Storage water heaters use gas (natural gas or propane), electricity or oil to heat and store water at a thermostatically controlled temperature (less than 180⁰ F) for delivery on demand. Federal appliance efficiency standards differentiate storage water heaters based on whether the rated storage volume is greater than 55 gallons or less than or equal to 55 gallons.

The U.S. Department of Energy (DOE) classifies consumer gas water heaters as having an input of 75,000 BTU per hour or less and has a storage capacity ranging between 20 and 100 gallons. A basic gas storage water heater is composed of a standing pilot ignition system, a burner, a combustion chamber, a flue baffle, a flue, an insulated water tank, a cold water inlet and hot water outlet, a sacrificial anode, a gas valve, a temperature and pressure relief valve, a thermostat, heat traps, and an outer case.

The DOE classifies consumer electric storage water heaters as having an input of 12 kilowatt (kW) or less and have a storage capacity ranging between 20 and 120 gallons. A basic electric storage consumer water heater differs from gas water heaters by using an electric resistance heating element. As noted in this chapter, electric storage water heaters are not allowed through the prescriptive approach to compliance but can be installed using the performance approach as long as the water heating energy budget is not exceeded.

DOE added a new category of water heaters called grid-enabled water heaters and is defined as an electric resistance water heater that has a rated storage tank volume of more than 75 gallons and is manufactured on or after April 16, 2015. The water heater must have an activation lock at the point of manufacture and is intended for use only as part of an electric thermal storage or demand response program.

A heat pump water heater (HPWH) is an electric water heater that works like a air conditioner in reverse. It uses a compressor to transfer heat from the surrounding air to the water tank. It includes all necessary auxiliary equipment such as fans, storage tanks, pumps, or controls. Typically, HPWHs include backup electric resistance elements to ensure hot water delivery when the air temperature is too cold or the hot water demand is too high. Some models entering the market use larger compressors to avoid the need for resistance elements.

The performance of HPWHs depends both on storage water temperature and inlet air temperature. Buildings in warm and cold climate zones, and different installation locations such as a garage or well vented outdoor closet, all have an impact on performance and must be considered. If the HPWH is installed in a confined water heater closet, carefully follow manufacturer instructions on the use of ducting to discharge exhaust air out of the closet. Without ducting, a confined closet can result in overcooling of the air, significantly diminishing HPWH performance.

HPWHs are most efficient in warmer climates, but even in cold climate zones such as Climate Zone 16, HPWHs still use only half as much electricity as conventional electric resistance water heaters. In addition to air temperature sensitivity, HPWH performance is affected by cold water inlet temperatures as introduction and mixing of inlet water during larger draws may trigger second stage electric resistance heating in the tank.

The Northwest Energy Efficiency Alliance (NEEA) Advanced Water Heater Specification was developed to address critical performance and comfort issues of HPWH in colder climates. Tiers are incorporated into this specification recognizing variations in product performance and configuration. An HPWH that meets the NEEA Advanced Water Heater Specification performs significantly better in real world conditions, and an HPWH that meets the NEEA Tier 3 or higher can be used to meet the prescriptive requirement for newly constructed buildings, addition, and alteration.

The list of qualified NEEA HPWH products can be found at

https://neea.org/img/documents/qualified-products-list.pdf

In 2020, the Energy Commission adopted Joint Appendix (JA) 13, which provides qualification requirements for HPWH demand management systems. Qualifying HPWHs have the capability to optimize operation to reduce normal water heater operation during on-peak periods by biasing operation prior to the peak period. Future opportunities include heating the storage tank above setpoint prior to the peak period, further improving the electrical load profile of these systems. A credit exists for these HPWHs within the compliance software. JA13 certified HPWHs, which must have a mixing valve installed to prevent any scalding risks, are currently listed at this website:

https://www.energy.ca.gov/rules-and-regulations/building-energyefficiency/ manufacturer-certification-building-equipment/ja13

Example 5-1 – HPWH:

Question:

Are there any additional compliance credits available for unitary HPWHs?

Answer:

NEEA Tier 3 and Tier 4 rated HPWHs are available in the performance compliance software and provide compliance credit compared to the standard design HPWH. Additionally, with the approval of JA13, there is now credit within the performance compliance software for HPWHs that are JA13-certified and provide the Basic Load Up demand management functionality. This credit varies with climate zone. Note that a mixing valve must be installed to ensure safe hot water delivery.

Air-to-water heat pumps provide space heating and cooling output via hydronic delivery. Some models also provide a water heating function. These products are emerging in the California market, but the residential compliance software can model the water heating capability of these units.

This appliance is essentially a commercial water heater that can be legally installed in a residential building. It is defined in the Federal Code of Regulations (10 CFR 431.102) as any gas-fired, electric, or oil storage or instantaneous commercial water heater that meets the following conditions:

1. Uses a single-phase external power supply for models that require electricity.
2. Is not designed to provide outlet hot water at temperatures greater than 180°F.
3. Is not excluded by the specified limitations regarding rated input and storage capacity as described in Table 5-2 below. In other words, a residential-duty commercial water heater must have rate input and rated storage volume below the value listed in Table 5-2.

A hot water supply boiler is industrial water heating equipment with a heat input rate from 300 to 12,500 kBTU per hour and at least 4,000 BTU per hour per gallon of stored water. A hot water boiler should have either the temperature or pressure control necessary for heating potable water for purposes other than space heating, or the boiler manufacturer’s literature should indicate that the intended uses of the boiler include heating potable water for purposes other than space heating. A hot water boiler could be fueled by oil or gas, and it must adhere to the minimum thermal efficiency and maximum standby loss as described in California’s Title 20 Appliance Efficiency Regulations.

Boilers are typically used for doing both space heating and water heating. Use of a boiler will typically require one or more unfired storage tanks to be installed as part of the system. Careful attention should be given to the layout of these systems due to the potential for high energy losses between the boiler and storage tanks. Boilers are not allowed through the prescriptive approach to compliance but can be installed using the performance approach as long as the water heating energy budget is not exceeded.

The primary maintenance activities for instantaneous water heaters are flushing the heat exchanger to remove scale buildup and inspecting and cleaning the inlet water filter screen, which helps minimize the amount of debris or sediment that enters the water heater.

Some manufacturers recommend a maintenance schedule, but the maintenance schedule users deploy may vary based on water quality. In areas with hard water, more frequent maintenance (every two years) is recommended. In areas where the water quality is relatively good, water heater maintenance is recommended every three to four years. Frequent inspection of the inlet water filter screen will enable a user to monitor the amount of sediment entering the water heater. If the filter tends to fill with sediment regularly, then more frequent flushing may be required. Users can also reference local water quality data to determine the level of water quality in their area to help guide maintenance schedules.

To assist in flushing the heat exchanger, manufacturers and plumbers recommend the installation of a drain kit (that is, isolation valves). (See Figure 5-2 below.) The installation of isolation valves on instantaneous units is mandatory in the Energy Codes (Section 110.3[c]6 and Section150.0(n)4). Isolation valves enable the unit to be isolated from both the inlet cold water and the outlet hot water lines, thereby allowing the heat exchanger to be flushed using a simple procedure. Integral to the kit are hose bibs that allow the flushing hoses to be attached. Instantaneous water heater that has integrated drain ports for servicing are acceptable to meet the requirements of Section 110.3(c)6 and will not require additional isolation valves.

Manufacturers recommend that a licensed professional flush the heat exchanger to avoid potentially damaging the water heater, though some manufacturers sell flush kits so that homeowners can maintain their own water heater. Flush kits consist of a submersible pump, two short hoses, hose connections, and a 5-gallon bucket. These components can be purchased separately or as a preassembled kit. A solution of white vinegar is widely recommended for flushing the heat exchanger as it is food-grade and very effective at removing scale.

In addition to flushing the heat exchanger, manufacturers recommend periodically inspecting and cleaning the inlet water filter screen, which helps minimize the amount of debris or sediment that enters the water heater. This can be done by running the filter screen under hot water and using a brush to remove debris. Replacement of the inlet water filter screen is not necessary unless it is damaged.

Figure 5-2: Isolation Valves

Source: BassCraft at http://www.brasscraft.com/products.aspx?id=266

For storage water heaters, the primary maintenance activities consist of draining the tank, inspecting the anode rod, and replacing the anode rod, if necessary. The recommended frequency of regular maintenance varies by manufacturer. Like instantaneous water heaters, the frequency of maintenance depends on water quality. Most manufacturers recommend draining the tank every six months to once per year to remove sediment that has accumulated in the bottom of the tank. Periodic inspections (every six months to once a year) of the burner, venting system, and temperature and pressure relief valves are also recommended by manufacturers.

Manufacturers typically recommend inspecting the anode rod every two years and replacing it when necessary to prolong tank life, but the frequency of inspection depends on local water conditions. If water is soft or a water softener is used, more frequent inspection of the anode is needed as softened water will corrode the sacrificial anode rod at a much faster rate than unsoftened water. If the setup of the water heater prevents an easy removal of the corroded anode rod, then it might be necessary to completely move the tank from the location to replace the anode rod.

HPWHs have a unique maintenance requirement that involves inspection and cleaning of the air filter. Since the system utilizes a fan to move air across the system’s refrigerant heat exchanger, the air filter must be checked on a regular basis (3-4 times per year). Many manufacturers provide filter cleaning alerts either through an app or on the HPWH display screen.