How to select a commercial water heater

As mechanical and plumbing engineers specify a water heater in education buildings, several considerations must be studied

By Michael Scruggs September 11, 2020


Learning Objectives

  • Learn about the detailed descriptions of the assumptions made during the water heater sizing process outlined in Chapter 51 of the ASHRAE HVAC Applications Manual.
  • Gather an explanation of the time-based evaluation method and examples of how it can be used to confirm appropriate water heater selections.
  • Review highlights for coordination with the owner, architect and users.

The water heater selection process is not overly complicated, but it is much easier when the design engineer has a comfort level with the players (e.g., design team, owner, local energy utility, original equipment manufacturer, etc.) and choices involved. The mechanical engineer’s ultimate responsibility when sizing a water heater is to provide the owner with a selection that meets specific needs while also working within the limitations of the building.

Chapter 51 of the 2019 ASHRAE Handbook — HVAC Applications describes several types of domestic hot water heaters that are capable of serving a commercial building. Most new commercial buildings still implement a storage-type water heater with immersed electric heating elements or gas-fired burners.

A storage-type water heater is defined by its storage tank size and its recovery rate. The storage tank size is the volume of the tank, presented in gallons. The recovery rate represents the amount of hot water the heater can continually generate in gallons per hour, for a specified temperature rise through the water heater. In general, gas-fired burners are capable of higher recovery rates than electric heating elements. The relatively high energy density of natural gas allows for gas-fired burners to generate more heating energy in the same volumetric footprint compared to electric heating elements.

Regardless of fuel, storage-type water heaters are typically sized by one of two methods: water demand per capita or water demand per fixture. The per capita approach is often used for smaller, light commercial facilities. Figures 16 – 23 in Chapter 51 of the 2019 ASHRAE Handbook — HVAC Applications illustrate suggested capacity profiles for office buildings, apartment buildings and restaurants, among others. These profiles can be used to select both a recovery rate and corresponding storage tank size based on a per person, per apartment unit or per meal basis. However, because of the high variability in occupancy and usage, sizing based on fixture count is more common in most commercial buildings.

Hot water demand per fixture

The 2019 ASHRAE Handbook — HVAC Applications provides the relevant parameters for sizing by fixture count in Table 10 of Chapter 51. The majority of the table presents empirical data on hot water demand, in gallons per hour, for common fixture types during a peak hour. It is important to realize that these values were informed by various studies between the 1930s and 1960s before the widespread installation of modern, low-flow plumbing fixtures. Therefore, the values can be very conservative for most new designs.

Table 10 categorizes fixture demand by building type, often resulting in difficulty determining which values are appropriate for more unique buildings. The fixture demand selected should always be the one corresponding to the building type that most closely matches the actual use of the plumbing fixture. It may be necessary to request feedback from the architect, owner or users to confirm which building type is most appropriate for each plumbing fixture. These decisions and any assumptions should be documented in writing for future reference.

Table 1: For this table, use the following formulas:
Fixture subtotal = fixture demand x number of fixtures
Possible demand = sum of fixture subtotals
Maximum demand = possible demand x demand factor
Suggested storage capacity = maximum demand x storage capacity factor
Courtesy: RMF Engineering[/caption]

Time-based evaluation for water heaters

Once the maximum demand and suggested storage capacity have both been identified, the water heater can be selected. If possible, a storage tank size should be selected just larger than the suggested storage capacity. The heater should also be selected at a recovery rate just above the maximum demand.

However, the engineer may encounter different restrictions requiring a water heater with a smaller tank size or lower recovery rate. Restrictions could include insufficient space, lack of the desired fuel source, budget limitations or even preference from facility maintenance personnel. If these constraints cannot be eliminated or they are deemed too costly to eliminate, time-based evaluation can be used to ensure the system’s needs are still being met.

Time-based evaluation describes the process of calculating the remaining available hot water at the end of each hour during a typical day. Assuming that the maximum demand is sustained throughout the day, the difference between the maximum demand and recovery rate will be equal to the change in available hot water each hour.

Table 2, with formulas for calculated values, shows an example of using time-based evaluation to assess a system’s capability over an eight-hour school day. This method can be used as a tool for the engineer, architect and owner to identify an acceptable length of time after which the hot water heater would be empty. While a higher education building typically functions around an eight-hour school day, other building types or owners may have different needs and expectations.

Table 2: Formulas for calculated parameters include:
0 hour available hot water = storage tank capacity
1 hour available hot water = first hour rating – maximum demand
2 hour available hot water (typical after 2) = previous hour available hot water – maximum demand + recovery rate (maximum value is storage tank capacity)
Courtesy: RMF Engineering

In this case, even though the suggested storage capacity of the system was 150 gallons, the building mechanical room only had enough space to accommodate a 65-gallon tank. Notice that although the storage tank size is less than the suggested storage capacity and the recovery rate is lower than the maximum demand, the selection will still sustain the system for the entire eight-hour school day.

A key point to recognize when using time-based evaluation is that water heaters are not capable of achieving the recovery rate when starting with a full tank of hot water. Therefore, manufacturers will publish “first hour” ratings documenting the amount of hot water that can be provided during the first hour of use if the water heater begins the hour full. Typically, a water heater’s first hour value is 90% to 95% of the sum of its tank size and recovery rate.

Time-based evaluation is a straightforward concept that usually allows even a nontechnical owner to understand the capabilities of the system. Once the concept has been reviewed with the owner, it can become a vital tool allowing the engineer to confidently choose a tank size and recovery rate that will accommodate both the system’s needs and the building’s limitations.

Water heater selection considerations

While the sizing process can identify any number of corresponding maximum demand and suggested storage capacity values, the engineer must be aware of the available sizes for both storage tanks and heaters. Adding to the complication, manufacturers often advertise dozens of different tank and heater combinations but depending on the building’s restrictions, there may only be a handful of appropriate selections. These limitations inherent to the building housing the water heater always need to be considered during a selection.

Two of the most common challenges facing the engineer are finding available space in the building for the water heater and determining the best fuel source based on the size and type of the available gas or electrical service.

As architects try to maximize space within the available building footprint, water heaters can often be relegated to small mechanical rooms, closets or even above ceilings. The design engineer should request additional space for the water heater. If that is not possible, manufacturers have a full range of designs including tall, thin tanks for closets and shorter tanks that could be installed within the ceiling plenum.

Figure 2: The natural gas water heater installation shown is typical for a commercial building, although it is capable of larger recovery rates than a similar size electric water heater. Courtesy: RMF Engineering[/caption]

Water heater selection tools

Key takeaways for mechanical engineers include:

  • The process for sizing a commercial water heater is a series of relatively simple decisions and calculations, but a detailed understanding of the building’s program, the owner’s expectations and available fuel sources are critical.
  • As a design engineer gains experience with different building types, especially those with hybrid function, they will become more at ease with the fixture type choices recommended by Chapter 51 of the 2019 ASHRAE Handbook — HVAC Applications. A familiarity with the architect or building owner provides another level of comfort for the design engineer.
  • Time-based evaluation gives the design engineer a tool with which to identify the owner’s expectations and confirm that the design meets them. Better awareness of the water heater sizes and recovery rates available from manufacturers will only further simplify this process.
  • Proper communication, early and often, with the architect and electrical engineer will help identify any known building constraints in time to address them or account for them in the water heater selection.

Over time, many of these decision points will be confirmed as standard practice for a given owner or campus, making the collaborative part of the process less time-intensive. Until that point, documentation is key to ensure all parties are in agreement as to the system’s requirements and abilities.

An informed, thorough design engineer should not feel intimidated when instructed to size a water heater for any type of commercial building.

Author Bio: Michael Scruggs is a mechanical engineer at RMF Engineering. He has experience in the design and analysis of mechanical and plumbing systems serving educational, laboratory, health care and commercial facilities.