Sizing a domestic water heater system is more of an art than an exact science. For anything but a small residential system, you can’t just go to a table or graph and make your selection. The codes only provide generalizations such as “Hot and cold water shall be supplied to all lavatories, sinks, showers, etc.” They do not indicate how much hot water is required. However, the codes provide information on pipe materials and minimum and maximum flow rates for fixtures (combined hot and cold water), and address safety issues such as maximum temperatures and required safety devices.
Several factors must be considered when sizing a system, and experience plays a large role. Each system is different, so the path that leads to the final selection is also different. Two buildings may be physically the same (ie have the same number of fixtures), but the amount of water required may be different if the buildings will be occupied by different groups of people. For example, a senior apartment building would have a different use profile than one that primarily houses families.
Information gathering
So how do you size and select a domestic hot water system? The first step is to collect the necessary information to define the system parameters and narrow down the options. Some of the information will be readily available, but some may require a bit of research. The following is quite a long list of questions, but not all of them apply to all situations.
• Where is the building located?
• What code or codes must be followed?
• Do local amendments apply?
• Does the owner or operator of the building have any unusual requirements?
• Are there other hot water systems in the building?
• What is the area used for?
• How many plumbing fixtures will there be?
• What kind of accessories will there be?
• Who will use them?
• Are there high-use fixtures, such as whirlpool tubs or washing machines?
• Are there plans to expand the facilities in the future?
• Will there be a laundry area or gym?
• If so, how many areas will be used simultaneously?
• How much space is available for the system?
• What power sources are available? (Natural gas? Propane? Steam? Hot water? Fuel oil? Heat demand?)
• What energy source is the most economical for the place?
• Where in the building will the equipment be located?
• Will the flues or combustion air be a problem due to the location?
• What is the source of cold water?
• What is the hardness of the water?
• Will the system be idle for long periods of time?
calculations
Using the information you have collected, you can calculate the load (that is, recovery rate and storage volume) required for the installation. Once again, this is not a simple process. There are several methods to calculate the load of a building. One method is not always better than another, although one may be more suitable for the particular application. There are several methods available; most of them are from the American Society of Plumbing Engineers (https://www.aspe.org/).
Selection of heating system
With the demand and storage volume established, the next step is to select the type of equipment to use. This is another point where experience plays a key role. Several factors must be weighed: the most practical or economical fuel source, expandability, available space, location relative to ceiling and wall locations, heater efficiency, budget constraints, and homeowner preference. are just some of them. Add in the various types of equipment available, and equipment selection can be a daunting undertaking. The best approach is to address each of the parameters and narrow down the possibilities to one or two that fit your application. Make the final selection based on your experience.
security devices
All domestic water heater systems must have safety devices. Water heaters and storage tanks must have temperature and pressure relief (T&P) devices, which are intended to prevent catastrophic failure of the vessels. If the system fails, the T&P devices relieve the pressure due to the expansion of the water within the storage tank and the hot water system. When the building service has a backflow prevention device, a thermal expansion tank is often required by code on the cold water inlet to the heaters; this compensates for the expansion of water within the system. It does not affect the operation of the T&P relief valve.
Many codes also require that the maximum allowable temperature in a bath or shower not exceed 115°F. To properly maintain the temperature, a thermostatic mixing valve is required. Most of the time, the heater thermostat is not considered an acceptable temperature regulating device. Mixing valves can be located centrally or on individual fittings.
Experience is vital to correctly sizing a domestic water heater system. There is only one way to get that experience: by trying new things. When sizing a system in a building that is a little different from your norm, try a couple of different methods to calculate the size. Talk to some of your coworkers or perhaps a sales representative. Investigate the ways in which different approaches to the same problem produce different results. You may be surprised to learn that a tried and true method is simply not the best fit for your system.