoid any moisture condensation on the walls.
Extreme humidity levels pose great risk to the frescoes.
Relative humidity above 70% can lead to mold and mildew, while levels below 30% can cause excessive dryness.
The original system was able to maintain the wall surface temperature requirement, but its relative humidity
control response was not quick enough as visitor levels
increased dramatically. The new system relative humidity would continue to be bounded between 50% and 60%
with space dew point 18°F (10°C) below the wall surface
dry bulb (±1.8°F [±1°C]). However, advanced control
schemes would be needed to improve system response.
Recent findings showing high CO2 concentrations in
the Chapel were particularly alarming to the Vatican
Museum conservators. Elevated CO2 levels in the presence of moisture can initiate a chemical process that
favors the formation of soluble calcium bicarbonate
that can result in a whitish patina.5 The CO2 concentrations measured during the Vatican Museum study with
the original HVAC system showed peak concentrations
approaching 2,000 ppm, and it was determined that a
level below 800 ppm is required.
Control of airborne pollutants is also vital. It has been
shown that the presence of sulfur dioxide can combine
with water to form sulfuric acid, which can attack the
fresco materials. Likewise, dust that is carried on visitors’ shoes and clothing is a threat to the Chapel because
potentially harmful chemical and biological agents can
be introduced with it. The new system is required to
maintain supply air dust concentrations (PM 2.5) below
0.003 µg/ft3 (0.1 µg/m3).
Exceptionally low sound level is required in the
Chapel. The new system would need to be designed for
an A-weighted sound pressure level in the 30 – 35 dBA
range by controlling all airborne and structure-borne
sources and transmission paths.
New HVAC System
The new system is designed to maintain the critical
microclimate conditions for the preservation of the frescoes and the comfort level within the Chapel for an average
loading of 1,500 visitors, with peaks of up to 2,000 visitors
at a time. The system cooling capacity and supply airflow
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TECHNICAL FEATURE 26
A S H R A E J O U R N A L ashrae.org J U N E 2015
FIGURE 2 New HVAC system showing air-handling units (top), chiller skids (center)
© 2014 MUSEI VATICANI
© 2014 MUSEI VATICANI
and dry coolers (bottom) during the installation phase of the project.
© 2014 MUSEI VATICANI
were increased by a factor of three to handle the thermal
load and CO2 emissions from the visitors under all weather
conditions. Redundant subsystems were designed, each
capable of managing conditions in the Chapel under nearaverage loading. All aspects of the HVAC system were integrated with the building, respecting the architecture and
the artwork, while remaining inconspicuous to occupants.
Extensive simulations were carried out to guide the
system design and to develop the control and air-distribution strategies. The range of simulations included
both steady-state and dynamic system modeling, along
with extensive computational fluid dynamics (CFD)
analysis. The steady-state system modeling was used to
support subsystem design, component selection and
verification of critical operational parameters. The analysis used a proprietary computer program that models
component and system level performance and all thermodynamic state points using an iterative solver.6
A schematic of the main subsystem from the steady-state
model is shown in Figure 1 (Page 21). Each redundant system
consists of a 165 ton (580 kW), R-134a variable speed watercooled screw chiller, an air-handling unit and a dry cooler
for heat rejection. Each air-handling unit has six levels of
filtration, variable speed supply fan, cooling coil, reheat
coil, variable capacity electric heater and a steam humidifier. A reheat coil is connected to the condenser water loop
to reduce the load of the dry coolers and improve system
efficiency. Condensate from the dehumidification process
is collected and sprayed into the dry cooler inlet airstream
for evaporative cooling enhancement and system efficiency improvement. In normal full-load operation, each
subsystem runs at 66% capacity, and each can manage 75%
of full load, twice the capacity of the original system.
Each air-handling unit has three filter stages that
apply six individual filters to treat return and outdoor
air. The filter stages address different airborne pollutants: one stage for mold, pollen and smoke particles;
one stage for indoor gaseous pollutants; and one stage
for outdoor gaseous pollutants. These filters reduce
the presence of airborne pollutants and manage sulfur
dioxide concentration below 0.35 ppb, nitrogen dioxide below 2.65 ppb, ozone below 0.94 ppb, total volatile
organic compounds (TVOC) below 0.006 µg/ft3 (0.2
µg/m3), and formaldehyde below 1.8 µg/ft3 (65 µg/m3).
A preassembled modular system approach was used to
enable complete system verification prior to shipment to
the site, and to simplify installation and commissioning.
Primary modules were chiller skids, air-handling units
and dry coolers. Furthermore, a temporary air-conditioning system was installed and operated during a fourmonth period while the original system was removed
and the new system was being installed.
All equipment was installed on terraces adjacent to the
Chapel’s south-facing masonry wall (Figure 2), and silencers
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TECHNICAL FEATURE were added in the common supply
air duct and in each individual return
duct for sound attenuation. All ducts
are stainless steel and the diffusers
include sound-absorbing insulation.
Additionally, the outdoor equipment
is surrounded with a sound enclosure
to minimize external sound radiation,
and the dry coolers are designed with
low-noise fans to reduce sound levels
generated by air movement in the
outdoor space.
FIGURE 3 (LEFT) CFD simulation of the Chapel. FIGURE 4 (RIGHT) Diffuser installation.
Air Distribution in the Chapel
At maximum load, the new system
was designed to deliver airflow of
31,785 cfm (15 m3/s), three times the
original system. The increased airflow was needed for cooling capacity,
dilution of CO2, and improved mixing during peak
occupancy. This was especially challenging due to the
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PHOTOS © 2014 MUSEI VATICANI
conflicting requirements of still air at the frescoes and
limited access points for air supply and return. Since the
entire Chapel is essentially covered in artwork, no new
openings could be added, and only the existing six penetrations under the windows on the south wall could be
used with minor modifications.
Extensive studies of options including numerous CFD
simulations of the Chapel were carried out using a commercial code.7 The objective was to seek methods to rapidly distribute the airflow in the space with a high level of
mixing, while maintaining low velocities near the fresco
surfaces. The CFD models were set up to approximate
the physics of the problem, including building external
thermal loads, conduction through the massive masonry
walls, thermal load and CO2 emissions due to people
in the space, diffuser discharge jet flow and buoyancy
effects. Special attention was given to jet shear layers and
trajectories to ensure that good estimates of near-wall
velocities were resolved. Most of the simulations were
steady-state using the k-ε turbulence model on a halflength symmetric approximation of the Chapel.
In the final design, air is delivered to the Chapel through
four diffusers below the four center windows in the south
wall. Two air returns are located under the outer two
windows so the system can use a combination of outdoor
and return air, while maintaining slight positive pressure in the Chapel. Figure 3 shows the simulated flow pattern from two of the supply diffusers at maximum load
and airflow. The jet can be seen to spread and fall due to
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buoyancy effects, creating a main recirculaFIGURE 5 Visitor analytics used to enhance system control.
tion zone in the lower section of the chapel
with a portion of the air diverting toward the
ceiling frescoes. All near-wall velocities are
maintained below 98.4 fpm (0.5 m/s) with
the highest velocities along the bottom half of
the north wall. Variable speed fans are used
to further reduce air velocities in the space as
occupancy decreases.
Specially designed diffusers, shown in Figure
4, were developed for the Chapel. Supply air
is ducted under the windows at high velocity.
The diffusers then rapidly expand the flow,
recovering dynamic pressure and producing low-speed discharge jets. The 2.5 area
ratio diffusers use horizontal and vertical
flow splitters to control diffusion and add
structural support. A diamond-shaped perforated outlet baffle is used to further augment
diffusion within the very limited space available. Detailed validation tests were carried out on a quarter-scale model
CFD modeling was performed for a single diffuser and
under isothermal conditions. Reynolds number was
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© 2014 MUSEI VATICANI
TECHNICAL FEATURE Advertisement formerly in this space.
TECHNICAL FEATURE matched and jet flow patterns were validated with good
results. The scale model was also used for diffuser selfgenerated noise evaluation.
System Control
Specialized controls were developed for the new system
using an extensive set of sensors and video monitoring. Sensors include 17 air dry-bulb, 13 dew-point and 16
surface temperature sensors distributed at key locations
on the walls of the Chapel, as well as two CO2 concentration sensors and two dry-bulb temperature and relative
humidity sensors installed in the return air ducts of the
air-handling units. A video system, shown in Figure 5 (Page
30), was developed to track the visitor level in the Chapel to
enable rapid response to the thermal, moisture, and CO2
loads. A first control layer uses a feed-forward approach to
set the correct supply airflow, outdoor air ratio, and chilled
water temperature as function of visitor level. A second
feed-back control layer adjusts system capacity to correct
temperature and humidity deviations from setpoint. Every
day a graphical report is automatically generated showing
the performance of the previous day in terms of system
control accuracy and stability.
Dynamic system models were built using a commercial
simulation code to evaluate transients and to support
control algorithm development.8 These simulations
included the effects of variable seasonal conditions,
latent and sensible loading, CO2 concentration, occupancy schedule and control time delays. The simulations
ensured proper control of temperature, relative humidity, pollutants and particles.
The new control system calculates a moving 24-hour
outdoor air temperature average and maintains indoor
air temperature equal to this average value when outdoor temperatures fall between the desired 68°F (20°C) to
77°F (25°C) range. If the outdoor air temperature is outside
this range, the system holds the Chapel interior temperature at 68°F (20°C) on cooler days and 77°F (25°C) on
warmer days. This control profile reduces the temperature
gradient within the Chapel and the potential for thermally
induced stress on the frescoes. CFD modeling was used to
analyze interior conditions during peak occupancy and to
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TECHNICAL FEATURE CO2 Concentration (ppm)
verify well-mixed thermal conditions
FIGURE 6 CFD analysis of temperature distribution within the Chapel.
as shown in Figure 6.
The system adjusts the total and
return airflows based on visitor
level from the video system to keep
CO2 levels near or below 800 ppm.
If concentrations exceed 800 ppm
the return air ratio is decreased
and the total airflow is increased.
As discussed earlier, an increase
in airflow three times the original
system was needed to dilute the
space with outdoor air during peak
FIGURE 7 CO2 levels in the Chapel with 1,100 visitors.
occupancy. Figure 7 shows the CO2 concentration com1,600
parison between the original and new systems during
Legacy System
New System
similar single days, each with 1,100 visitors.
1,400
The system continuously cycles the supply air through
1,200
the six filters to maintain acceptable dust and pollutant concentrations inside the Chapel, while adjusting
1,000
the ratio of return air to outdoor air to provide proper
800
CO2 dilution. During night operation, return air ratio is
increased to remove any remaining contaminants that
600
have accumulated during the day.
Summary
The new HVAC system began operation in October
2014. The Vatican Museums held a special conference:
“The Sistine Chapel Twenty Years Later: New Breath,
New Light,” Oct. 30–31, 2014, coinciding with the 450th
anniversary of the death of Michelangelo. The conference
reflected on the restoration of the frescoes, their current
health, the science and analysis regarding their continuing conservation, the new HVAC system, and a new lighting system for the Chapel. The conference attendees had
the opportunity to see the Chapel with its new HVAC system in operation, along with the new LED lighting. System
performance has been very good, operating as designed
based on numerous sensors and data. The system will be
monitored continuously in the months and years ahead to
ensure a healthy environment for the Sistine Chapel frescoes and their preservation for future generations.
Acknowledgments
This project was completed by United Technologies
Building & Industrial Systems and its Carrier operations.
The authors would like to thank the Vatican Museums for
their collaboration and confidence throughout the project,
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400
200
0
12a 4a 8a 12p3p 5p12a
Proger S.p.A. for their engineering and installation expertise, and the wide range of teams in United Technologies
and Carrier for their support and contributions to
this unique project. Finally, we would like to thank Dr.
Charles Bullock, a retired Carrier engineer and ASHRAE
member, for providing insight from his experience leading
the design of the original HVAC system for the Chapel.
References
1. Mancinelli, F. 1993. The Sistine Chapel. Edizioni Musei Vaticani.
2.Paolucci, A. 2010. The Sistine Chapel. Edizioni Musei Vaticani.
3.Bullock, C., F. Philip, S. Pennati. 1996. “The Sistine Chapel:
HVAC design for special use buildings.” ASHRAE Journal (4)42–56.
4.Cimino V., P. Mandrioli, M. Matteini, U. Santamaria. 2012.
Microclimatic Conditions of the Sistine Chapel. Vatican Museums
Internal Report.
5.Vatican Museums. 2014. The Sistine Chapel Twenty Years
Later: New Breath, New Light.
6.Carrier. IPM System Modeling Software. Syracuse, N.Y.: UTC
Carrier Corporation.
7. ANSYS. Fluent. Canonsburg, Pa.: ANSYS Inc.
8.University of Wisconsin. TRNSYS: Transient Systems Simulation Program. Madison, Wis.: University of Wisconsin.
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