Sizing a domestic water heater system constitutes an art form more than an exact science. For anything other than a small residential system, you cannot simply go to a table or chart and make your selection. Codes provide only generalizations such as “Hot and cold water shall be supplied to all sinks, lavatories, showers, etc.” They do not indicate how much hot water is required. Codes do, however, give information about pipe materials and minimum and maximum flow rates to fixtures (combined cold and hot water), and they address safety concerns such as maximum temperatures and required safety devices.
A number of factors must be considered when sizing a system, and experience plays a big part. Every system is different, so the path that leads to final selection is also different. Two buildings can be physically the same (i.e., have the same number of fixtures), but the quantity of water required may be different if the buildings will be occupied by different groups of people. For instance, an apartment building for older people 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 information needed to define the system parameters and narrow the options. Some of the information will be readily available, but some may require a little digging. The following is a fairly long list of questions, but not all of them apply to every situation.
• Where is the building located?
• What code or codes should be followed?
• Do any local amendments apply?
• Does the building’s owner or operator have any unusual requirements?
• Are there any other hot water systems in the building?
• What is the area used for?
• How many plumbing fixtures will there be?
• What kind of fixtures will there be?
• Who will be using them?
• Are there any high-usage fixtures, such as hot tubs or laundry machines?
• Are there plans to expand the facility in the future?
• Will there be a laundry or health club area?
• If so, how many areas will be used simultaneously?
• How much space is available for the system?
• What energy sources are available? (Natural gas? Propane? Steam? Hot water? Fuel oil? Heat reclamation?)
• Which energy source is most economical for the location?
• Where in the building will the equipment be placed?
• Will flues or combustion air be a problem, due to the location?
• What is the cold water source?
• What is the water hardness?
• Will the system be inactive for long periods of time?