PRESERVATION 

BRIEFS 

Heating, Ventilating, and Cooling 

Historic Buildings: 

Problems and Recommended Approaches 

Sharon C. Park, AlA 

u.s. Department of the Interior 

National Park Service 

Cultural Resources 

Heritage Preservation Services 

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-particularly for 

museums-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 

following: 

• 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 staining. 

• 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 boxedchases 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 tl}e 

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. 

History of Mechanical Systems 

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. 

Eighteenth Century. Early heating and ventilation in 

America relied upon common sense methods of managing the environment (see figure 1). 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. 

Nineteenth Century. The industrial revolution provided the technological means for controlling the environment for the first time (see figure 2). 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 introduc2 

1. Eighteenth century and later vernacular architecture depended on the 

siting of the building, deciduous trees, cross ventilation, and the placement 

of windows and chimneys to maximize winter heating and natural summer 

cooling. Regional details, as seen in this Virginia house, include external 

chimneys and a separate summer kitchen to reduce fire risk and isolate heat 

in the summer. Photo: NPS Files. 

2. Nineteenth century buildings continued to use architectural features such 

as porches, cupolas, and awnings to make the buildings more comfortable in 

summer, but heating was greatly improved by hot water or steam radiators. 

Photo: NPS Files 
tion 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. 

Twentieth Century. The twentieth century saw intensive development of new technologies and the notion 

of fully integrating mechanical systems (see figure 3). 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 highrise 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, has 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. 

3. The circa 1928 Fox Theater in Detroit, designed by C. Howard Crane, 

was one of the earliest twentieth century buildings to provide air conditioning to its patrons. The early water-cooled system was recently restored. 

Commercial and highrise buildings of the twentieth century were able, 

mostly through electrical power, to provide sophisticated systems that integrated many building services. Photo: William Kessler and Associates, 

Arch i tects. 

Climate Control and Preservation 

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. 

3 
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 (see figure 4). 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, freezethaw damage can result from excessive moisture in 

external walls. 

To avoid these types of damage to a 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 (see figure 5). 

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5. The installation of vapor retarders in walls of historic buildings in an 

effort to contain interior moisture can cause serious damage to historic 

finishes as shown here. In this example, all the wall plaster and lath have 

been stripped in preparation for a vapor barrier prior to replastering, Controlling interior temperature and relative humidity can be more effective than 

adding insulation and vapor barriers to historic perimeter walls. Photo: 

Ernest A. Conrad, P.E. 

inside 

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90 

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~ outside 80.'3 ~ sealant c. '" 

inside 

75 E 

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70 outlet -

65 condensation 

b. Hot, Humid Climate Condition air leak~~:+;i;) 

vapor retarder *,"~~'ir"'+~:-:::~~tTC. Moisture Condensation in Walls 

4. Mechanical heating and cooling systems change the interior climate of a building, Moisture in the air will dissipate from the warmer area of a building to 

the colder area and can cause serious deterioration of historic materials. Condensation can form if the dew point occurs within the building wall, particularly 

one that has been insulated (see a and b), Even when vapor retarders are installed (c), any non-continuous areas will provide spaces for moisture to pass. Wall 

Section Drawings: NPS files 

4 
Planning the New System 

Climate control systems are generally classified according to the medium used to condition the tem~erature: 

air, water, or a combination of both (see overvIew on 

page 6). The complexity of choices facinp a buildinp 

owner or manager means that a systematic approach IS 

critical in determining the most suitable system for a 

building, its contents, and its occup~nts. ~o m~tter . 

which system is installed, a change m the mtenor chmate 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 a historic buildi~g . 

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 build~g 

and costs to an owner of selecting the wrong mechamcal 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 u~er, the 

needs of the building, and the needs of a collectIon or 

equipment must be considered. It may not be neces- . 

sary to have a comprehensive climate control sy'stem if 

climate-sensitive objects can be accommodated m 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 desir~ble for ?istoric 

buildings. In each case, the lowest level of mterventlOn 

needed to successfully accomplish the job should be selected. 

Before a system is chosen, the following planning 

steps are recommended: 

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 arc~it~ctural 

spaces. What is the intensity of u~e of the buildmg: 

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 engine~r, and 

preservation consultants, each knowledgeable m 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), ten.ant requirements (quality of comfort, ease of operatIon), access (maintenance and future replacement), and the 

overall cost to the owner. 

3. Undertake a condition assessment of the existing 

building and its systems. What are the existing construction materials and mechanical systems? What condition are they in and are they reusable (see figure 6)? 

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, an? 

which of the historic features, e.g. shutters, awnmgs, 

skylights, can be reused (see. figure 7)? Evaluat~ air 

infiltration through the extenor envelope; momtor 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. 

6. A condition assessment during the planning stage would identify this 

round radiator in a small oval-shaped vestibule as a significant element of the 

historic heating system. In upgrading the mechanical system, the radiator 

should be retained. Photo: Michael C. Henry, P.E. , AlA. 

5 
Overview of HVAC Systems 

WATER SYSTEMS: Hydronic radiators, Fan coil, or 

radiant pipes 

Water systems are generally called hydronic and use a network 

of pipes to deliver water to hot water radiators, radiant pipes set 

in floors or fan coil cabinets which can give both heating and 

cooling. Boilers produce hot water or steam; chillers produce 

chilled water for use with fan coil units. Thermostats control the 

temperature by zone for radiators and radiant floors. Fan coil 

units have individual controls. Radiant floors provide quiet, 

even heat, but are not common. 

Advantages: Piped systems are generally easier to install in 

historic buildings because the pipes are smaller than ductwork. 

Disadvantages: There is the risk, however, of hidden leaks in 

the wall or burst pipes in winter if boilers fail. Fan coil condensate pans can overflow if not properly maintained. Fan coils 

may be noisy. 

Hydronic Radiators: Radiators or baseboard radiators are 

looped together and are usually set under windows or along 

perimeter walls. New boilers and circulating pumps can upgrade older systems. Most piping was cast iron although copper systems can be used if separately zoned. Modern cast iron 

baseboards and copper fin-tubes are available. Historic radiators can be reconditioned. 

cast iron baseboard 

cast iron radiator 

Fan Coil Units: Fan coil systems use terminal cabinets in each 

room serviced by 2, 3, or 4 pipes approximately 1-1/2" each in 

diameter. A fan blows air over the coils which are serviced by 

hot or chilled water. Each fan coil cabinet can be individually 

controlled. Four-pipe fan coils can provide both heating and 

cooling all year long. Most piping is steel. Non-cabinet units 

may be concealed in closets or custom cabinetry, such as benches, can be built. 

return piping Fan coil unit 

6 

CENTRAL AIR SYSTEMS 

The basic heating, ventilation and air conditioning (HVAC) system is all-air, single zone fan driven designed for low, medium 

or high pressure distribution. The system is composed of compressor drives, chillers, condensers, and furnace depending 

on whether the air is heated, chilled or both. Condensers, generally air cooled, are located outside. The ducts are sheet metal 

or flexible plastic and can be insulated. Fresh air can be circulated. Registers can be designed for ceilings, floors and walls. 

The system is controlled by thermostats; one per zone. 

Advantages: Ducted systems offer a high level of control of 

interior temperature, humidity, and filtration. Zoned units can 

be relatively small and well concealed. 

Disadvantages: The damage from installing a ducted system 

without adequate space can be serious for a historic building. 

Systems need constant balancing and can be noisy. 

Basic HVAC: Most residential or small commercial systems will 

consist of a basic furnace with a cooling coil set in the unit and 

a refrigerant compressor or condenser located outside the 

building. Heating and cooling ductwork is usually shared. If 

sophisticated humidification and dehumidification is added to 

the basic HVAC system, a full climate control system results. 

This can often double the size of the equipment. 

supply 

Heating furnace with cooling coil 

Basic Heat Pump/Air System: The heat pump is a basic HVAC 

system as described above except for the method of generating 

hot and cold air. The system operates on the basic refrigeration 

cycle where latent heat is extracted from the ambient air and is 

used to evaporate refrigerant vapor under pressure. Functions 

of the condenser and evaporator switch when heating is 

needed. Heat pumps, somewhat less efficient in cold climates, 

can be fitted with electric resistance coil. 

Cooling cycle 

compressor refrigerant flow 

he.li" ~ ,',"po .. lo. 

expansion valve 

cycle reverses for heating 
COMBINED AIR AND WATER SYSTEMS 

These systems are popular for restoration work because they 

combine the ease of installation for the piped system with the 

performance and control of the ducted system. Smaller air handling units, not unlike fan coils, may be located throughout a 

building with service from a central boiler and chiller. In many 

cases the water is delivered from a central plant which services I a complex of buildings. 

This system overcomes the disadvantages of a central ducted ; 

system where there is not adequate horizontal or vertical runs I 

for the ductwork. The equipment, being smaller, may also be 

I quieter and cause less vibration. If only one air handler is being 

utilized for the building, it is possible to house all the equipment in a vault outside the building and send only conditioned 

air into the structure. 

Advantages: flexibility for installation using greater piping runs 

with shorter ducted runs; Air handlers can fit into small spaces. 

Disadvantages: piping areas may have undetected leaks; air 

handlers may be noisy. 

Water-serviced Air Handlers: 

a supply air 

t) return air 

o 

fresh air intake 

Typical Systems Layout: 

IFI~~~~ ~~~~ heating coil supply air 

~~~~~~ cooling coil 

~ humidifier foi1 I Conditioned 

, ~ Air handler space 

n 

(J return air 

Chiller In 

{} 

Fresh air intake 

OTHER SYSTEM COMPONENTS 

Non-systems components should not be overlooked if they can 

make a building more comfortable without causing damage to 

the historic resource or its collection. 

Advantages: components may provide acceptable levels of 

comfort without the need for an entire system. 

Disadvantages: Spot heating, cooling and fluxuations in humidity may harm sensitive collections or furnishings. If an integrated system is desirable, components may provide only a 

temporary solution. 

Portable Air Conditioning: 

Most individual air conditioners are set in windows or through 

exterior walls which can be visually as well as physically damaging to historic buildings. Newer portable air conditioners are 

available which sit in a room and exhaust directly to the exterior 

through a small slot created by a raised window sash. 

~ :!~~~:t air 

(Air conditioner) 

Fans: Fans should be considered in most properties to improve 

ventilation. Fans can be located in attics, at the top of stairs, or 

in individual rooms. In moderate climates, fans may eliminate 

the need to install central air systems. 

exhaust fan 

Dehumidifiers: For houses without central air handling systems, a dehumidifier can resolve problems in humid climates. 

Seasonal use of dehumidifiers can remove moisture from damp 

basements and reduce fungal growth. 

(dehom;difie,) 

portable unit lA ~ 

Heaters: Portable radiant heaters, such as those with water 

and glycol, may provide temporary heat in buildings used infrequently or during systems breakdowns. Care should be taken 

not to create a fire hazard with improperly wired units. 

portable heater 

electric baseboard 

Compiled by Sharon C. Park. Sketches adapted from Architectural Graphic Standards with permission from John Wiley and Sons. 

7 
4. Prioritize architecturally significant spaces, finishes, and 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 (see 

figure 8). 

5. Become familiar with local building and fire 

codes. Owners or 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 begins. 

6. Evaluate options for the type and size of systems. 

A matrix 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 

7. Operable skylights and grilles that can be adapted for return air should be 

identified as part of the planning phase for new or upgraded mechanical 

systems. Photo: Dianne Pierce, NPS files. 

8 

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 

(see figure 9). 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. 

8. In considering options for new systems, existing spaces should be evaluated for their ability to house new equipment. This sketch shows several 

areas where new mechanical equipment could be located to avoid damaging 

significant spaces. Sketch: NPS files 

9. Improving ventilation through traditional means should not be overlooked 

in planning new or upgraded HVAC systems. In mild climates, good exhaust 

fans can often eliminate the need for air conditioning or can reduce equipment size by reducing cooling loads. Photo: Ernest A. Conrad, P.E. 
Designing the new system 

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 (see figure 10 a-f). 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: 

1. Establish specific criteria for the new or upgraded 

mechanical 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. 

2. Prioritize the requirements for the new climate 

control 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 a 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. 

3. Minimize the impact of the new HVAC on the 

existing 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. 

4. Balance quantitative requirements and preservation objectives. 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 (see fig 11). 

Systems Performance and Maintenance 

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. 

9 
10. The following photographs illustrate recent preservation projects where careful planning and design retained the historic character of the resources. 

before after 

a. Before and after of a circa 1900 school entrance. The radiators have been replaced with a two-pipe fan coil system built into bench seats. The 

ceiling was preserved and no exposed elements were required to add air conditioning. Piping runs are under the benches and there was no 

damage to the masonry walls. Photos: Notter Finegold + Alexander Inc. and Lautman Photography, Washington. 

historic after 

d. Auditors Buildings, Washington, D.C. This upper floor workspace had been modified over the years with dropped ceilings and partitions. In 

the recent restoration, the open plan workspace was restored, the false ceiling was removed, and the fireproof construction was exposed. A variable air volume (VA V) system using round double shell exposed ductwork is in keeping with the industrial character of the architectural space. 

Photo: Kenneth Wyner Photography, courtesy of Notter Finegold + Alexander Inc. Before view provided by Notter Finegold + Alexander/Mariani. 

10 
b. Central air conditioning was installed in the corridors of this circa 1900 

school building by adding an air handler over the entrance from a vestibule. 

The custom-designed slot registers provide linear diffusers without detracting 

from the architecture of the space. Photo: Lautman Photography courtesy of 

Notter Finegold + Alexander Inc. 

e. Town Hall, Andover, MA. The upstairs auditorium was restored and new 

mechanical systems were installed. Perimeter baseboard radiation provides 

heat and air handlers, located in the attic space provide air conditioning. The 

cast iron ceiling grille was adapted for return air and the supply registers 

were installed in a symmetrical and regular manner to minimize impact on 

the historic ceiling. Photo: David Hewitt/Anne Garrison for Ann Beha 

Associates. 

c. Conference room, Auditors Building, Washington, D.C. The historic 

steam radiators were retained for heating. The cast iron ceiling register was 

retained as a decorative element, but made inoperable to meet fire codes. 

Photo: Kenneth Wyner Photography courtesy of Notter Finegold + Alexander Inc. 

f. Homewood, Baltimore, MD. This elegant circa 1806 residence is now a 

house museum. The registers for the forced air ducted system seen behind the 

table legs, are grained to blend with the historic baseboards. The HVAC 

system uses a water/air system where chilled water and steam heat are 

converted to conditioned air. Photo: Courtesy Homewood Museum, Johns 

Hopkins University. 

11 
HVAC Do's and Don'ts 

DO's: 

• Use shutters, operable windows, porches, curtains, 

awnings, shade trees and other historically appropriate non-mechanical features of historic buildings 

to reduce the heating and cooling loads. Consider 

adding sensitively designed storm windows to 

existing historic windows. 

• Retain or upgrade existing mechanical systems 

whenever possible: for example, reuse radiator 

systems with new boilers, upgrade ventilation 

within the building, install proper thermostats or 

humidistats. 

• Improve energy efficiency of existing buildings by 

installing insulation in attics and basements. Add 

insulation and vapor barriers to exterior walls only 

when it can be done without further damage to the 

resource. 

• In major spaces, retain decorative elements of the 

historic system whenever possible. This includes 

switchplates, grilles and radiators. Be creative in 

adapting these features to work within the new or 

upgraded system. 

• Use space in existing chases, closets or shafts for 

new distribution systems. 

• Design climate control systems that are compatible 

with the architecture of the building: hidden system for formal spaces, more exposed systems possible in industrial or secondary spaces. In formal 

areas, avoid standard commercial registers and use 

custom slot registers or other less intrusive grilles. 

12 

• Size the system to work within the physical constraints of the building. Use multi-zoned smaller 

units in conjunction with existing vertical shafts, 

such as stacked closets, or consider locating equipment in vaults underground, if possible. 

• Provide adequate ventilation to the mechanical 

rooms as well as to the entire building. Selectively 

install air intake grilles in less visible basement, 

attic, or rear areas . 

• Maintain appropriate temperature and humidity 

levels to meet requirements without accelerating 

the deterioration of the historic building materials. 

Set up regular monitoring schedules. 

• Design the system for maintenance access and for 

future systems replacement. 

• For highly significant buildings, install safety monitors and backup features, such as double pans, 

moisture detectors, lined chases, and battery packs 

to avoid or detect leaks and other damage from 

system failures. 

• Have a regular maintenance program to extend 

equipment life and to ensure proper performance. 

• Train staff to monitor the operation of equipment and 

to act knowledgeably in emergencies or breakdowns. 

• Have an emergency plan for both the building and 

any curatorial collections in case of serious malfunctions or breakdowns. 

DON'TS: 

• Don't install a new system if you don't need it. 

• Don't switch to a new type of system (e.g. forced 

air) unless there is sufficient space for the new system or an appropriate place to put it. 

• Don't over-design a new system. Don't add air conditioning or climate control if they are not absolutely necessary. 

• Don't cut exterior historic building walls to add 

through-wall heating and air conditioning units. 

These are visually disfiguring, they destroy historic 

fabric, and condensation runoff from such units 

can further damage historic materials. 

• Don't damage historic finishes, mask historic features, or alter historic spaces when installing new 

systems. 

• Don't drop ceilings or bulkheads across window 

openings. 

• Don't remove repairable historic windows or replace them with inappropriately designed thermal 

windows. 

• Don't seal operable windows, unless part of a museum where air pollutants and dust are being 

controlled. 

• Don't place condensers, solar panels, chimney 

stacks, vents or other equipment on visible portions of roofs or at significant locations on the site. 

• Don't overload the building structure with the 

weight of new equipment, particularly in the attic. 

• Don't place stress on historic building materials 

through the vibrations of the new equipment. 

• Don't allow condensation on windows or within 

walls to rot or spall adjacent historic building 

materials. 
. 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 ex-

~mple, molds in condensate pans can block drainage 

lmes 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 ad~ition to regular cyclical maintenance, thorough 

mspectIons should be undertaken from time to time to 

evaluate the continued performance of the climate control s):,stem. As the system ages, parts are likely to fail, 

and SIgns of trouble may appear. Inadequately ventilat~d. areas may smell musty. Wall surfaces may show 

stammg, 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 c~osely monitored to ensure complete repair. 

EqUIpment must be accessible for maintenance and 

should be visible for easy inspection. Moreover, since 

~echanical systems last only 15-30 years, the system 

~tself mu~t be "reversible." That is, the system must be 

mstalled m such a way that later removal will not damage t~e building. In addition to servicing, the back-up 

momto~s that signal malfunctioning equipment must 

be routmely 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. 

a 

Conclusion 

The successful integration of new systems in historic 

buil~ings can be challenging. Meeting modern HVAC 

requrrem.ents for human comfort or installing contr~lled 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, ventila~ing, and air conditioning system eXIsts. In undertakmg 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 th<if should be 

preserved or reused, 

prioritize the preservation objectives, 

understand the impact of new interior climate conditions on historic materials, 

integrate preservation with mechanical and code 

requirements, 

maximize the advantages of various new or upgraded 

mechanical systems, 

understand the visual and physical impact of various 

installations, 

identify maintenance and monitoring requirements 

for new or upgraded systems, and 

plan for the future removal or replacement of the 

system. 

Too often the presumed climate needs of the occupants or collections can be detrimental to the long-term 

preservation of the building. With a careful balance 

?etw:en the preservation needs of the building and the 

mtenor temperature and humidity needs of the occupants, a successful project can result. 

11. Durin$ the restoration of this 1806 National Historic Landmark (photo a), a new climate control system was installed. The architects removed all the earli-

~; m~c7anlca~~1ulp7ent fr0

b

m

l 

the house and Installed new equipment in a 30' x 40' concrete vault located underground 150 feet from the house itself (photo . n y con I lOne air IS own Into the house reusing much of the circa 1930s ductwork. Photos: Thomas C. Jester. 

13 
Bibliography 

Banham, Reyner. The Architecture of the Well-Tempered Environment. 

London: The Architectural Press, 1969. 

Burns, John A., AlA. Energy Conserving Features Inherent in Older 

Homes. Washington: US. Department of Housing and Urban Development and US. Department of the Interior, 1982. 

Cowan, Henry J. Science and Building; Structural and Environmental 

Design in the Nineteenth and Twentieth Centuries. New York: John Wiley 

& Sons, 1978. 

Ferguson, Eugene S. "An Historical Sketch of Central Heating: 1800-

1860;' in Building Early America (Charles Peterson, editor) Philadelphia: Chilton Book Co., 1976. 

Fitch, James Marston. American Building; The Environmental Forces That 

Shape It. Boston: Houghton Mifflin Co., 1972. 

Giedion, Siegfried. Mechanization Takes Command; a Contribution to 

Anonymous History. New York: Oxford University Press, 1948. 

Merritt, Frederick S. Building Engineering and Systems Design. New 

York: Van Nostrand Reinhold Co, 1979. 

Smith, Baird M. Preservation Briefs 3: Conserving Energy in Historic 

Buildings. Washington, DC: US. Department of the Interior, 1978. 

Turberg, Edward. A History of American Building Technology. Durham: 

Durham Technical Institute, 1981. 

cover photo: This historic coal boiler continues in use after its conversion to 

an oil-fired boiler. Photo: NPS files 

14 

Acknowledgements 

The author gratefully acknowledges the invaluable assistance of 

Michael C. Henry, P.E., AlA, in the development and technical 

editing of this Preservation Brief. Technical review was also provided 

by Ernest A. Conrad, P.E. Thanks is also given to staff members of 

the National Park Service Cultural Resources Programs, induding 

Tom Keohan and Catherine Colby, Rocky Mountain Region; Michael 

Crowe, Western Region; Mark Chavez, Midwest Region; Randall J. 

Biallas, AlA, Chief, Park Historic Architecture Division, and George 

A. Thorsen, Historical Architect, Denver Service Center. SpeCial 

thanks is also given to Michael J. Auer of Technical Preservation 

Services tor his editorial assistance in preparing this paper and Tim 

Buehner for his assistance with the illustrations. 

This publication has been prepared pursuant to the National Historic 

Preservation Act of 1966, as amended, which directs the Secretary of 

the Interior to develop and make available information concerning 

historic properties. Preservation Brief 24 was developed under the 

editorship of H. Ward Jand!, Chief, Technical Preservation Services. 

Comments on the usefulness of this publication may be directed to 

Chief, Technical Preservation Services Branch, Preservation Assistance Division, National Park Service, P.O. Box 37127, Washington, 

D.C. 20013-7127. This publication is not copyrighted and can be 

reproduced without penalty. Normal procedures for credit to the 

author and the National Park Service are appreciated. 

Washington, DC 1991