Monday, 23 February 2015



Human Comfort & HVAC System Operation


Introduction



The goal of the heating, ventilating, and air conditioning (HVAC) system is to create and maintain a comfortable environment within a building. Depending on geographic location and building construction, various types of interior climate control systems help ensure that interior spaces are maintained at comfortable levels year-round. With today’s energy conservation concerns, buildings are constructed to be much tighter, reducing the level of natural exchange between indoor and outdoor air. As a result, more and more buildings rely on mechanical conditioning and distribution systems for managing air.

A properly operated HVAC system finds the often delicate balance between optimizing occupant comfort while controlling operating costs. Comfort is an important issue for occupant satisfaction, which can directly affect concentration and productivity. At the same time, controlling these comfort and health parameters directly affects HVAC system operating costs in terms of energy, maintenance and equipment life. Below will be analyzed six of these parameters:

  1. Temperature and humidity management
  2. Air movement (drafts)
  3. Radiant heating/cooling effects
  4. Airborne chemicals
  5. Different indoor building conditions
  6. Building occupant personal preferences


1. Temperature and humidity management



Temperature And Humidity Management


The sensation of being hot or cold depends on both temperature and humidity. Temperature and humidity must be controlled and monitored together. For example, on a very cold dry day, the facility may need to operate warmer than a cold damp day. If humidity in the building is low in the summer, a warmer set point will achieve adequate comfort. Conversely, if your facility is operating at the maximum humidity of 60%, a cooler temperature set point should be set in order to achieve adequate comfort. In many cases the control of both temperature and humidity is called “enthalpy control”.

Taking into account the above the obvious question is: what exactly are the ideal temperature set points? Well, while there is no specific answer to this question, for temperature and humidity management, ASHRAE’s standards 55 and 62 indicate that controlling to 25 – 50% relative humidity is ideal, with 60% as the upper limit to stay within the human comfort zone. Temperature set points are established based on the amount of humidity present. For office buildings, if building pressure is maintained and fresh air make-up is limited during unoccupied periods, humidity will not exceed 55%. However, sometimes reheat is necessary for controlling humidity. In those cases, occupant comfort should be enhanced by using innovative means. For example, direct cool air can be used near a source of heat such as a television, copier, or other appliances. Few buildings actively humidify the air to prevent low humidity excursions because of equipment vulnerability to mineral and mold accumulation; equipment would require dedicated and routine maintenance.



2. Air movement and drafts



Air Movement And Drafts

Excessive air movement in buildings is probably rare, but it can be a source of discomfort and complaints. In addition, noisy air handlers and supply ducts may give the sense of excessive air movement that can lead to complaints of drafts and uncomfortable temperature. Excessive air movement is generally not desirable in the winter or summer months.

Air movement and radiant energy losses require detailed work on specific areas where problems are being experienced. For air movement issues, facility management may retrofit higher efficiency discharge diffusers with designs better suited for the location and that direct supply air away from occupants. Lower duct velocities may be achieved by providing lower temperature conditioned air in the cooling season and warmer air in winter; thus requiring lower air volumes to be supplied. Sometimes air duct supply volume is simply a tuning problem, where variable air volume terminal air boxes’ minimum air flow setting is not enough to prevent excessive supply air volumes during low demand periods for heating/cooling. Radiant heat transfer discomfort can be addressed by adjustable window shades and curtains. If these are impractical, thin films are available to apply to the window to reduce infrared energy transmission.



3. Radiant heating and cooling



Radiant Heating And Cooling

Radiant heating and cooling is a sensation that is common with building occupants who work near windows and where there are high ceilings. Radiation is a form of heat transfer that occurs due to infrared radiation from warmer bodies to cooler ones. Unlike air flow (convective heating), radiation can penetrate windows easily. When applying this form of heat transfer in a building, heat can be either gained or lost through windows. Thus, occupants near windows can feel a sensation of warmth if it is hot outside or a sensation of being chilly on very cold days. The orientation of windows also plays a role. Heating may be lost or gained based on if the window faces a sunny, cloudy or a deep blue clear sky (cooling). Typically, occupants on the north side of a building feel a cooling affect while occupants on the south and west side feel a heating affect. Discomfort will be greatest near ‘single pane’ windows, where radiant heating or cooling has the greatest effect.

As a short term solution, the radiant cooling or heating effects can be addressed by adjusting building temperature set points to compensate. Of course, the long term and appropriate permanent solution is to address radiant cooling and heating losses with high efficiency windows. Double pane windows with “low-e” coatings – a microscopically thin, virtually invisible, metal or metallic oxide layer on a window or skylight glazing surface for the purpose of reducing radiative heat flow – greatly reduce radiant heat transfer, as do drapes and blinds.

Radiant heating and cooling also relates to passive solar design, which uses windows, walls, and floors to absorb and distribute the sun's heat in the winter and reject solar heat in the summer without the use of mechanical systems. It can also maximize the use of sunlight for interior illumination. Buildings designed for passive solar incorporate large south-facing windows, long walls running east to west, and a thermal mass to absorb and slowly release the sun's heat. Passive solar designs also incorporate natural ventilation and roof overhangs to block the sun's strongest rays during the summer, but allow heat to penetrate in the winter.

Passive solar design techniques are applied most easily in new construction because they involve integral design elements of the building. However, existing buildings can be adapted or "retrofitted" such as installing double-pane windows, thermal floors, and a new HVAC system to passively store solar heat and make the facility more energy efficient.



4. Airborne chemicals



Airborne Chemicals

Airborne contaminants have become a major issue in the last 20 years. This is a consequence of sealing buildings tightly for energy efficiency, while having older HVAC systems with improper fresh air make-up settings, as well as poor building operation due to inadequate or missing system operations manuals. The following are contributing factors to poor indoor air quality and suggestions on how airborne contaminants can be greatly reduced.

Biological toxins can be produced from live colonies of mold thriving due to excessive humidity in the occupied space and duct work. Mold can also flourish in condensate drip pans that drain poorly. These conditions cannot be tolerated due to the potential severe health impact. To combat these conditions, regular humidity monitoring and drip pan inspections are needed. Automation systems will monitor humidity minute by minute and drip pan inspections by operations staff should be performed quarterly – when filters are changed.

Volatile Organic Compounds (VOC’s) are airborne chemicals emitted from many building products used in new construction, renovation and restoration projects. These chemicals can cause respiratory irritation and distress in vulnerable people such as children, elderly, and people with allergies or immunodeficient conditions. The sources of these chemicals can be furniture, high-VOC paints, coatings, particle board, caulking, adhesives, fillers, janitorial cleaners and waxes. Even natural gas or propane fired heating and cooking systems can be a source, when poorly vented. It is important to manage VOC levels and keep them as low as possible. Facility managers should maintain adequate ventilation, with fresh air make-up to keep VOC’s below 1 ppm (parts per million) and with no odors.

Inadequate ventilation can be a significant problem when a building lacks sufficient fresh air make-up, particularly in areas of high occupant density such as theaters, meeting rooms, and classrooms, or areas where the concentration of airborne contaminants may be higher due to the nature of the workspace environment. The ASHRAE standard for adequate ventilation is a minimum of 15 cfm of outdoor air per occupant (20 cfm/person in office spaces). Excessive carbon dioxide (CO2) is a good indicator of inadequate fresh air make-up and a useful tool for monitoring indoor air quality. Areas of high density should be equipped with CO2 sensors that control outside air intake to enable effective control. Most operators maintain set fresh air make-up systems to open at 900 ppm. Humidity is also a substantial problem in high density populated areas, thus using humidity controls for congregating areas is also a good strategy for increasing ventilation efficiency.



5. Difference of indoor conditions in the building



Difference Of Indoor Conditions In The Building

Comfort complaints may result from rapid changes in indoor environmental conditions and/or large variations from one area of the building to the next. These swings in indoor environments may be due to thermostats, humidity sensors, and CO2 sensors not calibrated or operating properly, as well as poor design and/or missing sensors. Sometimes necessary equipment is value-engineered (applying various techniques in order to provide the necessary function at the lowest overall cost) out of new construction and renovations. Improper placement of sensors and not installing enough zones to properly control a space are other causes of variable conditions within a space. Even when these issues are properly covered, unevenness in building conditions may still occur from occupant activity such as office partitions (cubicles) and operating desk comfort heaters, which can bias building sensors.



6. Building occupants



Building Occupants

Building managers need to be sensitive to the types of people occupying their buildings and their level of activity. An older workforce, people with disabilities and people working in sedimentary jobs generally require a warmer building environment in order to be comfortable. Younger workers, people who are a bit over weight and those in active jobs usually are more comfortable in cooler environments.



References



The post was based on material found on the following ASHRAE handbooks and code standards:

ASHRAE - Fundamentals of HVAC Systems
ASHRAE - HVAC Systems And Equipment
ASHRAE 55 - Temperature Standards
ASHRAE 62 - Ventilation
ASHRAE 90.1 – Energy Efficiency in Commercial Buildings



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Mechanical Engineer (Ph.D. cand.), M.Sc. Cranfield University, Dipl.-Ing. Aristotle University, Thessaloniki - Greece.
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