5.5   Water Heating System Performance Compliance    

1.  Energy Budget Calculation

The computer performance approach allows for the modeling of water heating system performance taking into account building floor area, climate, system type, efficiency, and fuel type. The standard design water heating budget is defined by the corresponding prescriptive requirements. The performance method allows for modeling alternative water heater and distribution system combinations.  Some of these options will offer compliance credits and some result in penalties.

2.  Systems Serving Single Dwelling Unit

In the case of single family buildings, any type or number of water heaters can theoretically be installed.  The calculated energy use of the proposed design is compared to the standard design energy budget based on a single small storage water heater with a Standard hot water distribution system. Adding multiple water heaters to a single family design will generally result in an energy penalty in the water heating budget that must be offset elsewhere in the overall Title 24 compliance.

A standard distribution system serving a single dwelling unit does not incorporate a pump for hot water recirculation, and does not take credit for any design features eligible for energy credits. As per the prescriptive requirements, all mandatory pipe insulation requirements must be met such as all pipe lengths running to the kitchen must be insulated.  Alternative distribution systems are compared to the standard design case by using distribution system multipliers (DSMs), which effectively rate alternative options.

Table 5-7 provides a listing of all the recognized distribution systems that can be used in the performance approach with their assigned distribution multiplier. For more information or installation requirements on any of the systems refer to Section 5.3.

Table 5-7 – Applicability of Distribution Systems Options within a Dwelling Unit

Note: any system that does not meet the installation requirements 'listed in RA3 and RA4 for the specific system type in any way must either have the installation corrected or have the compliance run redone using the Non Compliance installation distribution multiplier.

3.  Systems Serving Multiple Dwelling Units

For systems serving multiple dwelling units with a recirculating pump the standard distribution system design is based on a central recirculation system with two recirculation loops which are controlled by a demand control technology. Systems designed with other options are allowed. But they require compliance verification through performance calculation.

Table 5-8 – Applicability of Distribution Systems Options within a Dwelling Unit Applicability of Distribution Systems Options for Central Distribution Systems in Multi-family Buildings

Central recirculation systems using only one recirculation loop are expected to have larger pipe surface areas than those of dual-loop designs, according to plumbing code requirements for pipe sizing. For large buildings, it may be better to use more than one recirculation loop with each serving a small portion of the building, even though the Standard does not provide additional credit for designs with more than two recirculation loops.

If demand control is not used, temperature modulation controls and/or continuous monitoring should be used as an alternative compliance method. Recirculation timer controls are not given any control credits because field studies revealed that they are usually not properly configured to achieve the intended purposes. Buildings with uncontrolled recirculation systems will have to install other efficiency measures to meet compliance requirements through the performance method.

Systems with all pipes insulated can claim compliance credit. The amount of credit is increased if the insulation is verified by a HERS rater. Increasing recirculation pipe insulation by 0.5 inch above the mandatory requirements can also result in compliance credit through performance calculation.

5.5.1    Treatment of Water Heater Efficiency

1.  Small Storage Water Heaters

Small storage gas and electric water heaters are modeled to reflect the dependency of their rated performance on the actual hot water recovery load (the estimated energy load on the water heater based on the CEC draw schedule).  This “load dependent Energy Factor” relationship decreases the water heater efficiency at loads lower than the EF test level and increases it for loads exceeding the EF test level.

 2.  Large Storage Water Heaters

For large storage water heaters, energy use due to hot water demand and distribution loss are calculated and listed according to thermal efficiency. The total water heater energy use is documented by also includes standby loss, which is reported either as a percentage or numeric value in Btu/hr. Both values are calculated based on a relationship of standby looses (which include thermal and electricity) compared to the total energy storage capacity of the water heater Heat Pump Water Heaters

Heat pump water heaters are modeled based on their rated EF. In 'addition to recognizing the performance impact of loads on annual efficiency, the heat pump water heater model also includes a climate zone adjustment to reflect changes to heat pump efficiency with the ambient temperatures in that climate zone.

3.  Instantaneous Gas Water Heaters

A PIER-sponsored evaluation of instantaneous (or tankless) gas water heaters was completed to assess whether the rated Energy Factor for these units accurately describes real world system performance. Results of the study indicate that the Energy Factor test procedure underestimates the impact of small volume hot water draws and heat exchanger cycling on annual system performance. Based on these findings, the 2008 Standards applied a 0.92 derating factor on the nominal EF of all gas instantaneous water heaters.  This derating was validated by further PIER field research completed in 2011.

Instantaneous water heaters are occasionally installed with small electric storage buffer tanks downstream of the tankless unit to mitigate the potential for cold water sandwich effects. If one of these units is installed, both the buffer tank and the instantaneous water heater must be modeled. The buffer tank must be listed in the CEC Appliance Directory and the listed standby wattage is used to model the buffer tank as a separate electric point of use water heater. In cases where the buffer tank in built into the instantaneous gas water heater the wattage of heating coil in the buffer tank must be modeled in the same manner as if the buffer tank where separate.

4.  Solar Water Heating Systems

Solar water heating can be used in compliance with single family or multi-family buildings. For treatment of solar water heating systems, please refer to Section 5.6.2.

5.5.2    Compliance Issues    

Water heating is becoming more and more important to overall compliance as building envelope performance and mechanical efficiency improved.  When the performance approach is used, a high efficiency water heater can significantly impact the overall performance margin of a building especially in the milder climates like climate zones 4 through 9, where water heating typically represents a larger fraction of the overall house energy budget.

Asking for a cut sheet on the installed equipment to verify efficiency is a simple shortcut to checking compliance.  Also note that when used in a combined hydronic system it is important to check the capacity of the system to verify that both space and water heating loads can both be met