2 FM quick reads on ventilation
1. Demand Control Ventilation Systems Can Save Energy, Achieve Occupant Satisfaction
Today's tip from Building Operating Management comes from Angela Cremeans and R. Stephen Spinazzola of RTKL: In facilities with wide swings in occupancy, demand ventilation systems — which strike a balance between occupant satisfaction, energy efficiency and cost savings — are worth a close look.
Demand ventilation systems — also known as dedicated outdoor air systems — are 100 percent outdoor air systems that include filtration along with heating and cooling coils. Normally, demand ventilation systems have some sort of heat recovery and are variable in volume, modulating down to 25 to 50 percent air flow. The systems provide humidity control for the entire building because they dehumidify outside air down to as little as 30 percent humidity, allowing the base building system to be dedicated strictly to cooling, not dehumidification.
Recent development of economical and accurate carbon dioxide sensors makes it feasible to use them as an integral part of a demand ventilation system. Model codes now allow the amount of outdoor air to be reduced below a constant 15 CFM per person.
Of course, there are issues that should be evaluated when deciding to proceed with a demand ventilation system.
The first question is whether local code allows outside air levels to be reduced. If the answer is no, determine whether the code official is willing to look at alternative schemes as part of a comprehensive design solution.
Depending on the base mechanical system, there may be some added first costs to implement a demand ventilation system. A demand ventilation system requires a parallel duct system to get outside air to individual spaces. The biggest savings in operating costs are in conference rooms and other assembly-type spaces.
Demand ventilation should not be considered unless the facility manager plans to do a full commissioning of mechanical systems prior to occupancy. Commissioning will ensure systems are performing as intended. Also, care should be taken to make sure the air balance is designed so that the minimum outside air doesn't drop below what is required to maintain positive building pressure.
This has been a Building Operating Management Tip of the Day. Thanks for listening.
2. Demand Control Ventilation Can Reduce Energy Costs
Today's tip from Building Operating Management comes from Angela Cremeans and R. Stephen Spinazzola of RTKL: In spaces that have widely varying levels of occupancy, demand control ventilation can reduce energy use and costs.
Many spaces are unoccupied for significant stretches of time, including churches, auditoriums, offices and retail spaces. Despite that fact, most HVAC systems are designed to bring in the same level of outside air continuously. But if a space is unoccupied, heating, cooling and moving outside air to that space can waste a significant amount of energy and money.
Demand control ventilation is a sophisticated strategy that supplies outside air to a space only when it is needed. Demand control ventilation relies on the fact that people in a space produce carbon dioxide. Sensors measure carbon dioxide levels. As those levels rise, more outside air is brought into that space. If levels fall, the amount of outside air is reduced, cutting the cost of conditioning and moving outside air. The amount of air is tailored to the specific needs of a given zone.
In a demand control ventilation strategy, the level of carbon dioxide is taken as a general indicator of the level of other contaminants. By increasing the amount of outdoor air brought into a space when carbon dioxide levels rise, the concentration of those other contaminants is reduced, improving indoor air quality.
Demand control ventilation can save energy costs in three ways:
1. Less outdoor air has to be conditioned by the HVAC system, so heating or cooling system energy is saved.
2. Less air has to be moved, so fan energy is saved.
3. If a building has a heat recovery system using building exhaust, that can further reduce the amount of energy consumed.
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