Your questions answered: HVAC system design

April Woods, PE, LEED AP BD+C, vice president at WSP USA, Orlando, Fla., and Jason Gerke PE, CxA, LEED AP BD+C, practice area leader – mechanical/plumbing, principal, GRAEF, Milwaukee, replied to questions from the July 21, 2020, webcast “HVAC system design.”

Question: Which standard or code reference to humidity in hospital application?

April Woods: ASHRAE Standard 170: Ventilation of Health Care Facilities Table 7.1 references the humidity levels required in each space type in a health care facility. These humidity values are given in either ranges that need to be met between minimums and maximums or maximum levels.

Question: Can you discuss the implications of HVAC systems tied to room occupancy sensors (adopted Jan. 1 in California)?

Jason Gerke: The opportunity to connect HVAC systems to room occupancy (or vacancy) sensors provides an opportunity for additional energy savings in a building. In general, not looking at any individual state energy code, a design engineer should look for opportunities on projects to use a single device typically used to only control one system to also provide input to another system — in this case, occupancy sensors for lighting — and also identify if the space is in use for HVAC. This sensor may provide an opportunity to close outside air supply to a space, set back the temperature or control other building systems.

Question: Would you explain more of what the condensate recovery is?

April Woods: Condensate collects during the dehumidification process on a cooling coil. Moisture condenses on the cooling coil and is typically collected in a drain pan underneath the coil section. Traditionally the condensate is just discharged into the sanitary system, however, it is can be captured and utilized for a different purpose, including cooling tower makeup, landscape irrigation or industrial process makeup.

Question: What is difference between EER and SEER?

Jason Gerke: EER is an older standard to rate the efficiency of air conditioning equipment. The EER rating system is based on a specific design condition and is most appropriate for cooling dominated regions. The SEER rating is a seasonal average rating of the air conditioning equipment and provides the average efficiency based on a wide range of seasonal conditions.

Question: Are there cases where following a newer code than your state requires would be a liability versus an advantage? Maybe where following the newer code would be a violation in the authority having jurisdiction’s opinion?

Jason Gerke: Typically, code updates are an improvement upon the code it is replacing. This could be an improvement to the language used or increases in energy efficiency requirements as examples. One reason to consider using a newer version of a code than what is currently adopted in a specific area is to take advantage of opportunities to improve building construction. An example is to use a newer version of the building code to avoid smoke vents on the top of an elevator shaft. Another opportunity might be to take advantage of an exemption for HVAC equipment efficiency or controls that is allowed in a newer version of the code. These opportunities do not typically create a higher risk of liability for the design engineer, as these are code language items and the local AHJ signs off on them through a variance process before construction implementation.

Question: What is the safety consideration for UV-C lights in air handling units?

April Woods: Door safety switches that de-energize the lights when opening the AHU access doors should be specified. In addition, warning labels should be posted on the exterior side of the door to alert personnel to potential eye and skin hazards. Safety training should occur so all personnel know the precautions that need to be taken.

Question: What filters should we use to protect from COVID-19?

Jason Gerke: There are many companies promoting various filtration and air cleaning devices over the past few months. It is important that design engineers take the time to research available products and investigate the equipment/material for appropriate application on a specific project. Some of these systems and materials may be better suited for a specific building type or process.

Question: What is direct expansion?

April Woods: Direct expansion systems are commonly referred to as DX systems and use the vapor-compression refrigeration cycle to provide conditioning to a building. It cools the air using a condensed refrigerant liquid. It comprises four basic components: a compressor, a condenser, an expansion valve and an evaporator. Typical equipment types are split systems, packaged rooftop units, variable refrigerant flow, commercial self-contained units and water-source heat pumps.

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