Heating, Ventilating, and Cooling Historic Buildings
History of Mechanical Systems
Climate Control and Preservation
Planning the New System
Overview of HVAC Systems
Designing the new system
Systems Performance and Maintenance
HVAC Do's and Don'ts
Conclusion
Bibliography
The need for modern mechanical systems is one of the most common reasonsto undertake work on historic buildings. Such work includes upgrading oldermechanical systems, improving the energy efficiency of existing buildings,installing new heating, ventilation or air conditioning (HVAC) systems,or--particularly for museums--installing a climate control system withhumidification and dehumidification capabilities. Decisions to installnew HVAC or climate control systems often result from concern for occupanthealth and comfort, the desire to make older buildings marketable, or theneed to provide specialized environments for operating computers, storingartifacts, or displaying museum collections. Unfortunately, occupant comfortand concerns for the objects within the building are sometimes given greaterconsideration than the building itself. In too many cases, applying modernstandards of interior climate comfort to historic buildings has provendetrimental to historic materials and decorative finishes. This Preservation Brief underscores the importance of careful planningin order to balance the preservation objectives with interior climate needsof the building. It is not intended as a technical guide to calculate tonnageor to size piping or ductwork. Rather, this Brief identifies some of theproblems associated with installing mechanical systems in historic buildingsand recommends approaches to minimizing the physical and visual damageassociated with installing and maintaining these new or upgraded systems. Historic buildings are not easily adapted to house modern precisionmechanical systems. Careful planning must be provided early on to ensurethat decisions made during the design and installation phases of a newsystem are appropriate. Since new mechanical and other related systems,such as electrical and fire suppression, can use up to 10% of a building'ssquare footage and 30%-40% of an overall rehabilitation budget, decisionsmust be made in a systematic and coordinated manner. The installation ofinappropriate mechanical systems may result in any or all of the following: For historic properties it is critical to understand what spaces, features,and finishes are historic in the building, what should be retained, andwhat the realistic heating, ventilating, and cooling needs are for thebuilding, its occupants, and its contents. A systematic approach, involvingpreservation planning, preservation design, and a follow-up program of monitoringand maintenance, can ensure that new systems are successfully added--orexisting systems are suitably upgraded--while preserving the historic integrityof 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 15-30years due to wear and tear or the availability of improved technology.Therefore, historic buildings should not be greatly altered or otherwisesacrificed in an effort to meet short-term systems objectives. The history of mechanical systems in buildings involves a study of inventionsand ingenuity as building owners, architects, and engineers devised waysto improve the interior climate of their buildings. Following are highlightsin the evolution of heating, ventilating, and cooling systems in historicbuildings. Eighteenth Century. Early heating and ventilation in America reliedupon common sense methods of managing the environment. Builderspurposely sited houses to capture winter sun and prevailing summer crossbreezes; they chose materials that could help protect the inhabitants fromthe elements, and took precautions against precipitation and damaging drainagepatterns. The location and sizes of windows, doors, porches, and the floorplan itself often evolved to maximize ventilation. Heating was primarilyfrom 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 theroom and to minimize annoying smoke. Thermal insulation was rudimentary--often wattle and daub, brick andwood nogging. The comfort level for occupants was low, but the relativelysmall difference between internal and external temperatures and relativehumidity 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 fromthe sun and kept the inside temperatures cool. Verandas, courtyards, porches,and high ceilings also reduced the impact of the sun. Hot and humid climatescalled for elevated living floors, louvered grilles and shutters, balconies,and interior courtyards to help circulate air. Nineteenth Century. The industrial revolution provided the technologicalmeans for controlling the environment for the first time.The dual developments of steam energy from coal and industrial mass productionmade possible early central heating systems with distribution of heatedair or steam using metal ducts or pipes. Improvements were made to earlywrought iron boilers and by late century, steam and low pressure hot waterradiator systems were in common use, both in offices and residences. Somelarge institutional buildings heated air in furnaces and distributed itthroughout the building in brick flues with a network of metal pipes deliveringheated air to individual rooms. Residential designs of the period oftenused gravity hot air systems utilizing decorative floor and ceiling grilles. Ventilation became more scientific and the introduction of fresh airinto buildings became an important component of heating and cooling. Improvedforced air ventilation became possible in mid-century with the introductionof power-driven fans. Architectural features such as porches, awnings, windowand door transoms, large openwork iron roof trusses, roof monitors, cupolas,skylights and clerestory windows helped to dissipate heat and provide healthyventilation. Cavity wall construction, popular in masonry structures, improved theinsulating qualities of a building and also provided a natural cavity forthe dissipation of moisture produced on the interior of the building. Insome buildings, cinder chips and broken masonry filler between structuraliron beams and jack arch floor vaults provided thermal insulation as wellas fireproofing. Mineral wool and cork were new sources of lightweightinsulation 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 internaland external temperatures. This was due, in part, to the limitations ofearly insulation, the almost exclusive use of single glazed windows, andthe absence of airtight construction. The presence of ventilating fansand the reliance on architectural features, such as operable windows, cupolasand transoms, allowed sufficient air movement to keep buildings well ventilated.Building materials could behave in a fairly traditional way, expandingand contracting with the seasons. Twentieth Century. The twentieth century saw intensive development ofnew 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 systemsin the latter half of the century. Forced air heating systems with ductsand registers became popular for all types of buildings and allowed architectsto experiment with architectural forms free from mechanical encumbrances.
The dropped ceilings covering an air conditioning system also cover the historic windows, altering their proportion and resulting in loss of the historic character. Photo: NPS files.History of Mechanical Systems
19th century buildings used porches, cupolas, and awnings to make them more comfortable in the summer. Photo: NPS files.
In the 1920s large-scale theaters and auditoriums introduced central airconditioning, and by mid-century forced air systems which combined heatingand air conditioning in the same ductwork set a new standard for comfortand convenience. The combination and coordination of a variety of systemscame together in the post-World War II high-rise buildings; complex heatingand air conditioning plants, electric elevators, mechanical towers, ventilationfans, and full service electric lighting were integrated into the building'sdesign. 
A return air grille is successfully screened behind the arch. Photo: NPS files.
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 systemswere promoted in construction journals. Caulking to seal out perimeterair around window and door openings became a standard construction detail.
The last quarter of the twentieth century has seen making HVAC systemsmore energy efficient and better integrated. The use of vapor barriersto control moisture migration, thermally efficient windows, caulking andgaskets, compressed thin wall insulation, has become standard practice.New integrated systems now combine interior climate control with fire suppression,lighting, air filtration, temperature and humidity control, and securitydetection. Computers regulate the performance of these integrated systemsbased on the time of day, day of the week, occupancy, and outside ambienttemperature.
Climate Control andPreservation
Although twentieth century mechanical systems technology has had a tremendousimpact on making historic buildings comfortable, the introduction of thesenew systems in older buildings is not without problems. The attempt tomeet and maintain modern climate control standards may in fact be damagingto historic resources. Modern systems are often over-designed to compensatefor inherent inefficiencies of some historic buildings materials and planlayouts. Energy retrofit measures, such as installing exterior wall insulationand vapor barriers or the sealing of operable window and vents, ultimatelyaffect the performance and can reduce the life of aging historic materials.
 Complex mechanical systems for institutional buildings may require a central control room. Photo: NPS files. |
In general, the greater the differential between the interior and exteriortemperature 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 thecolder area. Too little humidity in winter, for example,can dry and crack historic wooden or painted surfaces. Too much humidityin winter causes moisture to collect on cold surfaces, such as windows,or to migrate into walls. As a result, this condensation deteriorates woodenor metal windows and causes rotting of walls and wooden structural elements,dampening insulation and holding moisture against exterior surfaces. Moisturemigration through walls can cause the corrosion of metal anchors, angles,nails or wire lath, can blister and peel exterior paint, or can leave efflorescenceand salt deposits on exterior masonry. In cold climates, freeze-thaw damagecan result from excessive moisture in external walls.
To avoid these types of damage to a historic building, is importantto understand how building components work together as a system. Methodsfor controlling interior temperature and humidity and improving venationmust be considered in any new or upgraded HVAC or climate control system.While certain energy retrofit measures will have a positive effect on theoverall building, installing effective vapor barriers in historic wallsis difficult and often results in destruction of significant historic materials.
Planning the New System
Climate control syst
