Case study: College water heater
A hybrid campus building’s water heater was selected using the time-based evaluation method
In some instances, a higher education building used for practical instruction may function more like the industry it applies to than an actual classroom building. These hybrid buildings, housing anything from a chemistry lab to a welding instruction space, will present unique challenges to the engineer designing the domestic hot water system.
A significant portion of the building is dedicated to classroom learning with group and single restrooms for students. There is also an office suite for the faculty of the nursing program, with its own group restrooms and a break room. The remaining area is used for clinical simulation exercises and testing, with hand-washing sinks throughout. All three areas may be fully occupied at the same time or one could be used while the others are empty.
A challenge with a hybrid building like this one is correctly assigning fixture demands to each plumbing fixture. The value should be chosen based on the use of the fixture, not necessarily the classification of the overall building. The Health Science and Nursing Building required a mix of fixture demands appropriate for school, office and hospital building types.
Choosing appropriate demand factor and storage capacity factor values is also a struggle. An average value could possibly be obtained based on the number of plumbing fixtures in each area of the building, but the recommended approach is to use the most conservative value from the appropriate options. At OCTC, this resulted in a maximum demand of 245 gallons per hour.
After reviewing the available space in the mechanical room with the architect, a full-height 120-gallon water heater tank size was identified as the maximum footprint available. Because the building would already have a natural gas service for gas-fired hydronic boilers, both gas and electric heat were available. However, the recovery rates necessary to accommodate the maximum demand and the high natural gas delivery pressure from the local utility made gas the more attractive option. A gas-fired condensing water heater was identified as the preferred design and the table below shows the initial selection.
In this case, time-based evaluation was used extensively to inform the water heater selection; that process was affected by the hybrid nature of the building. While the classroom portion of the building functions around an eight-hour school day, the rest of the building does not. Clinical testing is commonly scheduled in four-hour windows while faculty hours may fluctuate with extremes approaching 10 or 12 hours.
With the help of the owner and architect, eight hours was chosen as a minimum requirement. As noted, the initial selection would have a 24-gallon deficit after eight hours. To meet the requirement, the next larger heater size was needed. It is important to recognize that in this case, correctly accounting for a reduced first-hour rating was the difference between the initial selection being just large enough or undersized. Table 4 shows the final water heater selection that was installed in the building.