Mary O ’ Brien Acciani 2017-05-12 01:52:13
How energy efficient upgrades trigger reactions to current systems Replacing outdated lighting in a building pays dividends in the form of reduced energy costs and a smaller carbon footprint. In addition to reducing the amount of electricity needed to illuminate a space, replacement of legacy lighting with high-efficiency lamps reduces the amount of load on the heating, ventilation and air-conditioning (HVAC) system. The smaller load decreases the amount of energy needed to cool a space. While this is generally a good thing, there can be negative impacts to the cooling operation of certain HVAC air systems when the lighting load is reduced. Because they are rendered “oversized” by the lighting load reduction, some HVAC systems without humidity control may be unable to remove the required amount of moisture. This results in higher than desired humidity levels which in turn can cause unwanted condensation and perhaps even mold, leaving facility managers wondering why humidity problems have developed in spaces that previously showed no problems. This humidity problem is most common in constant volume systems, however, variable air volume (VAV) systems can also be affected. Buildings cooled by VAV systems with reheat coils may not achieve the energy savings expected from the lighting retrofit. This is due to increased usage of reheat energy to overcome oversized terminal units even when at the minimum position. VAV systems without reheat may over cool spaces, resulting in complaints from occupants. The issues that can develop from both HVAC and VAV systems are solvable, but should be considered and addressed when planning lighting retrofit projects. SIZING AND OPERATION OF COOLING SYSTEMS To understand why oversizing cooling systems is a problem, it’s important to understand how they are designed and operate. In order to keep a space at a comfortable temperature, any heat transferred into or generated in the space must be removed. Also, any moisture transferred into or generated in the space must be removed. This will keep the humidity at a comfortable level. For simplicity, take a one-story open office as an example. During hot weather, heat is transferred into the space through the building envelope which includes windows, walls and the roof. Heat is generated in the space by the occupants and equipment such as computers, printers and copiers, as well as lights. Moisture is transferred into the space through ventilation air and infiltration entering through open windows, doors, and even tiny cracks around windows and doors. In some structures without proper vapor barriers, moisture can also be transferred into the space through the walls or floors. Occupants within the space and any process that produces steam or allows water to evaporate generate moisture. Examples include running water in a pantry sink, coffee pots, cooking, or even a fish tank. For most office spaces, the amount of moisture generated is fairly small, and most moisture comes from ventilation air or infiltration. In other spaces, like cafeterias and gyms with treadmills and elliptical machines, the amount of moisture generated can be significant. To maintain the desired temperature and relative humidity within a space, air systems introduce air that has been cooled and dehumidified. This air then “absorbs” enough of the heat and moisture in the space so that the end result is the desired temperature and humidity. (While not technically the correct term for this heat transfer process, the word “absorb” provides a helpful description for simplification.) The quantity of cooled and dehumidified air needed is calculated based on the heat gain in the space. The moisture gain in the space is addressed by delivering this calculated amount of dry air which absorbs the moisture. The HVAC system is designed to provide this gaged amount of air into the space. CONSTANT VOLUME SYSTEMS A building is divided into zones that have similar heat gain characteristics. When using the same previous example of a one-story office building, there would typically be a minimum of five zones — one for each of the compass directions — North, South, East and West, and one for the interior. Multiple HVAC units can be provided to handle multiple zones. Unitary systems consist of a rooftop HVAC unit with a DX (direct expansion) evaporator coil, compressor and condenser. These units also have a heat source. These constant volume units are typically found in smaller buildings and are usually designed to condition one zone of the building. These systems are customarily controlled by a single thermostat which cycles on the refrigerant system (evaporator, compressor, and condenser) when the temperature in the space rises above the set thermostat temperature. Whenever the refrigerant system is operational and the unit is cooling, moisture will be removed from the air stream. The cold evaporator coil in the HVAC unit causes the moisture in the airstream to condense out and be drained away, reducing the total amount of moisture in the air. Unfortunately, when the refrigerant system is not operating because the temperature in the space is below the set temperature, no moisture is removed from the air. This lack of evaporation removal becomes a problem if there is a significant source of moisture being produced in the space. The more oversized the HVAC unit is, the less time the refrigerant system needs to operate. This is sometimes referred to as short-cycling. Like unitary systems, constant volume fan coil systems are subject to similar issues when oversized. Unlike a unitary HVAC unit where a refrigerant system provides the cooling, a fan coil unit depends on chilled water circulating through a cooling coil. Instead of the thermostat controlling the refrigerant system, it controls a valve supplying the chilled water to the coil. Unitary systems and constant volume fan coil systems are similar in that moisture can only be removed from the airstream when the coil is cold. The heat gain from the lights in a space can also be a significant portion of the total air-conditioning load. In the mid-20th century, it was common for lighting to create loads of three watts per square foot. When these legacy lighting systems are replaced with more energy-efficient models like LED fixtures, the load from lighting can drop to one watt per square foot or even less. With this lighting replacement, a constant volume HVAC system sized appropriately for the three watts per square foot lighting load suddenly becomes oversized and will short cycle. This short cycling may not create significant problems in some spaces and the occupants may not notice much of a difference, but humidity issues may arise or worsen in other areas. Cafeterias are a good example of a space likely to have high humidity problems. Between the vapors from hot food, the large number of people, and moisture inadequately exhausted from the kitchen area, the concentration of moisture in the space can increase significantly. Often the HVAC units are sized to accommodate the full occupancy load, making them oversized any time the space isn’t at full capacity. Reduce the lighting load by replacing legacy lighting fixtures with new state-of-the-art high efficiency fixtures, and the HVAC units become even more oversized. The unit will short cycle because the space temperature drops too quickly, meeting the set temperature even as the moisture level in the space rises along with the relative humidity and dew point. Frequently, this buildup of moisture will result in condensation forming on metal supply registers or diffusers. This is because the HVAC unit is in cooling mode and the refrigeration system is operating releasing the supply of air generally gaged at 55 degrees Fahrenheit (F). When a space is at 75 F and 50 percent relative humidity, the dew point is just below 55 F. If the relative humidity of that same space rises to 60 percent, the dew point is just above 60 F. This means any surface cooler than the dew point will form condensation. Sometimes the register/diffuser will drip. Sometimes the airstream from the diffuser cools the adjacent ceiling surface, and mold will begin to form because of condensation on the ceiling surface. In spaces with exposed un-insulated ductwork, the ductwork itself can form condensation and drip. VARIABLE VOLUME SYSTEMS While constant volume HVAC systems are the most likely to suffer unintended consequences when high efficiency lighting replaces legacy systems, variable volume (VAV) systems are not immune to these issues. VAV systems operate by changing the amount of air supplied to a space based on the space’s thermostat. If the thermostat calls for more cooling, the VAV box or terminal unit provides more air to the space. If the thermostat calls for less cooling, the box provides less air to the space. Because a minimum amount of air is always required for ventilation, the VAV box never shuts off-entirely. These terminal units are sized to provide a maximum amount of air based on the heat load of the space. If the lighting load drops significantly, the minimum amount of air supplied may cause the space to overcool, resulting in discomfort among the occupants. Some VAV systems operate with reheat coils. These coils will heat the air to prevent the overcooling of the space described above. In this situation, these reheat coils are adding heat energy to the airstream to replace the heat energy previously provided by the legacy lighting system. Energy savings are realized in the amount of electricity needed to light the space, however, more energy is being used by the reheat coils. This is probably the least noticed of the unintended consequences because there are likely to be no obvious signs or occupant complaints. CONCLUSION AND RECOMMENDATIONS Energy-efficient lighting has come a long way in a relatively short period of time. As late as the 1980s, T-12 fluorescent fixtures were the standard in architectural lighting along with incandescent recessed can fixtures. The heat generated by these light fixtures was often a large portion of the air-conditioning load. When contemplating a lighting replacement project, look at the HVAC system in the proposed area and verify that it will not be negatively impacted. For many areas, the resulting findings will validate the use of HVAC systems. For those areas where it does not, proactive action to modify the HVAC system can help in avoiding the unpleasant issues described. MARY O’BRIEN ACCIANI, P.E., M.B.A., C.E.M., G.B.E., is the managing engineering principal at HDR Architecture in Princeton, New Jersey, USA. She is a licensed Professional Engineer, Certified Energy Manager and Certified Green Building Engineer with more than 35 years’ experience in the field of architectural engineering and facility management.
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