Sizing a home water heater system is an art form rather than an exact science. For anything other than a small residential system, you cannot just go to a table or graph and make your selection. The codes provide only generalizations such as “Hot and cold water must be supplied to all sinks, bathrooms, showers, etc.” They do not indicate how much hot water is needed. The codes, however, provide information on pipe materials and minimum and maximum flow rates to fittings (hot and cold water combination), and address safety issues such as maximum temperatures and required safety devices.
Several factors must be considered when sizing a system, and experience plays an important role. Each system is different, so the path that leads to the final selection is also different. Two buildings can be physically the same (that is, 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, an apartment building for the elderly would have a different usage profile than the one that primarily houses families.
Information gathering
So how do you size and select a domestic hot water system? The first step is to gather the information necessary to define the system parameters and limit the options. Some of the information will be available, but a little research may be required. The following is a fairly long list of questions, but not all of them apply in all situations.
• Where is the building located?
• What code or codes must be followed?
• Are any local amendments applied?
• Does the building owner or operator have 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 accessories, such as hot tubs or washing machines?
• Are there plans to expand the facility in the future?
• Will there be a laundry area or health club?
• If so, how many areas will be used simultaneously?
• How much space is available for the system?
• What sources of energy are available? (Natural gas? Propane? Steam? Hot water? Fuel oil? Heat recovery?)
• What source of energy is most economical for the location?
• Where in the building will the equipment be located?
• Will flue ducts or combustion air be a problem because of 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 that you have gathered, you can calculate the load (that is, the recovery rate and the storage volume) required for the installation. Again, this is not a straightforward process. There are several methods for calculating the load on 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/).
Heating system selection
With the demand and volume of storage established, the next step is to select the type of equipment to be used. 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 location, heater efficiency, budget constraints, and owner preferences. are just some of them. Add in the various types of equipment available, and you may find that selecting equipment can be a somewhat daunting undertaking. The best approach is to address each of the parameters and limit the possibilities to one or two that fit the application. Make the final selection based on your experience.
Safety 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 vessel failure. 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, the code often requires a thermal expansion tank at the cold water inlet to the heaters; This compensates for the expansion of the 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 does not exceed 115 ° F. To properly maintain the temperature, a thermostatic mixing valve is required. The heater thermostat is not often considered an acceptable temperature regulating device. Mixing valves can be located centrally or on individual accessories.
Experience is vital to properly sizing a home 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 slightly different from your norm, try a couple of different methods for sizing. Talk to some of your co-workers or perhaps a sales representative. Examine the ways that different approaches to the same problem produce different results. You may be surprised to learn that a tried and true method is not the best fit for your system.