Effectiveness of Germicidal UV Radiation for Reducing Fungal Contamination within Air-Handling Units

Levels of fungi growing on insulation within air-handling units (AHUs) in an office building and levels of airborne fungi within AHUs were measured before the use of germicidal UV light and again after 4 months of operation. The fungal levels following UV operation were significantly lower than the levels in control AHUs.
Fungal contamination of air-handling units (AHUs) is a widespread phenomenon in buildings with central heating, ventilation, and air-conditioning (HVAC) systems and is a potential source of contamination for occupied spaces (1, 8, 16, 20). Fungi have been found growing on air filters, insulation, and cooling coils, as well as in ducts. This contamination often contributes to building-related diseases, including both infectious diseases and hypersensitivity diseases, such as allergic rhinitis, asthma, and hypersensitivity pneumonitis (4, 11, 13). In addition, acute toxicosis and cancer have been attributed to respiratory exposure to mycotoxins (5).
Control of fungi in indoor environments has traditionally focused on source control, ventilation, and air cleaning. Source control emphasizes the reduction or elimination of moisture to limit fungal growth. Although this can be effective in many areas, it is not achievable in HVAC systems during cooling. By design, air-conditioning systems cause moisture to condense from air. As a result, other methods are needed to reduce fungal contamination. Ventilation relies on using filtered outdoor and recirculated indoor air. Ventilation is ineffective, however, when unfiltered outdoor air introduces outdoor bioaerosols or when the HVAC system itself is contaminated. Air cleaning has focused on using properly maintained high-quality filters within HVAC systems as well as portable air-cleaning devices. Recently, there has been renewed interest in the use of germicidal UV irradiation to disinfect indoor environments for control of infectious diseases in hospitals, other health care facilities, and public shelters (14, 15, 18, 19).
Although it has been known for many years that UV light has various effects on fungi (3, 9, 10), only a few studies have specifically focused on the effects of germicidal UV light (2, 7, 12, 17, 22, 23). Currently, various manufacturers are marketing germicidal UV lamps for controlling contamination, including fungal contamination in indoor environments, as well as AHUs and ducts. Studies have shown that these measures may be effective for controlling the spread of bacterial diseases (14, 15, 18, 19); however, little is known about the effectiveness of UV-C radiation for controlling fungal contamination. The present investigation was undertaken to determine the effectiveness of germicidal UV radiation for reducing fungal contamination within AHUs.
This investigation was conducted in a 286,000 square-foot office building in Tulsa, Okla. The building was originally constructed in the 1920s and was completely remodeled in 1976. Each floor of this four-story building is equipped with four primary AHUs and two perimeter units; these units were installed when the building was remodeled. Beginning in 1996, the air handlers were retrofitted with germicidal UV lamps. During the fall of 1996 all the AHUs in the building were inspected. At this time UV lamps were installed in AHUs on one floor, and work was progressing to install them on a second floor. Acoustical insulation within many of the AHUs exhibited abundant mold growth, as did drain pans. Preliminary air samples and insulation samples were collected to develop the sampling protocols used in this study.
AHUs on two floors were selected for further investigation; no UV lamps had been installed in these AHUs. The floors were designated the study floor and the control floor. Only the four main AHUs on each of these floors were used for the remainder of the investigation. In May 1997, air samples and insulation samples were collected from the eight AHUs. UV lamps were installed on both floors, but they were activated only in the AHUs on the study floor. Each AHU was retrofitted with 10 lamps, which were installed downstream of the coils. The output of each lamp was 158 μW/cm2 at 1 m or 10 μW/cm2 for every 2.54 cm of tube length at 1 m (21). The lamps were operated 24 h a day throughout the summer and early fall in the AHUs on the study floor. On the control floor, no UV lights were operated. Throughout the building, air conditioning was in use during this period. In late September, samples were collected from all eight AHUs.
Preliminary data showed that air sampling in the AHUs conducted while the AHUs were running resulted in collection of few or no fungal spores because the high airflow rate produced nonisokinetic conditions. For this reason the supply fan in each AHU was shut off prior to sampling. Although this action caused some mechanical disturbance, it provided a method for estimating the potential load of fungal propagules available for dispersal.
Air samples were collected in duplicate by using paired single-stage Andersen (N-6) samplers with malt extract agar plates for viable fungi and paired Burkard personal samplers for total spores. Two-minute Andersen samples and 5-min Burkard samples were collected approximately 40 cm downstream of the cooling coils 30 s after the supply air fan in each AHU was turned off. All samples were started simultaneously, but the Andersen samplers were switched off after 2 min. Samples were obtained from each AHU at least twice in both the spring and the fall.
Plates from the Andersen samplers were incubated at room temperature for 5 to 7 days. Colonies were counted, fungi were identified, and concentrations were expressed in CFU per cubic meter of air. Burkard slides were made permanent by using a lactophenol-polyvinyl alcohol mounting medium, and the slides were examined microscopically at a magnification of ×1,000. Spores were identified and counted. Counts were converted into atmospheric concentrations and expressed in numbers of spores per cubic meter of air. Data from all samples for each AHU were averaged for each time period.
For each AHU, pieces of fiberglass insulation (approximately 60 cm2) were cut from the insulation directly opposite the cooling coils, approximately 1 m from the base, 2 m from the end wall, and less than 30 cm from the UV lights. The insulation samples were individually sealed in sterile plastic bags for transport to the laboratory. In the laboratory, a smaller square of each insulation sample (6.5 cm2) was cut from the center of the larger piece. The small square was soaked in 10 ml of sterile distilled water for 20 min. The suspension was vortexed for 30 s and then dilution plated in triplicate on malt extract agar plates. The plates were incubated at room temperature for 5 to 7 days. Colonies were counted, fungi were identified, and concentrations were expressed in CFU per square centimeter. Data from replicate samples were averaged for each AHU.
For each type of sample collected (viable spores, total spores, and insulation) the concentrations obtained for each AHU were averaged to determine means for the study floor and means for the control floor. Mann-Whitney U tests were used to compare the means in May and in September by using Statistica 5.0 software.
The dominant fungi found within the AHUs for both the air samples and the insulation samples included Penicillium corylophyllum, Aspergillus versicolor, and a strain of an unidentified Cladosporium species which was somewhat similar to Cladosporium sphaerospermum (6) and may be a strain of this species. These three taxa accounted for more than 90% of all viable fungi isolated. Other fungi identified included Acremonium spp., Cladosporium cladosporioides, Cladosporium sphaerospermum, Cladosporium elatum, and Hyalodendron sp. Occasionally other Aspergillus and Penicillium species also occurred in the samples.
In May before the UV lights were turned on, the mean concentrations of the total fungi isolated from the insulation samples on the two floors were similar (Table (Table1),1), and there was no significant difference (P > 0.05). In the fall the mean concentration on the study floor had decreased, while on the control floor the concentrations had increased and were significantly greater than the concentrations on the study floor (P < 0.05). In September the mean concentrations of both A. versicolor and the unknown Cladosporium species were significantly lower in the AHUs on the study floor (P < 0.05). Similar results were obtained with the air samples (Table (Table2).2). In the spring before the UV lights were turned on, the mean concentrations of total viable airborne fungi in the AHUs on the two floors were not significantly different (P > 0.05). In the fall, the mean concentration of viable fungi in the AHUs on study floor was an order of magnitude lower, while on the control floor the concentration of viable fungi in the AHUs had increased. The total concentrations of viable fungi in the AHUs on the study floor and the control floor in the fall were significantly different (P < 0.05). Because many of the AHUs contained high concentrations of viable fungi, there were frequently multiple impactions and multiple colonies at each impaction point on a culture plate. As a result, it was not always possible to identify each colony to the species level. Therefore, the concentration data in Table Table22 are only genus-level data. The concentrations of Penicillium, Aspergillus, and Cladosporium were significantly lower in the AHUs on the study floor than in the AHUs on the control floor after the use of UV lights (P < 0.05). The total spore levels obtained with the Burkard samplers were far greater than the viable spore levels (Table (Table3).3). Prior to the use of UV lights, there was not a significant difference (P > 0.05) between the mean levels of total spores in the AHUs on the two floors. In September, the total concentrations on the study floor were significantly lower than the total concentrations on the control floor (P < 0.05). The fungal taxa identified were consistent with the data obtained with the Andersen sampler and also with the insulation data. However, because it is not possible to differentiate Penicillium and Aspergillus conidia without conidiophores, the two genera are combined as Penicillium-Aspergillus in Table Table3.3. The concentrations of Cladosporium and Penicillium-Aspergillus on the two floors were significantly different in September (P < 0.05).
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ACKNOWLEDGMENTS
Partial support for this project was provided by a grant from Steril-Aire, Inc., Cerritos, Calif.
We thank Melinda Sterling Sullivan, Jodi Keller, and Mary Pettyjohn for assisting with sampling and/or culturing activities. We also acknowledge the unending support and accommodations provided by Tom McKain, Building Supervisor, and Argel Johnson, Maintenance Director, throughout this study.
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Black Mold on your HVAC? Read this article

The presence of mold in an HVAC system is a common complaint. Mold is a sneaky little bugger. It can grow and proliferate and make building occupants sick without ever being seen. And the fastest way to spread mold through a building is through a forced-air HVAC system.
The reason this complaint is so common is that mold is always present in your buildings and your HVAC system to the extent that it is present in your building’s environment. There will be more mold in humid weather and less in dry weather. You will never get rid of it completely, but you can control it. All it needs to grow is moisture and food. Take those away and the mold goes away.
According to the U.S. EPA, you should routinely inspect your HVAC systems, not just for mold, but for moisture. Look at drain and condensate pans to make sure they are draining properly. If they are plugged, the moisture that accumulates will become a mold factory. Also make sure that all HVAC ducts and system components such as air handlers, blowers, plenums and the like are free of any moisture.
If, despite regularly inspecting your system, you are still getting complaints about it (mold starts to grow in as little as 48 hours), here are some tips you can share with your HVAC contractor for cleaning it up:
1.) Turn off your HVAC system.
2.) Everyone involved in this cleaning should wear at least an N-95 respirator
3.) Replace anything porous, such as filters or insulation that has become wet. Double-bag the waste using at 6-mil or thicker plastic bags.
4.) Use wet vacuums to clean out any standing water.
5.) Use an EPA registered disinfectant labeled for HVAC use to clean nonporous surfaces (Ductwork, coils, plenums, pans, etc) of mold, mildew and other dirt. BBJ MMR-II ready-to-use disinfectant and mold cleaner will kill and remove mold, mildew and odor-causing bacteria.
6.) As an added measure, isolate each section of ductwork you clean with bladders so the spores you stir up during cleaning don’t spread to other parts of the system or the building. Fog the area with an EPA registered disinfectant.
7.) Apply a mold and mildew inhibitor to all components of the HVAC systems. Again, this must be EPA registered and specifically labeled for use in HVAC systems to limit risks associated with using the wrong chemicals and cleaners in HVAC systems. Goodway’s CoilShine-BC is EPA registered for use in HVAC systems to control mold growth for up to 2 years.
8.) As a final step, HEPA vacuum anything that you cleaned up.
If you have mold, can it be cleaned safely?
If it is confirmed that you have a problem, the Environmental Protection Agency (EPA) suggests, “Do not run the HVAC system if you know or suspect that it is contaminated with mold – it could spread mold throughout the building.”
This is what the Center’s for Disease Control (CDC) advises if you have suspect that you have mold in your HVAC:
You may need to hire a professional to inspect your system. Any needed repairs or cleaning of vents and air ducts should be performed before restarting the system.
Throw away wet or water damaged filters.
Do not run your HVAC system if you know or think that it is contaminated with mold — it could spread mold throughout your home.
Turn off your HVAC system and cover vents and ducts during cleaning to prevent contaminating it.(3)
Of course, if it is the dead of winter in the cold states or high noon in the summer, it can be dangerous to not have heating or cooling running. With that said, you are going to have to get to work to handle this ASAP.
The first step is to determine if it can be cleaned properly and safely or if you have to replace the ducts in your home. If you are renting and there is no way to clean the system or replace it, then you will have to consider moving to a safe place.
In order to figure out the next step, you need to find out what materials your ducts are made of. This is crucial!
Many modern duct systems are made entirely of sheet metal. Others either have sheet metal with insulation on the exterior or with internal insulation and some are made entirely of fibrous glass insulation.
If you have a duct system that is made entirely of bare sheet metal or sheet metal with exterior insulation, you are most likely in luck. More often than not, they can be cleaned properly and safely if you hire a professional HVAC cleaner who has extensive experience with cleaning mold.
Please keep in mind that you do not want to hire amateurs to do this. Your health and life may be on the line here.
Sheet metal duct systems with an internal glass insulation or made entirely of insulation will have to be removed and replaced if they have water damage and or mold. There is no safe way around this fact and it can be very expensive. Here is what the EPA says, “If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.”

Building automation?

Building automation is the automatic centralized control of a building’s heating, ventilation and air conditioning, lighting and other systems through a building management system or building automation system (BAS). The objectives of building automation are improved occupant comfort, efficient operation of building systems, reduction in energy consumption and operating costs, and improved life cycle of utilities.
Building automation is an example of a distributed control system – the computer networking of electronic devices designed to monitor and control the mechanical, security, fire and flood safety, lighting (especially emergency lighting), HVAC and humidity control and ventilation systems in a building.
BAS core functionality keeps building climate within a specified range, provides light to rooms based on an occupancy schedule (in the absence of overt switches to the contrary), monitors performance and device failures in all systems, and provides malfunction alarms to building maintenance staff. ABAS should reduce building energy and maintenance costs compared to a non-controlled building. Most commercial, institutional, and industrial buildings built after 2000 include a BAS. Many older buildings have been retrofitted with a new BAS, typically financed through energy and insurance savings, and other savings associated with pre-emptive maintenance and fault detection.
A building controlled by a BAS is often referred to as an intelligent building, “smart building”, or (if a residence) a “smart home”. Commercial and industrial buildings have historically relied on robust proven protocols (like BACnet) while proprietary protocols (like X-10) were used in homes. Recent IEEE standards (notably IEEE 802.15.4, IEEE 1901 and IEEE 1905.1, IEEE 802.21, IEEE 802.11ac, IEEE 802.3at) and consortia efforts like nVoy (which verifies IEEE 1905.1 compliance) or QIVICON have provided a standards-based foundation for heterogeneous networking of many devices on many physical networks for diverse purposes, and quality of service and failover guarantees appropriate to support human health and safety. Accordingly, commercial, industrial, military and other institutional users now use systems that differ from home systems mostly in scale. See home automation for more on entry-level systems, nVoy, 1905.1, and the major proprietary vendors who implement or resist this trend to standards integration.
Almost all multi-story green buildings are designed to accommodate a BAS for the energy, air and water conservation characteristics. Electrical device demand response is a typical function of a BAS, as is the more sophisticated ventilation and humidity monitoring required of “tight” insulated buildings. Most green buildings also use as many low-power DC devices as possible. Even a Passivhaus design intended to consume no net energy whatsoever will typically require a BAS to manage heat capture, shading and venting, and scheduling device use.
Automation system
Main article: Building management system
The term building automation system, loosely used, refers to an electrical control system that is used to control a building’s heating, ventilation, and air conditioning (HVAC) system. Modern BAS can also control indoor and outdoor lighting as well as security, fire alarms, and basically everything else that is electrical in the building. Old HVAC control systems, such as 24 V DC wired thermostats or pneumatic controls, are a form of automation but lack the flexibility and integration of the modern system.
Buses and protocols
Most building automation networks consist of a primary and secondary bus which connect high-level controllers (generally specialized for building automation, but may be generic programmable logic controllers) with lower-level controllers, input/output devices and a user interface (also known as a human interface device). ASHRAE’s open protocol BACnet or the open protocol LonTalk specify how most such devices interoperate. Modern systems use SNMP to track events, building on decades of history with SNMP-based protocols in the computer networking world.
Physical connectivity between devices was historically provided by dedicated optical fiber, Ethernet, ARCNET, RS-232, RS-485 or a low-bandwidth special purpose wireless network. Modern systems rely on standards-based multi-protocol heterogeneous networking such as that specified in the IEEE 1905.1 standard and verified by the nVoy auditing mark. These accommodate typically only IP-based networking but can make use of any existing wiring, and also integrate powerline networking over AC circuits, power over Ethernet low-power DC circuits, high-bandwidth wireless networks such as LTE and IEEE 802.11n and IEEE 802.11ac and often integrate these using the building-specific wireless mesh open standard ZigBee).
Proprietary hardware dominates the controller market. Each company has controllers for specific applications. Some are designed with limited controls and no interoperability, such as simple packaged rooftop units for HVAC. The software will typically not integrate well with packages from other vendors. Cooperation is at the Zigbee/BACnet/LonTalk level only.
Current systems provide interoperability at the application level, allowing users to mix-and-match devices from different manufacturers, and to provide integration with other compatible building control systems. These typically rely on SNMP, long used for this same purpose to integrate diverse computer networking devices into one coherent network.
Types of inputs and outputs
Sensors
Analog inputs are used to read a variable measurement. Examples are temperature, humidity and pressure sensors which could be a thermistor, 4–20 mA, 0–10 volt or platinum resistance thermometer (resistance temperature detector), or wireless sensors.
A digital input indicates if a device is turned on or not – however, it was detected. Some examples of an inherently digital input would be a 24 V DC/AC signal, current switch, an air flow switch, or a volta-free relay contact (dry contact). Digital inputs could also be pulse type inputs counting the frequency of pulses over a given period of time. An example is a turbine flow meter transmitting rotation data as a frequency of pulses to an input.
Nonintrusive load monitoring is software relying on digital sensors and algorithms to discover appliance or other loads from electrical or magnetic characteristics of the circuit. It is, however, detecting the event by an analog means. These are extremely cost-effective in operation and useful not only for identification but to detect start-up transients, line or equipment faults, etc.
Controls
Analog outputs control the speed or position of a device, such as a variable frequency drive, an I-P (current to pneumatics) transducer, or a valve or damper actuator. An example is a hot water valve opening up 25% to maintain a setpoint. Another example is a variable frequency drive ramping up a motor slowly to avoid a hard start.
Digital outputs are used to open and close relays and switches as well as drive a load upon command. An example would be to turn on the parking lot lights when a photocell indicates it is dark outside. Another example would be to open a valve by allowing 24VDC/AC to pass through the output powering the valve. Digital outputs could also be pulse type outputs emitting a frequency of pulses over a given period of time. An example is an energy meter calculating kWh and emitting a frequency of pulses accordingly.
Infrastructure
Controller
Controllers are essentially small, purpose-built computers with input and output capabilities. These controllers come in a range of sizes and capabilities to control devices commonly found in buildings and to control sub-networks of controllers.
Inputs allow a controller to read temperature, humidity, pressure, current flow, air flow, and other essential factors. The outputs allow the controller to send command and control signals to slave devices, and to other parts of the system. Inputs and outputs can be either digital or analog. Digital outputs are also sometimes called discrete depending on the manufacturer.
Controllers used for building automation can be grouped into three categories: programmable logic controllers (PLCs), system/network controllers, and terminal unit controllers. However, an additional device can also exist in order to integrate third-party systems (e.g. a stand-alone AC system) into a central building automation system.
Terminal unit controllers usually are suited for control of lighting and/or simpler devices such as a package rooftop unit, heat pump, VAV box, fan coil, etc. The installer typically selects one of the available pre-programmed personalities best suited to the device to be controlled and does not have to create new control logic.
Occupancy
Occupancy is one of two or more operating modes for a building automation system. Unoccupied, Morning Warmup and Night-time Setback are other common modes.
Occupancy is usually based on time of day schedules. In Occupancy mode, the BAS aims to provide a comfortable climate and adequate lighting, often with zone-based control so that users on one side of a building have a different thermostat (or a different system, or subsystem) than users on the opposite side.
A temperature sensor in the zone provides feedback to the controller, so it can deliver heating or cooling as needed.
If enabled, morning warmup (MWU) mode occurs prior to occupancy. During Morning Warmup the BAS tries to bring the building to setpoint just in time for Occupancy. The BAS often factors in outdoor conditions and historical experience to optimize MWU. This is also referred to as an optimized start.
An override is a manually initiated command to the BAS. For example, many wall-mounted temperature sensors will have a push-button that forces the system into Occupancy mode for a set number of minutes. Where present, web interfaces allow users to remotely initiate an override on the BAS.
Some buildings rely on occupancy sensors to activate lighting or climate conditioning. Given the potential for long lead times before space becomes sufficiently cool or warm, climate conditioning is not often initiated directly by an occupancy sensor.
Lighting
Lighting can be turned on, off, or dimmed with a building automation or lighting control system based on time of day, or on occupancy sensor, photosensors, and timers. One typical example is to turn the lights in a space on for a half-hour since the last motion was sensed. A photocell placed outside a building can sense darkness, and the time of day, and modulate lights in outer offices and the parking lot.
Lighting is also a good candidate for demand response, with many control systems providing the ability to dim (or turn off) lights to take advantage of DR incentives and savings.
In newer buildings, the lighting control can be based on the field bus Digital Addressable Lighting Interface (DALI). Lamps with DALI ballasts are fully dimmable. DALI can also detect lamp and ballast failures on DALI luminaires and signals failures.
Air Handlers
Most air handlers mix return and outside air so less temperature/humidity conditioning is needed. This can save money by using less chilled or heated water (not all AHUs use chilled or hot water circuits). Some external air is needed to keep the building’s air healthy. To optimize energy efficiency while maintaining healthy indoor air quality (IAQ), demand control (or controlled) ventilation (DCV) adjusts the amount of outside air based on measured levels of occupancy.
Analog or digital temperature sensors may be placed in space or room, the return and supply air ducts, and sometimes the external air. Actuators are placed on the hot and chilled water valves, the outside air and return air dampers. The supply fan (and return if applicable) is started and stopped based on either time of day, temperatures, building pressures or a combination.
Constant volume air-handling units
The less efficient type of air-handler is a “constant volume air handling unit,” or CAV. The fans in Cavs do not have variable-speed controls. Instead, Cavs open and close dampers and water-supply valves to maintain temperatures in the building’s spaces. They heat or cool the spaces by opening or closing chilled or hot water valves that feed their internal heat exchangers. Generally, one CAV serves several spaces.
Variable volume air-handling units
A more efficient unit is a “variable air volume (VAV) air-handling unit”, or VAV.VAVs supply pressurized air to VAV boxes, usually one box per room or area. A VAV air handler can change the pressure to the VAV boxes by changing the speed of a fan or blower with a variable frequency drive or (less efficiently) by moving inlet guide vanes to a fixed-speed fan. The amount of air is determined by the needs of the spaces served by the VAV boxes.
Each VAV box supply air to a small space, like an office. Each box has a damper that is opened or closed based on how much heating or cooling is required in its space. The more boxes are open, the more air is required, and a greater amount of air is supplied by the VAV air-handling unit.
Some VAV boxes also have hot water valves and an internal heat exchanger. The valves for hot and cold water are opened or closed based on the heat demand for the spaces it is supplying. These heated VAV boxes are sometimes used on the perimeter only and the interior zones are cooling only.
A minimum and maximum CFM must be set on VAV boxes to assure adequate ventilation and proper air balance.
Air Handling unit (AHU) Discharge Air Temperature control
Air Handling Units (AHU) and Roof Top units (RTU) that serve multiple zones should vary the DISCHARGE AIR TEMPERATURE SET POINT VALUE automatically in the range 55 F to 70 F. This adjustment reduces the cooling, heating, and fan energy consumption. When the outside temperature is below 70 F, for zones with very low cooling loads, raising the supply-air temperature decreases the use of reheat at the zone level.
VAV hybrid systems
Another variation is a hybrid between VAV and CAV systems. In this system, the interior zones operate as in a VAV system. The outer zones differ in that the heating is supplied by a heating fan in a central location usually with a heating coil fed by the building boiler. The heated air is ducted to the exterior dual duct mixing boxes and dampers controlled by the zone thermostat calling for either cooled or heated air as needed.
Central plant
A central plant is needed to supply the air-handling units with water. It may supply a chilled water system, hot water system, and a condenser water system, as well as transformers and auxiliary power unit for emergency power. If well managed, these can often help each other. For example, some plants generate electric power at periods with peak demand, using a gas turbine, and then use the turbine’s hot exhaust to heat water or power an absorptive chiller.
Chilled water system
Chilled water is often used to cool a building’s air and equipment. The chilled water system will have a chiller(s) and pumps. Analog temperature sensors measure the chilled water supply and return lines. The chiller(s) are sequenced on and off to chill the chilled water supply.
A chiller is a refrigeration unit designed to produce cool (chilled) water for space cooling purposes. The chilled water is then circulated to one or more cooling coils located in air handling units, fan-coils, or induction units. Chilled water distribution is not constrained by the 100-foot separation limit that applies to DX systems, thus chilled water-based cooling systems are typically used in larger buildings. Capacity control in a chilled water system is usually achieved through modulation of water flow through the coils; thus, multiple coils may be served from a single chiller without compromising control of any individual unit. Chillers may operate on either the vapor compression principle or the absorption principle. Vapor compression chillers may utilize reciprocating, centrifugal, screw, or rotary compressor configurations. Reciprocating chillers are commonly used for capacities below 200 tons; centrifugal chillers are normally used to provide higher capacities; rotary and screw chillers are less commonly used, but are not rare. Heat rejection from a chiller may be by way of an air-cooled condenser or a cooling tower (both discussed below). Vapor compression chillers may be bundled with an air-cooled condenser to provide a packaged chiller, which would be installed outside of the building envelope. Vapor compression chillers may also be designed to be installed separately from the condensing unit; normally such a chiller would be installed in an enclosed central plant space. Absorption chillers are designed to be installed separately from the condensing unit.
Condenser water system
Cooling towers and pumps are used to supply cool condenser water to the chillers. Because the condenser water supply to the chillers has to be constant, variable speed drives are commonly used on the cooling tower fans to control temperature. Proper cooling tower temperature assures the proper refrigerant head pressure in the chiller. The cooling tower setpoint used depends upon the refrigerant being used. Analog temperature sensors measure the condenser water supply and return lines.
Hot water system
The hot water system supplies heat to the building’s air-handling unit or VAV box heating coils, along with the domestic hot water heating coils (Calorifier). The hot water system will have a boiler(s) and pumps. Analog temperature sensors are placed in the hot water supply and return lines. Some type of mixing valve is usually used to control the heating water loop temperature. The boiler(s) and pumps are sequenced on and off to maintain supply.
The installation and integration of variable frequency drives can lower the energy consumption of the building’s circulation pumps to about 15% of what they had been using before. A variable frequency drive functions by modulating the frequency of the electricity provided to the motor that it powers. In the USA, the electrical grid uses a frequency of 60 Hertz or 60 cycles per second. Variable frequency drives are able to decrease the output and energy consumption of motors by lowering the frequency of the electricity provided to the motor, however, the relationship between motor output and energy consumption is not a linear one. If the variable frequency drive provides electricity to the motor at 30 Hertz, the output of the motor will be 50% because 30 Hertz divided by 60 Hertz is 0.5 or 50%. The energy consumption of a motor running at 50% or 30 Hertz will not be 50%, but will instead be something like 18% because the relationship between motor output and energy consumption are not linear. The exact ratios of motor output or Hertz provided to the motor (which are effectively the same thing), and the actual energy consumption of the variable frequency drive/motor combination depend on the efficiency of the variable frequency drive. For example, because the variable frequency drive needs power itself to communicate with the building automation system, run its cooling fan, etc., if the motor always ran at 100% with the variable frequency drive installed the cost of operation or electricity consumption would actually go up with the new variable frequency drive installed. The amount of energy that variable frequency drives consume is nominal and is hardly worth consideration when calculating savings, however, it did need to be noted that VFD’s do consume energy themselves. Because the variable frequency drives rarely ever run at 100% and spend most of their time in the 40% output range, and because now the pumps completely shut down when not needed, the variable frequency drives have reduced the energy consumption of the pumps to around 15% of what they had been using before.