DHW system assumptions for early stage-models

The domestic hot water system, also called service hot water system, is the system that delivers hot water for non-space heating purposes like showers, sinks, dishwashers, and other domestic or commercial uses.

These systems include various components, such as:

• Water heaters or boilers: The main equipment that heats the water using gas, electricity, or other fuel sources.
• Storage tanks: Tanks where heated water is stored until needed.
• Circulation pumps: Used in larger systems to circulate hot water through pipes to reduce wait times at fixtures.
• Piping and insulation: To distribute hot water efficiently and minimize heat loss.

Service water heating systems are distinct from space heating systems, although in some cases, integrated systems may serve both purposes. For energy modeling and efficiency considerations, service water heating is often analyzed separately due to its specific load profile, temperature requirements, and usage patterns.

Typical DHW system inputs that impact energy consumption:

• Domestic hot water demand – gal/min (dependent upon use type estimations or installed fixture calculations)
• Fuel type – electric, natural gas, or oil
• Storage type – tank (size), or instantaneous
• Heating capacity – kW or Btu/h depending on fuel source
• Rating condition – (Btu/h)/gal based on tank size
• Efficiency – thermal efficiency of the heater
• Hot water temperature – typically 105-200F
• Piping insulation – thermal conductivity

For some building types, domestic hot water is a significant energy end use. For these buildings, such as residential buildings, food service, hotels and hospitals, attention to domestic hot water modeling inputs is important. The following resources provide estimates of hot water consumption rates for different types of buildings.  

• U.S. Department of Energy Commercial Reference Building Models of the National Building Stock
• COMNET; consumption rates and schedules