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As warmer weather slowly approaches we will begin a new cooling season with a rise in air temperature and relative humidity levels. Indoor relative humidity levels is a component along with temperature and pollutants that determine Indoor Air Quality (IAQ) within buildings. Poor indoor air quality is typically associated with Building Related Illness (BRI) and Sick Building Syndrome which can lead to performance losses of personnel. During the 1970’s, 1980’s and 1990’s significant changes in code requirements led to tighter building construction and use of different construction materials which can contribute to moisture buildup within the indoor environment. This moisture buildup or high indoor relative humidity conditions can have significant negative effects on the building and its occupants. Mold growth is a common concern as is the potential destructive nature of building components including within exterior walls that can’t be easily inspected. There are many different reasons why high indoor relative humidity conditions exists however, three (3) primary sources include infiltration through the building envelope (roofs, walls, windows, doors, attics, basements, etc.), generation of moisture in the building and the buildings air conditioning system. A leaky building envelope or how the envelope was designed/constructed are often easily visualized, felt and or diagnosed. Moisture generated by occupants or activities such as cleaning are also known.

All too often the building’s air conditioning system is the reason for high indoor relative humidity levels.

As building codes required tighter building envelope construction, reducing air leakage and increasing insulation requirements our buildings no longer breathe for the sake of reducing energy usage. These tighter more efficient buildings cause pollutants generated by occupants (e.g. carbon dioxide) to buildup. As a result of these findings, code required ventilation rates were increased. Ventilation essentially is outdoor air introduced into the building by the Heating, Ventilation, and Air Conditioning (HVAC) system to dilute these pollutants to provide indoor air quality that is acceptable to human occupants and that minimizes adverse health effects according to ASHRAE. Ventilation rates vary based on the floor area and use of the space (office, classroom, etc.). Occupant density (people per square foot) is also defined by code based on space function.

Atmospheric air used for ventilation air purposes is a combination of dry air and superheated steam (water vapor) in a low pressure condition (i.e. moisture).

Specific humidity is the mass of this water vapor present in a unit mass of dry air. Relative humidity expresses the amount of water vapor in the air relative to the amount of moisture it would hold if saturated at that temperature or percent saturated.

Air is a compressible fluid. The volume of air increases with increased temperature and decreases on a reduction of temperature. As we move into warmer weather conditions warmer air can hold more moisture relative to its saturation point or 100% relative humidity (raining). This is why our buildings are dry during the heating season and wet during the cooling season. As Engineers, we can easily calculate these conditions using a psychrometric chart and equations. Take for instance, on a 40°F day at 50% relative humidity outdoor air is introduced into a 70°F indoor space. The resulting indoor relative humidity is 15% as warmer air has more volume to hold molecules of moisture than the cooler outdoor air. During the summer however, outside air is warmer than the occupied space and if outside air is only cooled but not dehumidified the indoor relative humidity increases. As an example, when outdoor air is 85°F at 60% relative humidity is introduced into an occupied space that is air conditioned to 75°F the resulting indoor relative humidity is approximately 85%.

Dehumidification is typically a byproduct of air conditioning however, supply air needs to be cold enough leaving the cooling coil for dehumidification to occur. As HVAC Engineers we design for the peak or maximum cooling load condition yet these conditions only occur a few hours during the year. At part load conditions supply air can be cooled but not made cold enough for moisture to be extracted out of the air stream. The mixture of return air from the occupied space and outside air required for code required ventilation has to be cooled low enough for this moisture to be condensed out of the air stream which we call dehumidification. This condensation occurs when the cooling coil is at or below the dew point temperature of the mixed air stream. The condensation effect is no different than taking a can of your favorite beverage out of the refrigerator and placing it on your counter or taking it outside. If the can surface temperature is below the dew point temperature of the surrounding air then water droplets/condensation will form on the surface of the can until the can warms up.

When a building is experiencing high indoor relative conditions, primarily on part load conditions during the cooling season it could be that the HVAC unit is injecting moisture from outside into the building via the ventilation air and the air conditioning unit may be cooling the air to satisfy the space thermostat but not making the air cold enough to dehumidify. Most problematic areas will be large gathering spaces with a single thermostat or spaces with large fluctuation of occupants. Other conditions can be an unoccupied building that is operating as if it was occupied or spaces that are subcooled (i.e. less than 75°F).

If these conditions occur and condensate is not draining out of your air conditioning unit there are two (2) general approaches which should be looked into. First, try to make the supply air colder, typically between 55°F – 59°F without sub-cooling the space. The space air conditioning load is satisfied by the air flow rate and the supply air temperature. As the load varies either the supply air temperature or the supply air flow rate needs to vary. To achieve a lower supply air temperature condition, lower the fan speed/air flow. Sometimes air conditioning units have two (2) speed fans or ECM adjustable speed motors that can change air flow and larger air handling unit fans can be equipped with a variable speed drive.

Secondly, reduce the source of moisture by reducing your outside air flow rate by using space carbon dioxide sensors in a code compliant manor. When there are less occupants in the building or space the amount of outdoor air can be reduced accordingly. Additionally, if a space just needs to be cooled but is not occupied, mechanical ventilation is not required. This is particularly true for assembly spaces and/or buildings not used continuously but still cooled (churches, schools). An occupancy sensor can be used to determine occupancy or occupancy can be scheduled through a time clock feature of your air conditioning unit controller.

High indoor relative humidity levels can be very problematic for the building and its occupants during the air conditioning season. Sometimes understanding why it occurs and ways to reduce it is not simply recognizable.

Buildings including residential consume approximately 40% of the world’s energy followed by transportation and industry.

On January 1, 2015 the State of Maryland adopted the 2015 International Energy Conservation Code or IECC with adoption and enforcement on July 1, 2015. One tool to determine compliance is called COMcheck which allows the equivalent of ASHRAE (American Society of Heating Refrigerating and Air Conditioning Engineers) Standard 90.1-2013 Energy Standard for Buildings. Maryland and Vermont were the first two states which adopted this standard. As of this writing two additional states, Alabama and New Jersey have adopted this standard.

This baseline standard for high performance buildings has significantly changed how buildings are designed, constructed and operated over the past decade. While various types of buildings have different energy savings results, on average 30% energy savings were realized between ASHRAE 90.1-2004 and ASHRAE 90.1- 2010 standards. An additional +7% energy savings is anticipated for buildings designed to ASHRAE 90.1- 2013 standards.

It’s both a challenging time and exciting time for engineering innovative products and then applying them in a building design. Previous creative high performance building designs used in the past have been adopted by codes as requirements today. More than ever designing buildings to meet or exceed these code requirements is a holistic team approach.

There are several basic considerations and steps when designing buildings to optimize energy savings and comply with these mandatory requirements.

Reducing heat gains and losses through the building envelope (walls, windows, and roofs) is easily accomplished by increasing the insulating values of materials used, the reflectivity and thermal mass of those materials as well as the solar orientation of the building.

Reducing internal heat gains also significantly affects energy savings. Energy efficient lighting and products not only reduces the energy these devices consume but it also reduces the air conditioning loads. Lighting power density limits alone have been reduced from 2.0 watts per square foot or more to .9 watts per square foot or less depending on the application. LED lighting systems are quickly becoming standard design practice. The most significant changes are the requirements for lighting controls utilizing daylight harvesting/automatic dimming, occupancy and vacancy sensors, etc. to be able to automatically turn lights on or off. Additional controls are also being implemented to de-energize receptacles during unoccupied/night times.

The reduction of heat gains and losses through the building envelope coupled with the reduction of heat producing equipment and lights allows the heating and cooling equipment capacities to be reduced thus also conserving energy.

While significant changes have occurred through the type and control of lighting systems, the current code requirements also significantly changes how Heating, Ventilating and Air Conditioning (HVAC) systems are designed since these systems are the largest consumers of energy in a building. There are two primary components which affect energy usage, the generation of the heating and cooling medium and the transportation of the medium to where needed in the building. While manufacturers are responsible for developing energy efficient equipment that complies with the minimum efficiencies required by code to generate the heating or cooling medium it’s still the responsibility of the design engineer to apply this equipment in a system to maximize its performance at peak and part load conditions. The code has also become more stringent in limiting the energy consumed by building systems to distribute the energy, by fans and pumps, through the building. Pipe and duct sizes are larger today to meet reduced frictional pressure losses. Distribution equipment needs to be strategically located to reduce the most critical pressure drop path so as to minimize pump and fan motor brake horsepower. Together the design of the water and air distribution systems have to meet these power limitations.

Another way the code has significantly changed over the years and is a result of changes in technology is the use of variable speed drives (VSD) and electronically commutated motors (ECM) to vary the speed of motors. For example, a 10 HP fan operating at 50% capacity uses 1.25 HP of energy. ECM’s have burst on the scene replacing inefficient shaded pole and permanent split capacitor fractional horsepower motors typically used for fan coil units, heat pumps, exhaust fans, in-line circulators, etc. and are becoming available in larger horsepower motors. ECM motors can also vary in speed based on load (temperature sensor signal).

The mandatory use of heat recovery is becoming more stringent in each update of the code as well and we see that trend continuing in the future. Air side heat recovery uses building relief or exhaust air stream to pass through a heat recovery device to reclaim and reuse energy to precondition outside air for code required ventilation before wasting it to atmosphere. While air side heat recovery has been predominate the past decade, new equipment and system designs are incorporating water side heat recovery. Water side heat recovery can capture and reuse waste heat from air conditioning equipment to make useful heat for heating the building when both mechanical cooling and heating are needed. All in all, the codes are directing the system design to reuse every possible BTU of energy before wasting it to the atmosphere or ground heat sink.

As with lighting systems much is dependent on automatic controls to maximize energy savings for the equipment applied to an engineered system that is unique to each building. Carbon dioxide sensors are being required in smaller assembly spaces to control how much outdoor air is being introduced into the building to meet ventilation requirements. Robust weather sensors to determine ambient air conditions, devices controlling speeds of large motors and meters used to measure flow all play an important role to sequence equipment in the operation of the system while meeting all code setpoints and reset requirements.

Most importantly however is selection of system type, its associated equipment, and the system design for a building. There is no silver bullet solution as each building is unique to itself. Geothermal systems are a popular choice since for every one BTU of energy consumed, on average three BTU’s of useful energy are free. Central heating and cooling systems using boilers, chillers and air handling equipment have long been the system of choice. A relatively new system to the US market, a variable refrigerant volume (VRV) or variable refrigerant flow (VRF) offers a new energy efficient choice. This system is refrigerant based, uses a variable speed compressor to transport energy and has the fewest amount of heat exchangers which increases the efficiency of a system.

The use of energy modeling at the earliest concept stages of design and through life cycle cost analysis can help in determining the type of system.

Industry standard equipment, building design, conventional design and operating parameters and how systems are controlled are in the state of change. Today’s building design engineers need to be innovative and creative in the development of systems while using the laws of engineering to maximize the coefficient of performance (COP) for each building.

Alban Engineering is a Mechanical/ Electrical / Plumbing Consulting Engineering firm specializing in LEED Certified and High Performance design for Public and Educational Facilities. Jeff Alban has won several Regional ASHRAE Technology Awards for innovative designs which incorporate ASHRAE standards.