The purpose of this PBMP analysis is to address the waste of water that occurs in a typical household between the time one turns on a tap or a fixture and when the water arrives at the same location at the desired useful hot water temperature. The key question that needs to be answered in order to properly frame the discussion is:
How much water can or should be wasted while waiting for hot water to arrive?
The amount of water that is wasted is directly related to the volume and temperature of water that is in the pipe between the source of hot water and point of need, i.e., the fixture or appliance. That volume of waste can be either cold or warm water. That water is not at the desired useful hot water temperature at the beginning of the hot water event and can vary from practically zero to many gallons.
The amount of water waste also relates to time and energy. For a given flow rate, the smaller the volume that needs to be purged from the line, the less the wait. Less wait generally increases customer satisfaction.
Energy is used in buildings both to pressurize and heat the water. When needed, cold water is pressurized, e.g., using well pumps in remote locations and using pumps to lift the water in tall buildings. The energy so-consumed needs to be accounted for in the building and in the water and wastewater treatment systems. The energy required for heating the water is based on the delivery volume, the use volume, the volume of water that cools off between hot water events and on the efficiency of the water heater. The configuration of the hot water distribution system – length, diameter, environmental conditions, insulation, etc. – is directly related to its water delivery performance and to the energy needed to support this level of performance.
Every gallon that is not wasted while waiting for hot water to arrive means that a gallon does not need to be delivered to the water-using customer and that same gallon does not need to be treated as wastewater. The energy intensity of the water use cycle has been studied by the California Energy Commission (CEC). In their staff report (CEC-Klein, 2005), the CEC reported that energy intensity can vary from 2,000 to 20,000 kilowatt-hours/million gallons (kWh/MG). In northern California the typical energy intensity is roughly 4,000 kWh/MG and in southern California it is roughly 13,000 kWh/MG. Saving a gallon of water - hot or cold - in southern California typically saves more energy than in northern California.