5.2   Water Heating Equipment    

5.2.1    Water Heater Types

There are several different types of equipment used for producing domestic hot water.  Any water heater type used for compliance must be recognized under the appliance regulations.  The most commonly used water heater for single family homes is either small storage gas or instantaneous (tankless) gas units.  For multi-family buildings, two options are commonly used: either one or more commercial storage water heaters or one or more boilers coupled with a storage tank to serve the entire building. Alternatively, individual water heaters are installed in each dwelling unit (similar to single family).

1.   Small Storage Gas Water Heaters

Storage water heater means a water heater that heats and stores water within the appliance at a thermostatically-controlled temperature for delivery on demand, and that has an input less than 4,000 Btu per hour per gallon of stored water.

Small Storage Water Heater is defined as a water heater that is a gas storage water heater with an input of 75,000 Btu per hour or less, an oil storage water heater with an input of 105,000 Btu per hour or less, an electric storage water heater with an input of 12 kW or less.  All small storage water heaters are rated using an Energy Factor.  This value represents the combination of the units firing efficiency and standby loss over a 24 hour period.

 Application Issues

In California, a vast majority of small water heaters are atmospheric natural gas water heaters, despite the fact that they are relatively inefficient due to high standby losses. Historically these units are the cheapest to install and operate in California.  Due to observed California hot water loads that are considerably lower than assumed in the Energy Factor rating test the typical California performance of small storage water heaters is lower than their ratings.

2. Large Storage Gas

Large Storage Gas is a storage gas water heater with input capacity greater than 75,000 Btu/h. They are rated with an AFUE and either a total standby loss numeric value or a percent standby loss..

Application Issues

These units offer higher capacity and generally greater storage volume, and are therefore better suited for high load situations, including combined hydronic space and water heating applications.  Many of the products available in this category are higher efficiency.  These units typically require an electrical connection for controls and combustion air blowers.

3. Small Storage Electric

A Storage Electric water heater is an electric water heater designed to heat and store water at less than 180°F. Water temperature is controlled with a thermostat. Storage electric water heaters have a manufacturer's specified storage capacity of at least two gallons.

Application Issues

Storage electric water heaters represent less than 6% of the installed residential water heaters in California, and are often found in areas where natural gas is unavailable.  For most of the state, relatively inexpensive natural gas is a much more economical water heating approach for the consumer.

4. Storage Heat Pump

A Storage Heat Pump is an electric water heater that uses a compressor to transfer thermal energy from one temperature level to a higher temperature level for the purpose of heating water. It includes all necessary auxiliary equipment such as fans, storage tanks, pumps or controls.

Application Issues 

Energy Factors for heat pump water heaters are found in the Energy Commission’s Appliance Database under Certified Water Heaters.  In recent years, heat pump water heaters have started to gain a greater national presence, since they offer roughly 50% savings relative to a standard storage electric water heater.  None of the prescriptive packages allow the use of heat pump water heaters but they can be applied for water heating using the performance approach. 

Heat pump water heater performance, more than other water heater types, is sensitive to a variety of factors including:  operating mode and set point, hot water loads and load intensity (short, intense draws reduce efficiency), climate, and unit location (environment temperature).  Most manufacturers suggest that the unit be installed in space with a volume of at least 700 ft3 to provide a sufficient quantity of air to allow for more efficient heat pump operation.  Indoor located units may offer space conditioning benefits in hotter climate areas, although noise may be a potential concern.

5. Instantaneous (Tankless) Gas

Instantaneous gas water heaters are defined as a water heater that has an input rating of at least 4,000 Btu per hour per gallon of stored water.  These units, commonly referred to as gas tankless water heaters, operate their burner in response to water flow heating the water flowing through the heat exchanger (typical volumes around 0.5 gallons).  The main efficiency benefit of associated with these units is the elimination of standby losses common to storage water heaters. Virtually all of these units require an electrical connection for controls and combustion air blower.

Application Issues

Instantaneous units are recognized as being optimistically rated by the Energy Factor test, due to the test procedures specification of only six draws during the 24 hour test period.  Field data suggests a ~10% degradation in performance due to real world loads and draw patterns.  Although performance is sensitive to the number of draw events and average draw volume size, in general instantaneous efficiency is much less sensitive to daily hot water load than storage water heaters.  Installation issues related to instantaneous units include the need for a larger gas line (typical input ratings of 140 – 200 kBtu/hr), alternative venting systems, and electrical connection.  For retrofits, this can be a major added cost. 

Instantaneous water heaters have minimum flow rates for initiating burner firing, resulting in some low flow rate hot water draws not being satisfied.  In 'addition, firing from a cold start requires an additional 15-30 seconds of delay before fully heated water leaves the unit.  This has implications in terms of water waste and occupant satisfaction.  Care must also be taken in matching tankless unit with recirculation pumps (pumps must be sized to overcome high unit pressure drop) and solar systems (tankless firing becomes intermittent as inlet temperature approach the setpoint temperature).  Finally, maintenance of instantaneous units is more critical than for a conventional water heater, especially in areas with hard supply water, due to potential heat exchanger scaling problems.  Despite these potential issues, instantaneous sales have increased dramatically over the past ten years, with a recent study suggesting that 25% of new California homes have gas instantaneous water heaters.

Instantaneous water heaters are occasionally installed with small electric storage buffer tanks either internally or downstream of the instantaneous unit to mitigate the potential for cold water sandwich effects, an effect which may cause fluctuating delivery temperatures.  If a buffer tank is installed the buffer tank must be modeled as a separate electric water heater.   If the buffer tank is installed downstream of the instantaneous system the buffer tank must be listed in the CEC Appliance Directory and the listed wattage of the unit will be entered into the compliance software.  For instantaneous units with integral buffer tanks the rated wattage on the manufacturer’s cut sheet should be used.

6.  Instantaneous (Tankless) Electric

An Instantaneous Electric water heater is an electric water heater controlled automatically by a thermostat, with a manufacturer's specified storage capacity of less than 2 gallons.

Application Issues

Instantaneous electric water heaters are not generally designed for use with solar water heating systems or as heat sources for indirect fired water heaters. They are also typically inappropriate for use with recirculation systems. Consult manufacturer's literature when considering these applications.

Instantaneous electric units offer several advantages over electric storage water heaters:  smaller size, reduction in standby losses, and ability to locate remotely resulting in reduced distribution losses.  Countering this is the need for upsized electrical service to handle the demands (up to 30 kW) associated with instantaneous heating of the water.  In areas without natural gas, these systems may provide some operating cost savings, however for the majority of California, the abundance and low cost of natural gas creates a difficult environment for this technology to succeed.

7.  Hot Water Supply Boiler

A hot water supply boiler is industrial water heating equipment with a heat input rate from 300 kBtu/hr to 12,500 kBtu/hr and at least 4,000 Btu/hr per gallon of stored water. A hot water boiler should have either the temperature and pressure control necessary for heating potable water for purposes other than space heating, or the boiler manufacturer’s literature should indicate that the boilers’ intended uses 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 Standards in effect.

Application Issues

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.

5.2.2    Mandatory Requirements for Water Heaters

8.  Equipment Certification

Manufacturers must certify that their products comply with the Appliance Efficiency Regulations at the time of manufacture. Regulated equipment which applies to all of the aforementioned system types in Section 5.2.1 must be listed in the California Energy Commission appliance database.

 9.  Equipment Efficiency

Small water heaters are regulated by federal efficiency standards. The efficiency requirements for such equipment are given in Table 5-1 below. Note that on April 16, 2015 the federal standards change, requiring higher efficiencies for most product classes and also classifying storage water heaters into two categories:  ≤ 55 gallons volume, and > 55 gallons.  The larger volume units will require higher performance.  For gas water heaters with > 55 gallon storage volume, the efficiency levels dictate condensing performance, while for electric storage water heaters > 55 gallons, the efficiency level suggest performance comparable to a heat pump water heater.

Table 5-1 – Minimum Energy Factor Requirements

The energy efficiency of equipment that is larger than the Table 5-1 specifications is regulated by the California Appliance Efficiency Regulations. Energy Factor is not applicable for this equipment, but rather minimums are specified for thermal efficiency and standby loss as shown in Table 5-2.

Table 5-2 – Minimum Energy Factor Requirements - Large Water Heaters

3. Storage Tank Insulation

A minimum R-12 external tank wrap is a mandatory requirement for minimum efficiency storage water heaters.

Any unfired tanks (used as a back-up for solar water heating or as storage for a boiler) must either be insulated externally with R-12 or have a label indicating the tank is internally insulated with R-16. Alternatively, a tank can comply with this mandatory measure if calculations are provided that show that the average heat loss is less than 6.5 Btu/hr-ft² when there is a temperature difference of 80°F between the water in the tank and the ambient air.

4. High Efficiency Water Heater Ready

In order to facilitate future installations of high efficiency equipment, the Standards has implemented the following requirements for systems using gas or propane water heaters to serve individual dwelling units:

a.   A 120 V electrical receptacle that is within three feet of the water heater and accessible to the water heater with no obstructions; and,

b.   A Category III or IV vent, or a Type B vent with straight pipe between the outside termination and the space where the water heater is installed; and,

c.   A condensate drain that is no more than 2 inches higher than the base of the installed water heater, and allows natural draining without pump assistance, and,

d.   A gas supply line with a capacity of to provide at least 200,000 Btu/hr to the water heater.

These requirements make it easier for someone to retrofit high efficiency gas water heaters in the future.  Virtually all high efficiency gas water heaters require an electrical connection and wiring during initial construction stage is much less costly than trying to retrofit it later.

Table 5-3 below summarizes venting requirements for different types of water heaters.  Higher efficiency water heaters often require different vent materials due to the presence of acidic condensation from flue gases.  The standard Type B vent installed for conventional atmospheric gas water heaters is made of steel and would soon be destroyed by the condensate. As a result, this standard requires that the only time one can use a Type B vent for the water heater is when there is a straight shot between the water heater and where the vent leaves the building. The application of the word straight is intended to require that installation meet all code and manufactures guidelines. Because category III and IV pipes are usually smaller than those for Type B vents, a straight Type B vent can be easily modified into a category III or IV vent by simply inserting a new vent pipe through the existing Type B vent pipe. A flue pipe that makes bends though the building structure is not easy to retrofit and thus these flues must be either category III or IV vent pipes. Please note that only stainless steel category III and IV vents are compatible with typical atmospheric combustion storage water heaters. The requirement for the condensate drain being placed near the water heater and no higher than the base of the tank allows the condensate to be removed without relying on a sump pump.

Designing the gas line to provide 200,000 Btu/hr gas supply capacity to the water heater is required to accommodate future retrofit to a tankless (instantaneous) water heater, which usually has a heat input capacity of 199,000 Btu/hr or above. Similar to the electrical requirement, installing a larger gas line during new construction is very inexpensive relative to a future gas line retrofit. Gas pipe sizing for the building needs to consider piping layout and gas supply requirements for other gas appliances as well, such as gas clothes dryers, gas furnaces, gas ranges and ovens, and gas fireplace burners. The tradition practice of using a ½ inch gas pipe in a single family house to serve a storage water heater will NOT be able to meet the new standard requirement. The minimum gas pipe size for water heaters will be ¾ inch. However, the exact gas piping system should be designed following the applicable plumbing code. 

Table 5-3 – Summary of Acceptable Vent Material by Appliance Category