HVAC

Cooling towers serve as the heat-rejection device for the building’s cooling system. Warm water carries waste heat from the chiller to the cooling tower, which is frequently located on the roof. Through thermal interaction with the outdoor air and evaporation, the water is cooled and returned to the chiller to pick up more heat. A fan often aids in the mixing of air and water to maximize heat transfer in minimal space. Make up water must be added when water leaves the cooling tower via evaporation, leaks, or wind-driven losses. The water is frequently flushed away through a process called “blow down” to remove contaminant building up. Blow down can use a lot of water, so use a conductivity meter to ensure that water is flushed away only when needed.

Because cooling towers are exposed to the outdoor elements, maintenance is especially important to ensure efficient operation.

System Relationships

Water

• Conductivity meters can ensure that blow-down water is used only when necessary.

Ducts convey conditioned air from the air handling unit or AHU out through the building and return it back to be conditioned again, or exhaust it from the building. They are usually constructed of sheet metal and are often insulated to reduce heat loss and lesson noise transfer to the space.² The size of ductwork, along with the number of turns and size transitions, affects the amount of energy needed by the fans to deliver air. Think of trying to use a drinking straw: a large, straight straw will require less effort than a narrow, bendy straw.² Regularly clean your ducts to reduce friction that must be overcome by fans. Also ensure that dampers, which are like valves that adjust how much air enters a duct, are working properly to avoid unnecessary blockage of airflow pathways.

Air delivered by ductwork enters a space via diffusers and registers, which are usually on the ceiling or wall. Air is returned to the AHU or exhausted through grills or perforated openings in the wall or ceiling. Overhead air distribution is most common way that conditioned air enters a room; however, this method frequently produces drafty conditions and makes individual control more difficult. Underfloor air distribution systems serve the space from below, often with many adjustable openings to allow for lower air speeds and increased controllability by occupants at their workstations. Displacement ventilation strategies may be paired with underfloor air distribution to save energy in certain applications.

System Relationships

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• Clean ductwork is more likely to be free of mold and other allergens.
• Properly sized ductwork and diffusers are less noisy than when undersized.

By the time you have taken free air from the outdoors and filtered it, heated or cooled it, and adjusted its humidity, you have a building full of relatively expensive air. Much of this expensive air must be exhausted from our bathrooms and other spaces. The value lies not in the air itself, but the energy that went into conditioning that air. Much of this value can be saved through energy recovery systems. Outside air that is brought into the building can first be made to interact with exhaust air to exchange heat and/or humidity without the two airstreams actually mixing. Energy recovery systems can save 50% to 80% of the energy used to condition incoming outdoor air,² and can come as runaround coils, heat pipes, and the following:

• Plate-and-frame heat exchangers – exhaust ducts and outdoor air ducts are brought together to pass heat to one another, typically across a series of metal plates. These systems are fairly low cost, but only transfer sensible heat from one stream to the other.
• Energy recovery wheels – exhaust ducts and outdoor air ducts are exposed to a rotating wheel of desiccant material. The warmer, more humid air stream deposits moisture and temperature on the material as it passes through. The wheel then rotates to the other air stream for energy “pickup.” These systems tend to be more expensive than other systems, but save enough energy to have attractive payback period.

System Relationships

Water

• When recapturing humidity with desiccant wheels, water is saved that would otherwise be needed for humidification.

Fans move air throughout the building. Several types of fans are available to serve a variety of applications, from delivery of air through ductwork to exhaust of air from a parking garage. Some examples of fan types include centrifugal, axial, and propeller. Fans are typically sized for the “worst case scenario,” but many situations allow for a reduced flow of air, such as mild weather or weekend operation. Variable frequency drives or VFDs can be added to new or existing fans to throttle back the amount of air delivered while reducing the amount of energy used.

Fans move four main types of air:

• Outdoor – fresh air brought from outside the building, used for breathing, building pressurization, and replacement of air that leaves the building. Outdoor air is sometimes referred to as “make up air,” typically to replace air being exhausted via kitchen, laboratory, or other process exhaust.
• Supply – conditioned air that is used to condition the building. Supply air is typically heated or cooled, and can be a mix of both outdoor air and return air.
• Return – air that has already been used to condition the space, and is returned at room temperature back to the air handling unit for conditioning.
• Exhaust – air that is no longer suitable for reuse and is ejected to the outside of the building. Examples of exhaust air include air from restrooms, kitchens, laboratories, and specialty spaces where air quality is a concern.

System Relationships

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• When tied to CO2 monitors, fans can deliver increased outdoor air when needed, such as a large meeting in a conference room.

HVAC systems do more than make up for the temperature and humidity differences between indoors and outdoors. Internal loads, such as computers, lights, and other equipment contribute waste heat to the space. This heat must be removed from the building in order to maintain a comfortable indoor environment. As a result, where internal loads can be reduced, cooling needs can also be reduced. Specify high efficiency lighting, educate building occupants about turning off and unplugging unneeded devices, employ energy saving computer and monitor settings, and purchase ENERGY STAR® appliances and equipment. When renovating a space, reduce internal loads first before sizing HVAC equipment; you may find that you can buy a smaller, less expensive system to handle the reduced loads.

System Relationships

Lighting

• Combine daylight sensors with HVAC zones near the exterior windows during time periods when daylight harvesting is possible to reduce HVAC and artificial lighting loads.

An energy saving, stormwater managing, and attractive alternative to traditional roofing surfaces is the planted roof. These roofs employ layers of waterproofing material, soil, and plant life to increase insulation values and promote evaporation and transpiration of rainwater before it reaches our sewers. Planted roofs can even serve as rooftop parks and leisure spaces for building occupants to enjoy.

The two major types of planted roofing are intensive and extensive. Intensive planted roofs have thick layers of soil — 6 to 12 inches or more — that can support a broad variety of plant or even tree species. These are typically the more expensive option, but can provide greater insulation and water treatment benefits. Extensive roofs are simpler planted roofs with a soil layer of 6 inches or less to support turf, grass, or other ground cover. Whichever type you choose, be sure to select plant species that can thrive with your climate’s rainfall to avoid the need for irrigation.³

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• Planted roofs reduce the amount of water needing treatment by storm and sewer systems.

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• If your roof is accessible, consider adding seating so that occupants can enjoy their rooftop garden during breaks.

The color, insulation value, and orientation of roof and walls have a great impact on the amount of energy lost through these surfaces. Lighter colored walls and roofs reflect more heat from the sun. A reflective, “cool roof” can lower the roof temperature by up to 50°F.⁴ Tightly constructed buildings allow less air to escape, reducing infiltration of unwanted outside air that must then be conditioned. Insulation can be added to framing materials, sprayed in gaps, or be integral to the walls and roof, as is the case with structural insulated panels or SIPs.⁵

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• Well insulated walls reduce cold spots for those sitting at the building perimeter.

Windows are a great way to allow daylight to enter a room, but they also are subject to heat loss. New, multi-paned windows have higher insulating values than ever before, but should still be placed strategically along the façade. Design window openings to allow visible light into the space while avoiding glare to work areas. Operable windows also let occupants control the temperature and fresh air in their spaces, but can let in undesirable humidity at times. Connect sensors from the operable windows to the building control system to alert occupants and maintenance staff to close windows when conditions might promote mold growth or moisture damage. Metal window frames also transfer heat more quickly into and out of a space. Choose frames with thermal breaks to slow down this heat loss.

Shading strategies let light into our spaces when it is of most use and block it when we want to avoid glare or additional heat gain. Window films can reflect direct sunlight away from the building. External overhangs let the sun shine in during winter months, when the sun is low on the horizon, and block it during summer months, when the sun is high overhead. Manual blinds give control to building occupants, and automated shades can be tied into the building control system. The presence of neighboring buildings and trees can also help cool a building, so these should be considered when designing both windows and heating and cooling system.

System Relationships

Lighting

• Daylighting through windows combined with lighter reflective surfaces can reduce the need for artificial light, lowering the amount of heat given off by fixtures.

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• Consider translucent panels as a source of daylight with higher insulation values than windows.

In its simplest form, an air handling unit or AHU is a mechanical device which combines a fan and a source of heating or cooling. Air is drawn across an electric heating element or through a coil of hot or chilled water pipes, conditioning the air to the proper temperature and humidity for distribution throughout the building. Most AHUs include a combination of heating and cooling elements, dampers for mixing outside and return air, filters to clean the air, drainage for dehumidification condensate, and a variety of controls that shift AHU operation in response to temperature, humidity, and even CO2 changes in the space. You may also find humidifiers, smoke detectors, or other accessories to ensure comfort and safety for building occupants. AHUs may be in mechanical rooms or packaged units on the roof. AHUs typically are classified as “constant volume”, or CV, or “variable air volume”, or VAV, delivering either a consistent volume of conditioned air or a varying amount of air depending on space needs. Both systems have advantages, but VAV systems typically save significant amounts of energy when compared to CV.

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• AHU fans can be loud. Place AHUs away from regularly occupied spaces or install sound insulation.

Boilers use electricity, natural gas, or other energy sources to heat water or create steam for heating systems throughout a building. As water evaporates within the boiler, dissolved solids become more concentrated, causing scale to build up in the system that reduces system efficiency. To reduce scale buildup, water is rejected from the system in a process known as “blow down” and freshwater, known as “make-up,” is added. The ratio of impurities⁶ in the blow down to make-up is known as “cycles of concentration”. The higher the cycles of concentration, the less make-up is required. Consider the following measures to increase the cycles of concentration and water efficiency of boilers:

• Institute water treatment program to reduce the buildup of dissolved solids and increase cycles of concentration
• Ensure that there is a well-maintained condensate return loop on steam systems that sends captured condensate via steam traps back to the boiler, which will in turn reduce energy, chemical and water use
• Monitor condensate return and automatic blow down systems closely for leaks to prevent wasted water use
• Install an automatic blow down and chemical feed system that rejects water based on specific water quality thresholds

If you are in need of a new boiler, choose a high efficiency option. The average existing boiler loses 24% of the energy intended for heating right out the flue, before it does any useful water heating. Modern boilers can exceed efficiencies of 90%.² Maintenance is key to proper boiler operation.

There are many different ways to provide space cooling to a facility. A chiller is a device that generates chilled water and distributes it throughout the facility to cool the space. Chillers are most frequently used in large facilities, providing space cooling in 20% of all commercial building floor space and 32% of office building floor space.⁷ Most chillers use electric motors to drive a mechanical vapor-compression cycle that generates chiller-water. However, absorption chillers can generate chilled water using heat energy as a driver.

Many opportunities exist to increase the efficiency of existing chillers. However, if you are in need of a new chiller, choose a high-efficiency option. The DOE Federal Energy Management Program, also known as FEMP, provides guidance on efficiency requirements for water-cooled and air-cooled electric chillers. Building automation controls are key to minimizing energy consumption.

System Relationships

Lighting

• More efficient lighting systems can reduce the load on the space cooling system. This can mean a smaller chiller size required to cool the facility, as well as lower space cooling costs.

Water

• Water-cooled chillers use cooling towers to reject waste heat to the environment through evaporation. Air-cooled chillers do not use water but are less efficient than water-cooled chillers.

Mechanical systems provide heating or cooling in response to signals from control systems. Depending on the system type, these controls may change the supply temperature, humidity, or amount of outdoor air to an individual room, larger zone, or the building as a whole. Buildings may be divided into multiple zones according to HVAC requirements, size, and location. Thermostats and temperature sensors send temperature information to the air handling unit, which alters the amount or condition of delivered air to meet established set points. Some buildings have demand control ventilation, providing just enough outside air to meet the needs of the occupants. Outside air is expensive to condition, so during times where few people are in a space, as determined by CO₂ monitors, less outside air is delivered.

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• Individual thermal controls allow for better comfort.

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• Changes to some control sequences in a multi-sensored automated HVAC system can affect each other. Ensure that a device behaves appropriately by paying careful attention to the system as a whole.

When outdoor temperature and humidity conditions are just right, tremendous energy savings can be achieved through use of an economizer. With air side economizers, for example, if your space requires 60 degree air for cooling and it is 60 degrees outside, it may make sense to just use outdoor air rather than activating the chiller to condition the return air. If conditions are not quite perfect, control systems can activate dampers to utilize a mix of outdoor and return air to achieve the needed supply air conditions while using little or no mechanical cooling. Water-side economizers work on a similar principle, but use the cooling towers to leverage “free cooling” rather than running the chiller. Because economizer operation requires a fine balance between controls, equipment, and dampers, these systems should be regularly checked for proper operation. A functioning economizer can save a lot of energy, but a malfunctioning economizer can use even more energy than not having one at all.

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• Air-side economizer use can increase the amount of fresh, outside air for occupants.

When the air we breathe is clear of dust, pollen, and other particles, we are more likely to be comfortable at work and perform well on the job. Filters, usually found in or near the air handling unit, help clean the air as well as protect HVAC equipment from build-up that can reduce performance. Certain filters are better at removing small particles than others. Look for the filter efficiency as a percentage or the MERV rating — typically between 1 and 20 — when selecting a filter; the higher the MERV number, the smaller the particle trapped by the filter. Filters with lower MERV ratings may be appropriate for residential use, medium ratings for commercial spaces, and high ratings for hospitals and clean rooms.⁸

Higher filtration efficiency typically comes with a higher price and a greater strain on building fan systems, so choose a filter appropriate for your application. Many filters are disposable, recyclable, or washable. Electrostatic filters capture particles through an electronically charged surface, and must be washed regularly. If using a more expensive, more efficient filter, consider putting a low-cost pre-filter upstream. This technique will help extend the life of the more expensive filter, catching big particles with the less expensive pre-filter.

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• High efficiency filters remove particles that might cause allergy or asthma attacks.
• Filters add strain to building fans, so choose just enough filtration for your application.

Furnaces heat air, typically with electricity or through combustion of natural gas or other fossil fuels, which is then distributed throughout the building for warmth. The average existing furnace is only 76% efficient, but newer models can exceed efficiencies of 90%.² Furnaces are usually less complex heating systems than boilers, so replacement with a more efficient model can have an even more attractive return on investment. Single stage electric systems can be replaced with two-stage controls, so that the system is not merely on or off, but can have an intermediate setting to meet milder heating needs.

Piping establishes the path through which water and other liquids will travel throughout and beyond the building. Piping can carry chilled water, heating hot water, steam, domestic drinking water, fire protection water, and wastewater. For HVAC systems, avoid loss of energy used to heat and chill water by adding pipe insulation. Efficient pipe routing with minimal pipe bends throughout the building helps reduce first costs and the pumping energy needed. Specifying a larger size of pipe will add somewhat to first costs, but pumping energy may be greatly reduced due to lower friction.⁹

System Relationships

Water

• Leaky HVAC pipes contribute to increased need for makeup water.
• Chilled water and steam are often treated with chemicals, so fixing leaks saves on these materials as well.

Pumps are devices that use a motor to move a fluid from one place to another. In the context of buildings, pumps typically move water through pipes vertically or across long horizontal distances for use, such as cold water originating from a chiller that needs to get to the air handling unit in order to cool spaces in the building. Pipe size, material, and routing through the building all affect the workload for a pump. Pumps should be sized to operate at peak efficiency under conditions they will experience most often. Pumping systems can be made more efficient by using variable speed drives to use less energy during partial loads.