Calculation Factors and Sources
This is an illustration of the impact of water heater placement on the wait time necessary to get hot water at a particular point of use. The impact of moving the water heat from a common location like the garage in this case to a more central location will substantially reduce wait time, and will also make a big difference in total water consumption and fuel consumption.
We make the following assumptions about the home:
- The home is occupied by a family of 4
- The entering water temperature is 60° F
- We assume the following use pattern:
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- Showers per day – 4, 2 gpm @ 108 ° F
- Uses of a lav sink per day – 8, 1 gpm @108° F
- Uses of a kitchen sink per day – 8, 1.5 gpm @120° F
- Uses of the dishwasher per day – 1, 1.5 gpm @120° F
- We assume that at the start of each use the hot water supply line is cold
- All hot water line are ¾” ID
- Wait times for the Rinnai include a 4 second cold start allowance
When calculating carbon dioxide emissions we utilize:
- Carbon dioxide emissions for electrical generation per state based on April 2002 “Updated State-level Greenhouse Gas Emission Coefficients for Electrical Generation 1998 – 2002” US Department of Energy
- Carbon dioxide emission rates for Natural Gas and LP gas Based on US Energy Information Administration Emission Reporting
- Natural gas – 11.7 lbs CO2 / therm consumed
- LP gas – 12.5 lbs CO2 / gallon consumed
- Natural gas or LP gas consumption based on US Department of Energy efficiency calculations. Rinnai EF is assumed to be 0.82. Gas DHW tank EF is assumed to be 0.60. Electric DHW tank EF is assume to be 0.92
Energy Calculations are based on the following assumptions
- The energy in the hot water supply line is lost at the end of use cycle. Temperature of hot water pipe chase is assumed to be 75°
- Natural gas or LP gas consumption based on US Department of Energy efficiency calculations. Rinnai EF is assumed to be 0.82. Gas DHW tank EF is assumed to be 0.60. Electric DHW tank EF is assume to be 0.92
- Reducing water heater output water temperature has a significant impact on wait time, water, and energy waste. Whenever possible use Rinnai control pads at points of use to get the maximum savings.