5.5   Performance Approach Compliance for Water Heating

5.5.1    Energy Budget Calculation

The computer performance approach allows for the modeling of water heating system performance by 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 others will result in penalties.

5.5.2    Systems Serving Single Dwelling Unit

In the case of single dwelling units, 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 natural gas instantaneous water heater with a standard 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 total energy budget.

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 additional DHW design features. As per the prescriptive requirements, all mandatory pipe insulation requirements must be met, such as insulating all pipes running to the kitchen. Alternative distribution systems are compared to the standard design case by using distribution system multipliers (DSMs), which effectively rate alternative options.

Table 5-8 lists all the recognized distribution systems that can be used in the performance approach with the assigned distribution multiplier. The standard distribution system has a multiplier of 1.0. Distribution systems with a multiplier less than 1 represent an energy credit, while distribution systems with a multiplier greater than 1 are counted as an energy penalty. For example, pipe Insulation with HERS Inspection Required (PIC-H) has a multiplier of 0.8. That means that it is modeled at 20 percent less distribution loss than the standard distribution system. For more information or installation requirements on any of the systems, refer to Section 5.6.

Table 5-8 – Applicability of Distribution Systems Options Within a Dwelling Unit

5.5.3    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 that are controlled by a demand control technology. Systems designed with other options are allowed, but they require compliance verification through performance calculation.

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 additional credit for designs with more than two recirculation loops is not provided.

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.

5.5.4    Treatment of Water Heater Efficiency

1.  For information on how water heater efficiency is considered in terms of modeling energy performance using the compliance software tool, please refer to the Residential Alternative Calculation Method (ACM) Reference Manual.

5.5.5    Compliance Issues

Water heating is becoming more important to overall building compliance as building envelope performance and mechanical efficiency improve. When the performance approach is used, a high-efficiency water heater and an efficient distribution system can significantly affect 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 energy budget.

Asking for a cut sheet on the installed equipment to verify efficiency is a simple shortcut to checking compliance. 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.