When sizing water heaters for an existing or new building, it is important to provide an adequate supply of hot water in all building types. A water heater must provide hot water for various hot water uses such as industrial processes, bathing, washing, cleaning and cooking.

Sizing water heaters is the key to a hot water system that performs well and does not run out of hot water or provide temperature fluctuations. That could lead to high temperatures, causing scald injuries, or low temperatures promoting Legionella bacteria growth. 

Most water heaters are sold as replacement water heaters. Commercial and residential water heaters have a life expectancy of about 8 to 12 years, or about 10 years on average (https://home-tech.com/44342/life-expectancy-of-a-water-heater/); buildings last much longer than water heaters. Some exceptions for water heater life expectancy include when the water heater is not exposed to aggressive water conditions, excessive thermal cycling, condensation or heat stress. 

There is not much difference in life expectancy between a tank-type water heater and a tankless water heater, but tankless water heaters generally cost more. 

The average life expectancy of a steel-framed building can be 100 to 150 years; concrete structures can last about 100 years; and commercial/wood frame structures can last 60 to 80 years on average, depending on their location, weather and building usage (theconstructor.org). 

Based upon the life expectancy of a building verses the life expectancy of a water heater, a steel frame structure will see, on average, about 15 water heater replacements over its life. A concrete structure will see up to about 10 water heater replacements, and a wood frame structure will see about seven to eight replacements over its life. This demonstrates the fact that most water heaters are sold as replacement water heaters. 

It is important when sizing and selecting a replacement water heater to determine the storage size and energy input capacity of the existing water heater. More importantly, determine what hot water fixtures there are in the building and what quantity of hot water they use. Also, check to see if the water heater is also used for building heating. Building modifications can cause the existing water heater to be undersized for the application.

Water heaters are the most dangerous piece of equipment in a building. If they are not installed correctly, they can cause: 

A fire;

An explosion; 

A scald injury or death;

A carbon monoxide leak;  

Legionella bacteria growth contributing to Legionnaires’ Disease;

A lack of hot water, which can lead to turning “up” the thermostat dial, leading to scald injuries;

A waste of water and energy when recirculation piping is not done properly;

Loss of building heat and frozen pipes (when combined systems fail). 

Sizing and selecting a water heater for either an existing building or a new building is the same process. The most critical thing to consider is the required hot water demand, and this should be based on a survey of the existing building fixtures or proposed fixtures on new plans. 

Choosing a replacement water heater based on price, not size or capacity, often leads to an undersized water heater. This is dangerous because when complaints of “no hot water” start pouring in during peak periods, it leads to the raising of the temperature of the thermostat dial, which can lead to scalding injuries.

Sizing for a new or replacement water heater requires counting hot water fixtures, determining flow rates or gallons per hour per fixture, and the building usage to determine the total peak demand for hot water in the building. The size of a water heater is determined by the amount of hot water it can produce over a specific period, usually expressed in hours, which is sufficient to meet the required peak demand. 

Determine the temperature rise required by subtracting the incoming water temperature from the desired hot water temperature. The incoming water temperature is usually around 40 F to 60 F, while the desired hot water temperature is around 105 F to 140 F. Therefore, the temperature rise can be determined.

The Fundamental Water Heating Formula

To determine the required British thermal units per hour (BTU/hour), we first establish the hot water load by counting the hot water usage fixtures, then convert into gallons per hour per fixture. Multiply the hot water demand in gallons x 8.34 pounds per gallon x the degree rise x 60 minutes in an hour to get the required BTU/hour. 

See the American Society of Plumbing Engineers’ (ASPE) Plumbing Engineering Design Handbooks or the water heater manufacturer’s sizing literature. The type of water heater selected will affect the system design. 

If the water heater is going to be an instantaneous/tankless water heater, then the flow in gallons per minute (gpm) must be determined using the fixture unit method and adding correction factors to account for concentrated simultaneous usage. The energy input can be calculated based on raising the water temperature up to the desired temperature at the peak flow rate. 

When storage water heaters are used, the combination of stored gallons plus the energy input must satisfy the peak hot water demand period. In storage-type water heaters, the storage capacity of a water heater is measured in gallons; it’s usually indicated on the manufacturer’s unit specifications. 

The usable amount of hot water in the storage tank is about 70% of the actual storage amount. This is because after a draw of hot water, the incoming cold water mixes with the hot water remaining in the tank. The last 30% of water in the tank is typically too cool to be of a useable temperature. Having a few extra gallons of hot water is a good thing.

Heat pump water heaters use electricity to transfer heat from the surrounding air or ground water to the water tank. These water heaters are generally storage-type water heaters and are highly efficient because they do not have a high energy input. However, some may not be suitable for cold climates. The recovery rate for these water heaters is much less than a standard electric or gas-fired storage water heater; therefore, a much larger tank is required.

Ultra-high-efficient water heaters (more than 93% to 95% efficiency) rarely save enough energy over the life of the water heater to cover their increased initial cost. 

As the name suggests, solar water heaters are powered by solar energy. They harness the sun’s energy to heat water, reducing your energy bills considerably. Solar water heaters are highly efficient and can last for up to 20 years.

Condensing water heaters are powered by gas and are highly efficient. They use a condensing process to extract heat from the hot gases produced by the gas burner, resulting in substantial energy savings. However, they are usually more expensive than conventional gas water heaters.

Water heaters are also used for building heating applications with combined heating/domestic hot water systems. Some water heaters have a double-wall heat exchanger coil; hot water flows through the coil inside the hot water tank and then flows to the heating coil in an air conditioner unit. 

These types of water heaters do two jobs. When they fail, a building will lose both domestic hot water service and building heating. The life expectancy of a water heater in a combined heating/domestic hot water system is even less than that mentioned for water heaters because of the excessive thermal cycling and heat stress on the equipment. 

Replacements Are Typically Done in Crisis Mode

When a water heater is replaced, it is often right after it begins leaking or stops working and is no longer producing hot water; this creates an urgent need to get hot water, generally putting the building owner in emergency mode. This is especially true in office buildings, apartments or hotels where many people may be complaining about not having hot water. 

In many cases, the owner has little to no knowledge of plumbing or how to size or select a water heater. Owners typically don’t understand the BTU/hour heat input requirements for instantaneous or tankless water heaters or the storage capacity required to satisfy the peak hot water demand period for the building.

When owners are in panic mode, they typically just want to buy the cheapest water heater without understanding any of the other water-heating or space-heating applications or the safety issues. 

Beware of equipment changes! If the size of the storage tank or the BTU/hour input capacity changes with a replacement water heater, it will change the operating temperatures of the entire hot water distribution and recirculation system. This would require readjusting the maximum temperature limit stop on all code-compliant bathtub-shower valves to the code-mandated maximum of 120 F, or less, to minimize the risk of scalding. 

Every time a water heater is replaced, the maximum temperature limit-stop on a shower and tub-shower valve, and any temperature-actuated mixing valve downstream of the water heater must be checked and adjusted to ensure safe hot water temperatures flowing from every fixture. 

• Storage-type water heaters. If you have a storage-type water heater, a temperature-actuated mixing valve conforming to ASSE 1017 is recommended to be installed on the water heater outlet. This allows the storage temperature to be raised to extend the amount of hot water available and provide a relatively constant hot water distribution temperature. 

• Tankless water heaters. There are two basic types: point-of-use water heaters (ASSE 1084), which are designed to only serve one fixture or a small group of lavatories; and hot water distribution system water heaters (ASSE 1082), which are intended to serve the whole building or large groups of fixtures. 

Either type typically only heats water up to the usage temperature of about 105 F and the maximum energy input is fixed, so when peak flows exceed the water heater’s rating, there will be corresponding drops in temperature. Some models have staged burners or heating elements that allow some level of temperature modulation, but when flows exceed the maximum energy input, the temperature will fall off. 

Other models have a built-in flow-modulating valve to slow down the flow through the water heater to maintain the outlet temperature. Modulating valves cause a drop in the hot water system pressure. When using low-flow fixtures, each fixture must flow enough water to activate the flow switch that turns on the burner or heating element on a tankless heater. Tankless water heaters are designed for instantaneous flow rates. 

• Temperature-actuated mixing valves. To ensure that there is enough hot water for the peak demand coverage while storing hot water at a pasteurizing temperature
(140 F will kill Legionella bacteria in 32 minutes and 150 F will kill Legionella bacteria in about two minutes), install a temperature-actuated mixing valve on the outlet pipe of the water heater. 

This allows the stored hot water to be mixed down to a stable and constant distribution temperature. ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, and ASHRAE Guideline 12 recommend keeping the water temperature above the Legionella bacteria growth temperature of 122 F in the entire distribution system.

This is measured at the lowest temperature location, which would be at the circulated hot water return pipe just before the water heater. A temperature gauge should be installed at this location to ensure the minimum distribution temperature is above the Legionella bacteria growth temperature. I recommend that, depending on the heat loss throughout the distribution system, the mixing valve be set to maintain 124 F at the hot water return temperature gauge location. 

• Balancing. Balancing of hot water return branches is essential to maintain hot water at the end of each branch. This can be done with manual balancing valves or thermostatic (automatic) balancing valves. Balancing valves add flow restriction to closer branches to equalize the flow to all branches. Without balancing valves, the flow will short-circuit through closer branches and farther branches will see little to no flow.

Hot Water Temperatures and the Codes

The model plumbing codes provide very little information when it comes to water heater storage capacity, energy input and other sizing requirements, and storage and distribution temperatures. In both model plumbing codes — the International Plumbing Code and the Uniform Plumbing Code — Chapter 4, Fixtures, is where temperature limits are mandated for the hot water flowing from various fixtures. 

Not all fixtures have temperature limits. There is no code language that addresses hot water storage or distribution temperatures — except one. It mentions a requirement for a thermostatic mixing valve set to a maximum of 140 F for the domestic hot water distribution systems where there is a combined heating/domestic hot water system. 

This lack of direction on hot water system storage and distribution temperatures is understandable. There is a hot water temperature loss that occurs across a hot water distribution system that will be different in each building based upon pipe size, pipe material, insulation type and thickness, circulated flow rate, etc. 

It would be impossible to maintain a specific temperature at all points in a hot water distribution system. I have recorded temperature losses across such systems in excess of 40 F. It is for this reason that neither the water heater thermostat dial nor master mixing valves located at the water heater can be used for scald prevention to control the hot water temperatures flowing from various fixtures. 

Safety and Health Concerns

Safety must be designed, built and maintained into every hot water system. While many of the integral water heater safety features operate automatically, the thermostat dial is set by the building maintenance personnel after installing a water heater. If the water heaters are not in a locked room, inaccessible to the tenants, there should at least be a locked cover over the thermostat to prevent tenants from adjusting the temperature. 

There should be regular, documented checks of the entire hot water system for the following issues:

Document the hot water system check list at regular intervals or at the manufacturer’s recommended maintenance interval, whichever is shorter. 

Check/document the water heater thermostat set points. 

Check/document the water heater inlet and outlet temperatures.

Check/document any mixing valves inlet and outlet temperatures. 

Check/document fixture outlet temperatures (points of use to assure proper system temperatures and balancing). 

Check/document that the temperature and pressure-relief valve is operational, and that the relief piping is independent to a safe and visible location with no tees, no traps, no plugs or caps and no valves.

Check/document the water heater drain. Flush water and sediment from the drain at the bottom of the water heater at intervals recommended by the manufacturer.

Check/document the flame arrester combustion air inlet plates to assure they are clear of dust and lint.

Check/document the burner flame to see if it is burning properly. Look for yellow flame, soot or air blockages.

Check/document any leaks in the draft hood or     combustion gas flue vents.

Check/document for proper flue materials on the water heater, based on the category of venting required for the water heater and the category listed on the flue material. Plastic pipe that is not listed to UL1738 or other approved standard listed in the code is not allowed.

Check/document any excessive pressures that will cause a water heater tank to bulge and crack. Excessive pressure can cause T&P relief valve discharges or rupture an expansion tank bladder. 

Check/document combined systems for domestic hot water system contamination by high-temperature steam or boiler water additives. This can happen if a heat exchanger wall corrodes, erodes or ruptures from high-velocity or high-differential-pressure steam or heating hot water fluid (gas in the case of a heat recovery system). Contamination/cross-connection should be avoided by using double-wall heat exchangers or by installing a pressure-gradient monitoring system.

Check/document excessive temperatures that can cause erosion of copper pipe and soften plastic pipe, reducing their pressure ratings. 

Check/document any relief valve openings from plugged T&P relief valves or relief valve piping being blocked. Valves installed in relief valve lines can be closed, rendering the relief valve useless when there is a burner or heat source issue that could cause an explosion.

Check/document for scalding water at fixtures. Adjust in-line temperature controls or temperature limits at fixtures.

These checks are to ensure the water heater and the hot water system are performing as intended. Regular maintenance check lists should be posted in a conspicuous location to assist maintenance personnel with recording data and inspecting the system. 

Sizing a water heater is an essential part of ensuring comfort and adequate amounts of hot water in a building. A properly sized water heater will provide an adequate supply of hot water to meet the peak hot water demands of a building. 

Factors such as building size, hot water usage habits and water temperature, together with the different types of water heaters available, influence the size of the water heater you need. Proper design would be to have a thermostatic mixing valve on storage-type water heaters to allow higher, disinfecting storage temperatures. 

By using the steps outlined here, in water heater manufacturers’ sizing guidelines and in the ASPE Plumbing Engineering Design Handbooks, you should be able to determine the right water heater size for your building and have safe, efficient and reliable amounts of hot water. 

A hotel manager once told me that he never had someone call him up at 6:30 a.m. to tell him they had plenty of hot water, but he had a lot of calls when the hot water temperature fell off due to high peak demands.