Historic Preservation - Technical Procedures

Preservation Briefs: 24 Heating, Ventilating And Cooling Historic Buildings: Problems And Recommended Approaches
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Preservation Briefs 24, National Park Service, Pad
Basic Mechanical Requirements
Last Modified:
Preservation Briefs: 24 Heating, Ventilating And Cooling Historic Buildings: Problems And Recommended Approaches
Last Modified:




The link immediately below connects to the latest version of Preservation Brief 24:



Sharon C. Park, AIA

This standard includes the bulk of information contained in the
original Preservation Brief developed by the National Park Service.
To obtain a complete copy of this brief, including figures and
illustrations, please contact:  

              Superintendent of Documents
              P.O. Box 371954
              Pittsburgh, PA  15250-7954

              GPO #024-005-01090-4

Please call the Publication Order Information Desk at 202/783-3238
or FAX 202/512-2250 to verify price and availability.  


The need for modern mechanical systems is one of the most common
reasons to undertake work on historic buildings.  Such work
includes upgrading older mechanical systems, improving the energy
efficiency of existing buildings, installing new heating,
ventilation or air conditioning (HVAC) systems, or installing a
climate control system with humidification and dehumidification
capabilities.  Decisions to install new HVAC or climate control
systems often result from concern for occupant health and comfort,
the desire to make older buildings marketable, or the need to
provide specialized environments for operating computers, storing
artifacts, or displaying museum collections.  Unfortunately,
occupant comfort and concerns for the objects within the building
are sometimes given greater consideration than the building itself.
In too many cases, applying modern standards of interior climate
comfort to historic buildings has proven detrimental to historic
materials and decorative finishes.

This Preservation Brief underscores the importance of careful
planning in order to balance the preservation objectives with
interior climate needs of the building.  It is not intended as a
technical guide to calculate tonnage or to size piping or ductwork.
Rather, this Brief identifies some of the problems associated with
installing mechanical systems in historic buildings and recommends
approaches to minimizing the physical and visual damage associated
with installing and maintaining these new or upgraded systems.

Historic buildings are not easily adapted to house modern precision
mechanical systems.  Careful planning must be provided early on to
ensure that decisions made during the design and installation
phases of a new system are appropriate.  Since new mechanical and
other related systems, such as electrical and fire suppression, can
use up to 10% of a building's square footage and 30%-40% of an
overall rehabilitation budget, decisions must be made in a
systematic and coordinated manner.  The installation of
inappropriate mechanical systems may result in any or all of the

    -  Large sections of historic materials are removed to install
    or house new systems.

    -  Historic structural systems are weakened by carrying the
    weight of, and sustaining vibrations from, large equipment.

    -  Moisture introduced into the building as part of a new
    system migrates into historic materials and causes damage,
    including biodegradation, freeze/thaw action, and surface

    -  Exterior cladding or interior finishes are stripped to
    install new vapor barriers and insulation.

    -  Historic finishes, features, and spaces are altered by
    dropped ceilings and boxed chases or by poorly located
    grilles, registers, and equipment.

    -  Systems that are too large or too small are installed
    before there is a clearly planned use or a new tenant.

For historic properties it is critical to understand what spaces,
features, and finishes are historic in the building, what should be
retained, and what the realistic heating, ventilating, and cooling
needs are for the building, its occupants, and its contents.  A
systematic approach, involving preservation planning, preservation
design, and a follow-up program of monitoring and maintenance, can
ensure that new systems are successfully added--or existing systems
are suitably upgraded--while preserving the historic integrity of
the building.

No set formula exists for determining what type of mechanical
system is best for a specific building.  Each building and its
needs must be evaluated separately.  Some buildings will be so
significant that every effort must be made to protect the historic
materials and systems in place with minimal intrusion from new
systems.  Some buildings will have museum collections that need
special climate control.  In such cases, curatorial needs must be
considered--but not to the ultimate detriment of the historic
building resource.  Other buildings will be rehabilitated for
commercial use.  For them, a variety of systems might be
acceptable, as long as significant spaces, features, and finishes
are retained.

Most mechanical systems require upgrading or replacement within
15-30 years due to wear and tear or the availability of improved
technology.  Therefore, historic buildings should not be greatly
altered or otherwise sacrificed in an effort to meet short-term
systems objectives.


The history of mechanical systems in buildings involves a study of
inventions and ingenuity as building owners, architects, and
engineers devised ways to improve the interior climate of their
buildings.  Following are highlights in the evolution of heating,
ventilating, and cooling systems in historic buildings.


Early heating and ventilation in America relied upon common sense
methods of managing the environment.  Builders purposely sited
houses to capture winter sun and prevailing summer cross breezes;
they chose materials that could help protect the inhabitants from
the elements, and took precautions against precipitation and
damaging drainage patterns.  The location and sizes of windows,
doors, porches, and the floor plan itself often evolved to maximize
ventilation.  Heating was primarily from fireplaces or stoves and,
therefore, was at the source of delivery.  In 1744, Benjamin
Franklin designed his "Pennsylvania stove" with a fresh air intake
in order to maximize the heat radiated into the room and to
minimize annoying smoke.

Thermal insulation was rudimentary--often wattle and daub, brick
and wood nogging.  The comfort level for occupants was low, but the
relatively small difference between internal and external
temperatures and relative humidity allowed building materials to
expand and contract with the seasons.

Regional styles and architectural features reflected regional
climates.  In warm, dry and sunny climates, thick adobe walls
offered shelter from the sun and kept the inside temperatures cool.
Verandas, courtyards, porches, and high ceilings also reduced the
impact of the sun.  Hot and humid climates called for elevated
living floors, louvered grilles and shutters, balconies, and
interior courtyards to help circulate air.


The industrial revolution provided the technological means for
controlling the environment for the first time.  The dual
developments of steam energy from coal and industrial mass
production made possible early central heating systems with
distribution of heated air or steam using metal ducts or pipes.
Improvements were made to early wrought iron boilers and by late
century, steam and low pressure hot water radiator systems were in
common use, both in offices and residences. Some large
institutional buildings heated air in furnaces and distributed it
throughout the building in brick flues with a network of metal
pipes delivering heated air to individual rooms.  Residential
designs of the period often used gravity hot air systems utilizing
decorative floor and ceiling grilles.

Ventilation became more scientific and the introduction of fresh
air into buildings became an important component of heating and
cooling.  Improved forced air ventilation became possible in
mid-century with the introduction of power-driven fans.
Architectural features such as porches, awnings, window and door
transoms, large open-work iron roof trusses, roof monitors,
cupolas, skylights and clerestory windows helped to dissipate heat
and provide healthy ventilation.

Cavity wall construction, popular in masonry structures, improved
the insulating qualities of a building and also provided a natural
cavity for the dissipation of moisture produced on the interior of
the building.  In some buildings, cinder chips and broken masonry
filler between structural iron beams and jack arch floor vaults
provided thermal insulation as well as fireproofing.  Mineral wool
and cork were new sources of lightweight insulation and were
forerunners of contemporary batt and blanket insulation.

The technology of the age, however, was not sufficient to produce
"tight" buildings.  There was still only a moderate difference
between internal and external temperatures.  This was due, in part,
to the limitations of early insulation, the almost exclusive use of
single glazed windows, and the absence of air-tight construction.
The presence of ventilating fans and the reliance on architectural
features, such as operable windows, cupolas and transoms, allowed
sufficient air movement to keep buildings well ventilated.
Building materials could behave in a fairly traditional way,
expanding and contracting with the seasons.


The twentieth century saw intensive development of new technologies
and the notion of fully integrating mechanical systems.  Oil and
gas furnaces developed in the nineteenth century were improved and
made more efficient, with electricity becoming the critical source
of power for building systems in the latter half of the century.
Forced air heating systems with ducts and registers became popular
for all types of buildings and allowed architects to experiment
with architectural forms free from mechanical encumbrances.  In the
1920s large-scale theaters and auditoriums introduced central air
conditioning, and by mid-century forced air systems which combined
heating and air conditioning in the same ductwork set a new
standard for comfort and convenience.  The combination and
coordination of a variety of systems came together in the
post-World War II high rise buildings; complex heating and air
conditioning plants, electric elevators, mechanical towers,
ventilation fans, and full service electric lighting were
integrated into the building's design.

The insulating qualities of building materials improved. Synthetic
materials, such as spun fiberglass batt insulation, were fully
developed by mid-century.  Prototypes of insulated thermal glazing
and integral storm window systems were promoted in construction
journals.  Caulking to seal out perimeter air around window and
door openings became a standard construction detail.

The last quarter of the twentieth century has seen making HVAC
systems more energy efficient and better integrated.  The use of
vapor barriers to control moisture migration, thermally efficient
windows, caulking and gaskets, compressed thin wall insulation,
have become standard practice.  New integrated systems now combine
interior climate control with fire suppression, lighting, air
filtration, temperature and humidity control, and security
detection.  Computers regulate the performance of these integrated
systems based on the time of day, day of the week, occupancy, and
outside ambient temperature.


Although twentieth century mechanical systems technology has had a
tremendous impact on making historic buildings comfortable, the
introduction of these new systems in older buildings is not without
problems.  The attempt to meet and maintain modern climate control
standards may in fact be damaging to historic resources.  Modern
systems are often over-designed to compensate for inherent
inefficiencies of some historic buildings materials and plan
layouts.  Energy retrofit measures, such as installing exterior
wall insulation and vapor barriers or the sealing of operable
window and vents, ultimately affect the performance and can reduce
the life of aging historic materials.

In general, the greater the differential between the interior and
exterior temperature and humidity levels, the greater the potential
for damage.  As natural vapor pressure moves moisture from a warm
area to a colder, dryer area, condensation will occur on or in
building materials in the colder area.  Too little humidity in
winter, for example, can dry and crack historic wooden or painted
surfaces.  Too much humidity in winter causes moisture to collect
on cold surfaces, such as windows, or to migrate into walls.  As a
result, this condensation deteriorates wooden or metal windows and
causes rotting of walls and wooden structural elements, dampening
insulation and holding moisture against exterior surfaces.
Moisture migration through walls can cause the corrosion of metal
anchors, angles, nails or wire lath, can blister and peel exterior
paint, or can leave efflorescence and salt deposits on exterior
masonry.  In cold climates, freeze/thaw damage can result from
excessive moisture in external walls.

To avoid these types of damage to an historic building, it is
important to understand how building components work together as a
system.  Methods for controlling interior temperature and humidity
and improving ventilation must be considered in any new or upgraded
HVAC or climate control system.  While certain energy retrofit
measures will have a positive effect on the overall building,
installing effective vapor barriers in historic walls is difficult
and often results in destruction of significant historic materials.


Climate control systems are generally classified according to the
medium used to condition the temperature: air, water, or a
combination of both.  The complexity of choices facing a building
owner or manager means that a systematic approach is critical in
determining the most suitable system for a building, its contents,
and its occupants.  No matter which system is installed, a change
in the interior climate will result.  This physical change will in
turn affect how the building materials perform.  New registers,
grilles, cabinets, or other accessories associated with the new
mechanical system will also visually change the interior (and
sometimes the exterior) appearance of the building.  Regardless of
the type or extent of a mechanical system, the owner of an historic
building should know before a system is installed what it will look
like and what problems can be anticipated during the life of that
system.  The potential harm to a building and costs to an owner of
selecting the wrong mechanical system are very great.

The use of a building and its contents will largely determine the
best type of mechanical system.  The historic building materials
and construction technology as well as the size and availability of
secondary spaces within the historic structure will affect the
choice of a system.  It may be necessary to investigate a
combination of systems.  In each case, the needs of the user, the
needs of the building, and the needs of a collection or equipment
must be considered.  It may not be necessary to have a
comprehensive climate control system if climate-sensitive objects
can be accommodated in special areas or climate-controlled display
cases.  It may not be necessary to have central air conditioning in
a mild climate if natural ventilation systems can be improved
through the use of operable windows, awnings, exhaust fans, and
other "low-tech" means.  Modern standards for climate control
developed for new construction may not be achievable or desirable
for historic buildings.  In each case, the lowest level of
intervention needed to successfully accomplish the job should be

Before a system is chosen, the following planning steps are

1.   DETERMINE THE USE OF THE BUILDING.  The proposed use of the
    building (museum, commercial, residential, retail) will
    influence the type of system that should be installed.  The
    number of people and functions to be housed in a building will
    establish the level of comfort and service that must be
    provided.  Avoid uses that require major modifications to
    significant architectural spaces.  What is the intensity of
    use of the building: intermittent or constant use, special
    events or seasonal events?  Will the use of the building
    require major new services such as restaurants, laundries,
    kitchens, locker rooms, or other areas that generate moisture
    that may exacerbate climate control within the historic space?
    In the context of historic preservation, uses that require
    radical reconfigurations of historic spaces are inappropriate
    for the building.

2.   ASSEMBLE A QUALIFIED TEAM.  This team ideally should consist
    of a preservation architect, mechanical engineer, electrical
    engineer, structural engineer, and preservation consultants,
    each knowledgeable in codes and local requirements.  If a
    special use (church, museum, art studio) or a collection is
    involved, a specialist familiar with the mechanical
    requirements of that building type or collection should also
    be hired.

    Team members should be familiar with the needs of historic
    buildings and be able to balance complex factors: the
    preservation of the historic architecture (aesthetics and
    conservation), requirements imposed by mechanical systems
    (quantified heating and cooling loads), building codes (health
    and safety), tenant requirements (quality of comfort, ease of
    operation), access (maintenance and future replacement), and
    the overall; cost to the owner.

    ITS SYSTEMS.  What are the existing construction materials and
    mechanical systems?  What condition are they in and are they
    reusable?  Where are existing chillers, boilers, air handlers,
    or cooling towers located?  Look at the condition of all other
    services that may benefit from being integrated into a new
    system, such as electrical and fire suppression systems.
    Where can energy efficiency be improved to help downsize any
    new equipment added, and which of the historic features, e.g.
    shutters, awnings, skylights, can be reused? Evaluate air
    infiltration through the exterior envelope; monitor the
    interior for temperature and humidity levels with
    hygrothermographs for at least a year.  Identify building,
    site, or equipment deficiencies or the presence of asbestos
    that must be corrected prior to the installation or upgrading
    of mechanical systems.

    FEATURES TO BE PRESERVED.  Significant architectural spaces,
    finishes and features should be identified and evaluated at
    the outset to ensure their preservation. This includes
    significant existing mechanical systems or elements such as
    hot water radiators, decorative grilles, elaborate
    switchplates, and nonmechanical architectural features such as
    cupolas, transoms, or porches.  Identify non-significant
    spaces where mechanical equipment can be placed and secondary
    spaces where equipment and distribution runs on both a
    horizontal and vertical basis can be located.  Appropriate
    secondary spaces for housing equipment might include attics,
    basements, penthouses, mezzanines, false ceiling or floor
    cavities, vertical chases, stair towers, closets, or exterior
    below-grade vaults.

    their representatives should meet early and often with local
    officials.  Legal requirements should be checked; for example,
    can existing ductwork be reused or modified with dampers?  Is
    asbestos abatement required?  What are the energy, fire, and
    safety codes and standards in place, and how can they be met
    while maintaining the historic character of the building?  How
    are fire separation walls and rated mechanical systems to be
    handled between multiple tenants?  Is there a requirement for
    fresh air intake for stair towers that will affect the
    exterior appearance of the building?  Many of the health,
    energy, and safety code requirements will influence decisions
    made for mechanical equipment for climate control.  It is
    importance to know what they are before the design phase

    or feasibility studies should be developed to balance the
    benefits and drawbacks of various systems. Factors to consider
    include heating and/or cooling, fuel type, distribution
    system, control devices, generating equipment and accessories
    such as filtration, and humidification.  What are the initial
    installation costs, projected fuel costs, long-term
    maintenance, and life-cycle costs of these components and
    systems?  Are parts of an existing system being reused and
    upgraded? The benefits of added ventilation should not be
    overlooked.  What are the trade-offs between one large central
    system and multiple smaller systems?  Should there be a forced
    air ducted system, a 2-pipe fan coil system, or a combined
    water and air system?  What space is available for the
    equipment and distribution system? Assess the fire-risk levels
    of various fuels.  Understand the advantages and disadvantages
    of the various types of mechanical systems available.  Then
    evaluate each of these systems in light of the preservation
    objectives established during the design phase of planning.


In designing a system, it is important to anticipate how it will be
installed, how damage to historic materials can be minimized, and
how visible the new mechanical system will be within the restored
or rehabilitated spaces.  Mechanical equipment space needs are
often overwhelming.  In some cases, it may be advantageous to look
for locations outside of the building, including ground vaults, to
house some of the equipment, but only if it there is no adverse
impact to the historic landscape or adjacent archeological
resources. Various means for reducing the heating and cooling loads
(and thereby the size of the equipment) should be investigated.
This might mean reducing slightly the comfort levels of the
interior, increasing the number of climate control zones, or
improving the energy efficiency of the building.

The following activities are suggested during the design phase of
the new system:

    SYSTEM.  New systems should be installed with a minimum of
    damage to the resource and should be visually compatible with
    the architecture of the building.  They should be installed in
    a way that is easy to service, maintain, and upgrade in the
    future.  There should be safety and back-up monitors in place
    if buildings have collections, computer rooms, storage vaults
    or special conditions that need monitoring.  The new systems
    should work within the structural limits of the historic
    building.  They should produce no undue vibration, no undue
    noise, no dust or mold, and no excess moisture that could
    damage the historic building materials.  If any equipment is
    to be located outside of the building, there should be no
    impact to the historic appearance of building or site, and
    there should be no impact on archeological resources.

    SYSTEM.  The use of the building will determine the level of
    interior comfort and climate control.  Sometimes, various
    temperature zones may safely be created within an historic
    building.  This zoned approach may be appropriate for
    buildings with specialized collections storage, for buildings
    with mixed uses, or for large buildings with different
    external exposures, occupancy patterns, and delivery schedules
    for controlled air. Special archives, storage vaults or
    computer rooms may need a completely different climate control
    from the rest of the building.  Determine temperature and
    humidity levels for occupants and collections and ventilation
    requirements between differing zones.  Establish if the system
    is to run 24 hours a day or only during operating or business
    hours.  Determine what controls are optimum (manual, computer,
    preset automatic, or other).  The size and location of the
    equipment to handle these different situations will ultimately
    affect the design of the overall system as well.

    ARCHITECTURE.  Design criteria for the new system should be
    based on the type of architecture of the historic resource.
    Consideration should be given as to whether or not the
    delivery system is visible or hidden.  Utilitarian and
    industrial spaces may be capable of accepting a more visible
    and functional system.  More formal, ornate spaces which may
    be part of an interpretive program may require a less visible
    or disguised system.  A ducted system should be installed
    without ripping into or boxing out large sections of floors,
    walls, or ceilings.  A wet pipe system should be installed so
    that hidden leaks will not damage important decorative
    finishes.  In each case, not only the type of system (air,
    water, combination), but its distribution (duct, pipe) and
    delivery appearance (grilles, cabinets, or registers) must be
    evaluated.  It may be necessary to use a combination of
    different systems in order to preserve the historic building.
    Existing chases should be reused whenever possible.

    The ideal system may not be achievable for each historic
    resource due to cost, space limitations, code requirements, or
    other factors beyond the owner's control.  However,
    significant historic spaces, finishes, and features can be
    preserved in almost every case, even given these limitations.
    For example, if some ceiling areas must be slightly lowered to
    accommodate ductwork or piping, these should be in secondary
    areas away from decorative ceilings or tall windows.  If
    modern fan coil terminal units are to be visible in historic
    spaces, consideration should be given to custom designing the
    cabinets or to using smaller units in more locations to
    diminish their impact.  If grilles and registers are to be
    located in significant spaces, they should be designed to work
    within the geometry or placement of decorative elements.  All
    new elements, such as ducts, registers, pipe-runs, and
    mechanical equipment should be installed in a reversible
    manner to be removed in the future without further damage to
    the building.


Once the system is installed, it will require routine maintenance
and balancing to ensure that the proper performance levels are
achieved.  In some cases, extremely sophisticated, computerized
systems have been developed to control interior climates, but these
still need monitoring by trained staff.  If collection exhibits and
archival storage are important to the resource, the climate control
system will require constant monitoring and tuning.  Back-up
systems are also needed to prevent damage when the main system is
not working.  The owner, manager, or chief of maintenance should be
aware of all aspects of the new climate control system and have a
plan of action before it is installed.

Regular training sessions on operating, monitoring, and maintaining
the new system should be held for both curatorial and building
maintenance staff.  If there are curatorial reasons to maintain
constant temperature or humidity levels, only individuals
thoroughly trained in how the HVAC systems operates should be able
to adjust thermostats.  Ill-informed and haphazard attempts to
adjust comfort levels, or to save energy over weekends and
holidays, can cause great damage.

Maintenance staff should learn how to operate, monitor, and
maintain the mechanical equipment.  They must know where the
maintenance manuals are kept.  Routine maintenance schedules must
be developed for changing and cleaning filters, vents, and
condensate pans to control fungus, mold, and other organisms that
are dangerous to health.  Such growths can harm both inhabitants
and equipment.  (In piped systems, for example, molds in condensate
pans can block drainage lines and cause an overflow to leak onto
finished surfaces).  Maintenance staff should also be able to
monitor the appropriate gauges, dials, and thermographs.  Staff
must be trained to intervene in emergencies, to know where the
master controls are, and whom to call in an emergency.  As new
personnel are hired, they will also require maintenance training.

In addition to regular cyclical maintenance, thorough inspections
should be undertaken from time to time to evaluate the continued
performance of the climate control system.  As the system ages,
parts are likely to fail, and signs of trouble may appear.
Inadequately ventilated areas may smell musty.  Wall surfaces may
show staining, wet patches, bubbling or other signs of moisture
damage.  Routine tests for air quality, humidity, and temperature
should indicate if the system is performing properly.  If there is
damage as a result of the new system, it should be repaired
immediately and then closely monitored to ensure complete repair.

Equipment must be accessible for maintenance and should be visible
for easy inspection.  Moreover, since mechanical systems last only
15-30 years, the system itself must be "reversible."  That is, the
system must be installed in such a way that later removal will not
damage the building In addition to servicing, the back-up monitors
that signal malfunctioning equipment must be routinely checked,
adjusted, and maintained.  Checklists should be developed to ensure
that all aspects of routine maintenance are completed and that data
is reported to the building manager.


The successful integration of new systems in historic buildings can
be challenging.  Meeting modern HVAC requirements for human comfort
or installing controlled climates for museum collections or for the
operation of complex computer equipment can result in both visual
and physical damage to historic resources.  Owners of historic
buildings must be aware that the final result will involve
balancing multiple needs; no perfect heating, ventilating, and air
conditioning system exists.  In undertaking changes to historic
buildings, it is best to have the advice and input of trained
professionals who can:

    -  Assess the condition of the historic building,

    -  Evaluate the significant elements that should be preserved
    or reused,


Last Reviewed 2012-09-06