Hospitals are very complex environments that require special HVAC system design, maintenance and repair considerations. HVAC systems in healthcare facilities provide a broad range of services in support of populations who are uniquely vulnerable to an elevated risk of health, fire, and safety hazard. These heavily regulated, high-stakes facilities undergo continuous maintenance, verification, inspection, and recertification; typically, operate 24 hours per day, 7 days per week; and are owner-occupied for long life cycles. HVAC systems are responsible for keeping indoor air quality (IAQ) high and providing a safe temperature for patients and staff alike. The air treatment in healthcare facilities is of prime importance. The patients feel more comfortable and heal early if the air circulation, temperature and humidity are maintained as per prescribed standard.  Hospitals need a functional HVAC system to stay operational. This is especially true for places like a clean room or an operating room. Air-exchange, humidity, pressure and temperature have to be precise in such environments to minimize the risk of infection for patients. Every hour of the day, the HVAC system needs to contribute to the overall goal of suppressing hospital-acquired infections. According to research by the Centers for Disease Control, one in 25 patients in the U.S. will suffer at least one infection acquired in the healthcare environment.

However, even outside of the operating room, HVAC systems are required to not only keep high IAQ levels, but maintain specific temperatures to minimise bacterial growth throughout the hospital. And not to mention, hospitals have to run 24/7 which leads to huge levels of energy consumption. A well-maintained HVAC system can help reduce operating costs in addition to providing a safe environment.

There are stringent standards and many industry guidelines when it comes to the performance of a hospital HVAC system.

HVAC in Healthcare Facility

Increasing interest has been expressed towards intelligent heating, ventilation and air conditioning (HVAC) systems in hospital environments. Hospitals require efficient heating, ventilation and air conditioning (HVAC) systems to maintain good indoor air quality (IAQ), aseptic conditions, and to secure healthy, safe and suitable indoor thermal conditions (i.e. temperature, humidity, air quality and airflow) for the hospital personnel and the patients.

Hospital ventilation must be effective for controlling airborne transmission and preventing outbreaks of infectious diseases. A correlation exists between ventilation, air movements in buildings and the transmission of infectious diseases. Poorly designed, maintained (i.e. contaminated) and used HVAC systems are common in hospitals and often lead to poor IAQ.

Effect of breakdown of Hospital HVAC System

  • Patient feel uncomfortable
  • Risk of hospital infection spread
  • Surgical procedures delay
  • Emergency situations rerouted to other hospitals
  • Fog in the hallways and operating rooms
  • Damages to supplies which require refrigeration or lower temp
  • Equipment failure
  • Financial loss

HVAC & Infection Control

In a hospital environment, there tend to be high concentrations of harmful micro-organisms. From an infection control perspective, the primary objective of hospital design is to place the patient at no risk for infection while hospitalised. The special technical demands include hygiene, reliability, safety and energy-related issues.

Infections, which may result from activities and procedures taking place within the facility, are a cause for great concern. Three main routes responsible for infections are contact, droplet, and airborne transmission, which are quite affected by room design and construction factors.

An airborne infectious isolation room is constructed to minimise the migration of air from an isolation room to other areas of healthcare facilities. The risk of being infected through the airborne route is a function of particle concentration. The chance of a particle that is carrying an organism falling into an open wound increases with particle concentration. By reducing the concentration, one can reduce the chance of infection and, hence, the number of patients infected.

Recommendations for engineering controls to contain or prevent the spread of airborne contaminants center on general ventilation, air cleaning (primary and secondary filtration), and local exhaust ventilation (source control)

General Ventilation

The most effective means of controlling contaminants, odour and indoor air pollution is through ventilation, which requires simultaneous control of number of conditions:

  • Air change rates
  • Pressure gradient appropriate with class of isolation
  • Appropriate air distribution in the compartments being air conditioned
  • High quality air filtration including absolute filtration
  • Precise temperature and humidity control ensuring maintenance of the intended microclimate

Air Change Rates

Ventilation supply rates for healthcare facilities require large expenditure of fresh air to dilute and remove the contaminants generated in the space. The ventilation rates for healthcare facilities is expressed as air changes air per hour (ACH), which is a measure of how quickly the air in an interior space is replaced by outside (or conditioned) air. For example, if the amount of air that enters and exits in one hour equals the total volume of the space, the space is said to undergo one air change per hour. Air flow rate is measured in appropriate units such as cubic feet per minute (CFM) and is given by

In this equation,

Q is the volume flow rate of air being calculated, and ACH is the number of air changes per hour

To determine the airflow required to adequately ventilate an area,

1)  Calculate the room volume to be ventilated Width x Length x Height = ft3 (cubic feet).

2)  Calculate the air volume requirement by multiplying the room volume by the air change rate per hour = ft3/h.

Studies carried out be AIA indicate that just one air-change with fresh air can remove 63 per cent of suspended particles from the room air. If a ventilation system can perform 10 air changes per hour (ACH), it takes 14 minutes to remove 90 per cent of airborne contaminants in a room and 28 minutes to remove 99 per cent. Thus, increased number of fresh air changes per hour is effective for cleaning airborne contaminants. However, the higher air change rate (>20 ACH) may cause turbulence and the cost for ventilation itself will be too high. Therefore, a recommended compromise of 12 ACH is proposed which should be achievable when the filters have reached their maximum pressure drop. Higher ACR also equates to higher energy use.

The selection of 12 air changes per hour is largely a matter of convention. Ventilation rates are voluntary unless a state or local government specifies a standard in healthcare licensing requirements. These standards typically apply to only the design of a facility, rather than its operation. Based on the scientific knowledge and professional judgment reflected in the AIA guidelines, ASHRAE and the American National Standards Institute (ANSI) have developed design recommendations for ventilation and pressure relationships for various patient-care areas. Healthcare facilities without specific ventilation standards should follow ANSI/ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality or otherwise in the absence of any specified supply air change/hour following guidelines may be used:

  • For the space to be maintained under negative pressure exhaust 10 to 15 per cent more air than the supply.
  • For the space to be maintained under positive pressure, exhaust 10 to 15 per cent less air than the supply air.

Room Pressure Control

Building room pressurisation is a critical factor to monitor in a hospital as it can greatly affect the controllability of the environment. If the building pressure is allowed to become negative due to supply filters being loaded, supply fans running too slow, or return fans running too fast, humid and dirty air can be drawn into the building through cracks and openings. This air is completely unconditioned and can provide several of the necessary ingredients to promote mold growth (e.g., moisture, more spores, and nutrients.)

Building room pressure gradient is achieved by controlling the quality and quantity of intake and exhaust air, maintaining differential air pressures between adjacent areas, and designing patterns of airflow for particular clinical purposes.

Special Challenges

A unique challenge occurs when a patient needs both the positive and negative room isolation. For example when there is an immune compromised patient who also has a communicable infectious disease such as TB. Studies indicate that approximately 15 per cent of HIV patients also suffer from TB, and this presents a unique design problem. This patient needs to be in a protective environment for his own health but also needs to be isolated to protect others from his communicable disease.

The solution is to house such patients in a positive pressure protective environment room with an anteroom that is under negative pressure relative to the corridor and protective environment.

Caution: Avoid designing systems to switch between positive and negative pressure.

Laminar Flow in Healthcare Facilities

Other mechanism of air distribution in ultra-clean areas of hospitals is the laminar or unidirectional airflow distribution. Laminar airflow ventilation systems are designed to move air in a single pass with parallel streamlines, usually, through a bank of HEPA filters either along a wall or in the ceiling.

Laminar flow systems use perforated ventilation grills across the entire ceiling or side wall at air flow rates significantly greater than normal to force a steady constant stream of air across the entire room, similar to a smooth steady flow out of an open water faucet versus one that splashes as the water comes out of the faucet.

Laminar flow distribution requires a very high volume of air flow and is designed for an air velocity of 90 + 20 ft/min. This unidirectional approach optimises airflow and minimises air turbulence and ensures that any contamination that is generated within the area is quickly and effectively removed. Laminar airflow systems are often used in operating rooms to help reduce the risk for healthcare-associated airborne infections.

The data that demonstrates a bonafide application and support of laminar airflow in PE rooms is lacking. Given the high cost of installation and operation, the value of laminar airflow is questionable and shall be ascertained through lifecycle analysis.

Air Filtration

All of the air that is drawn into an air handling system is ‘contaminated’ to some degree. It is commonly accepted that airborne particles (solid particles, liquids, fumes, smoke, or bacteria) that are larger than 5 microns in size tend to settle quickly out of the air onto horizontal surfaces. Airborne particles that are less than 5 microns in size (especially, those less than 2 microns in size) tend to settle slowly out of the air and remain suspended (airborne) for larger period of time.

Concerns over hospital-acquired infections have propelled filtration solutions into the forefront as a primary tool for infection control.

All public areas of healthcare facilities are required to have two banks of filters — a 30 per cent (ASHRAE 52.1) pre-filter and 90 per cent final filter. Provided that the final filter is properly installed and maintained and provided that there is little or no bypass around the filter, the combined efficiency of two bank filters is nearly 100 per cent in removing particles of 1µm – 5 µm in diameter. This filtration system is adequate for most patient care areas in ambulatory care facilities, and the operating room environment.

A common metric for filter performance is the minimum efficiency reporting value (MERV), a rating derived from a test method developed by ASHRAE. The MERV rating indicates a filter’s ability to capture particles between 0.3 and 10.0 microns in diameter. A higher MERV value translates to better filtration, so a MERV-13 filter works better than a MERV-8 filter. In healthcare facilities, a final filter of MERV-14 is satisfactory.

High Efficiency Particulate Air (HEPA) Filters

HEPA filters have a minimum initial efficiency of 99.97 per cent for removing particles 0.3 microns in size. This is a critical point as these filters are being used to remove mold and bacteria, typically 1 to 5 microns in size when airborne, as well as viral particles which are submicron in size (as a reference, Aspergillus spores are 2.5 – 3 µm in diameter). Each HEPA filter is individually tested at the factory in order to confirm their conformance to this standard. They may also be field-tested in order to confirm their ongoing adherence to efficiency requirements.

HEPA filters to be monitored (with manometers or other pressure indicating devices) on regular basis and replaced in accordance with the manufacturer’s recommendations and standard preventive maintenance practices. Gaps in and around filter banks and heavy soil and debris upstream of poorly-maintained filters have been implicated in healthcare-associated outbreaks of aspergillosis, especially during times of nearby construction.

HEPA filters are a costly budget item. In order to extend the life of a HEPA filter and reduce ongoing replacement costs, it is strongly recommended to provide a roughing prefilter prior to the HEPA. Studies indicate that a low-efficiency prefilter may extend the life of a HEPA filter by 25 per cent, while adding higher efficiency intermediate filters such as a MERV 14 (95 per cent by ASHRAE 52.1 dust spot test) filter can extend the life of the HEPA filter by as much as 900 per cent. This concept, called ‘progressive filtration,’ allows HEPA filters in special care areas to be used for 10 years or more. HEPA filter efficiency is monitored with the dioctylphthalate (DOP) particle test using particles that are 0.3 µm in diameter.

Caution: HEPA filters replacements require bag-in or bag-out procedures to minimise risk of exposure of the maintenance personnel to the infectious material.

Odour Control

There are several areas within a healthcare facility where odour or gaseous contaminants are common. Some of these contaminants may only be nuisance or comfort related, while others may represent a threat to personal health.

Fumes and smells can be removed from air by chemical processes such as ‘gas sorption’ which control compounds that behave as gases rather than as particles (e.g., gaseous contaminants such as formaldehyde, sulfur dioxide, ozone, and oxides of nitrogen). Gas sorption involves one or more of the following processes with the sorption material (e.g., activated carbon, activated alumina or chemically treated active clays).

Gas sorption units are available in variety of chemical treated clays, each performing differently for different gases. A prefilter is recommended upstream of the gas sorption unit to ensure that filter pores are not blocked with particulates. There are currently no standards for rating the performance of gaseous air cleaners, making the design and evaluation of such systems problematic.

Air Filtration to Protect HVAC Equipment

Accumulation of dust and moisture within HVAC systems increases the risk of spread of healthcare-associated environmental fungi and bacteria. The components of air handling units such as cooling coil, filters, and ductwork can be the ideal environments for breeding bacteria, fungus and mold. It not properly maintained, these will become reservoirs for infection causing molds. Common practices for protecting HVAC coils include locating the filter upstream of the coil and having a filter rating of at least MERV 8. The filters should fit snugly into the holding frames; be of rigid, moisture resistant construction; and be constructed from materials that will not support microbial growth. Care should be taken to ensure that no air bypasses around the filters. Bypass can be reduced by gasketing the filters to seal them in place and by installing filter blanks in spaces were the filter track does not contain filters.

Local Exhaust Ventilation

Local exhaust ventilation is designed to capture toxic gases, vapors, dusts, fumes and mists near its source, before the contaminant has a chance to disperse into the workplace air. A proper design of an exhaust ventilation system is necessary for effective removal of airborne contaminants that would otherwise pollute the work environment resulting in health hazards.

A local exhaust ventilation system usually consists of number of separate exhaust hoods applied to several different operations and connected by system of branch and main ducts to a central air cleaning device and exhaust fan and discharge stack to the outside.

Laboratories and Special Procedure Rooms

Laboratories and special procedure rooms that are known to contain toxic and hazardous contaminants are typically designed under negative pressure to prevent these gases from spreading throughout the facility. Examples of these areas include cytology labs where xylene and toluene may be part of the process, autopsy exhaust, ethylene oxide (ETO) steriliser exhaust, X-ray film processing areas, infectious materials in waste (including regulated medical waste), steam sterilisers, areas using high-level disinfectants or morgues, where formalin may be used. These chemicals are both irritants and carcinogenic. Such areas typically employ 100 per cent pass-through ventilation where no air is recirculated within the facility.

Temperature & Humidity Control

Local temperature distributions greatly affect occupant comfort and perception of the environment. Temperature should be controlled by change of supply temperature without any airflow control; the temperature difference between warm and cool regions should be minimised to decrease airflow drift. Efficient air distribution is needed to create homogenous domain without large difference in the temperature distribution. The laminar airflow concept developed for industrial clean room use has attracted the interest of some medical authorities. There are advocates of both vertical and horizontal laminar airflow systems. For high-contaminated areas, the local velocity should be greater than or at least equal to 0.2 m/s. For patient rooms 0.1 m/s is sufficient in the occupied area. The unidirectional laminar airflow pattern is commonly attained at a velocity of 0.45 ± 0.10 m/s.

Two essential components of conditioned air are temperature and humidity. After outside air passes through a low – or medium-efficiency filter, the air undergoes conditioning for temperature and humidity control.

Temperature Control

Control of temperature includes the operation of both heating and cooling systems to maintain temperature setpoints in the different areas of the building. Cool temperatures (68°F – 73°F) are usually associated with operating rooms, clean workrooms, and endoscopies suites. A warmer temperature (75°F) is needed in areas requiring greater degrees of patient comfort. Most other zones use a temperature range of 70°F – 75°F. Temperatures outside of these ranges may be needed on limited occasions in limited areas depending on individual circumstances during patient care.

HVAC systems in healthcare facilities have either single-duct or dual-duct systems. A single-duct system distributes cooled air (55°F) throughout the building, and uses thermostatically controlled reheat boxes located in the terminal ductwork to warm the air for individual or multiple rooms.

The more common dual-duct system consists of parallel ducts, one with a cold air stream and the other providing a hot air stream. A mixing box in each room or group of rooms mixes the two air streams to achieve the desired temperature. Temperature standards are given as either a single temperature or a range, depending on the specific healthcare zone.

Humidity Control

Efforts to limit excess humidity and moisture in the infrastructure and on-air stream surfaces in the HVAC system can minimise the proliferation and dispersion of fungal spores and waterborne bacteria throughout the indoor air. Control of humidity includes the operation of both humidification and dehumidification systems to maintain a minimum and maximum humidity level in the facility.

Four measures of humidity are used to quantify different physical properties of the mixture of water vapor and air. These are relative humidity, specific humidity, dew point and vapor pressure. The most common of these is ‘relative humidity,’ which is the ratio of the amount of water vapor in the air to the amount of water vapour air can hold at that temperature. At 100 per cent relative humidity, the air is saturated. For most areas within healthcare facilities, the designated comfort range is 30 per cent- 60 per cent relative humidity. Relative humidity levels greater than 60 per cent in addition to being perceived as uncomfortable, promote fungal growth.

Contingencies for HVAC design

Hospital shall be fully functional under any circumstance, regardless of the unpredictability or severity of the situation. Understanding code requirements, performing a hazard vulnerability analysis, and understanding what role the medical facility has in the community during an emergency are all essential to designing a safe and reliable HVAC system for a hospital.

Energy Conservation

Depending on the climate, between 35 per cent and 60 per cent of the annual energy costs of the typical healthcare facility are related to the operation of the HVAC systems.

Intelligent HVAC System

An intelligent work environment should be able to sense the interaction between users and space, process this information and understand the context data, react in a way that adjusts to users’ needs and enhances their endeavours in the environment, be active and autonomous, omnipresent and enhance the worker’s flow of work and perception of their physical and psychological well-being. An intelligent hospital HVAC system should take into account local climate and facility type of the hospital, conserve energy, increase safety, decrease the number of indoor air symptoms and improve the work atmosphere and efficiency. Intelligent HVAC systems have been increasingly implemented into hospital buildings.

Challenges concerning intelligent HVAC systems:

  • Poor hospital design
  • Lack of adequate sensors
  • Poor data collecting, archiving and visualisation by building automation systems
  • Complexity of airborne infection spread prevention
  • Conflicting indoor air preferences between patients and staff
  • Insufficient knowledge on HVAC systems


The air is not just a medium but it can be regarded as a guard in the critical health applications. The proper direction of the airflow increases the possibilities of successful pollutant scavenging from healthcare applications. The numerical tool, used here, was found to be so effective to predict the airflow pattern in the healthcare facilities at reasonable costs and acceptable accuracy. Good architectural design allows the HVAC system designers to properly locate the supply outlets and extraction ports in the optimum locations. Hospital HVAC repair is critical to operations. A hospital can’t function without an HVAC system. Downtime simply is not an option for hospitals. Hospital HVAC repair and preventative maintenance to avoid downtime is crucial. Choosing a commercial HVAC partner you can trust pays dividends every day a hospital is operational.

Leave a Reply