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The costs of electricity and natural gas must be considered in the economics of IWHs and CIWHs.
IWHs had an energy cost range of $63 to $292 per year, with an average cost savings of 33%.
Residents rated StWHs higher than IWHs or CIWHs on delay in delivery and need to increase flow rate to get hot water at low flows (Table 11).
This study increases the real-world data available on the comparative performance of StWHs, IWHs, and CIWHs.
For example, it has been postulated that occupants might use more hot water with IWHs or CIWHs because there would be no risk of running out of hot water, or that they might develop a habit of walking away from faucets while waiting for hot water with IWHs or CIWHs because of longer delays.
Each site had multiple water heaters and extensive instrumentation installed, making it hard to estimate the typical installed costs of IWHs and CIWHs from this project.
Simple paybacks for the IWHs and CIWHs installed in this study are estimated to range from 10 to 60 years.
IWHs and CIWHs have been shown to save a significant amount of natural gas.
This study shows that the EF ratings for StWHs, IWHs, and CIWHs are greater than actual measured efficiencies, and the difference between EF and actual measured efficiency differs for the different technologies, so that comparisons of EF ratings between technologies do not accurately represent relative consumption.
Nine of ten households participating in the tests indicated that there were no performance issues that would definitely prevent residents from purchasing IWHs and CIWHs, though participants did rate time to get hot water at the fixtures and flow rate required to get hot water less favorably than for StWHs.
Even though a large percentage of water heating energy use can be saved by IWHs and CIWHs, their high installed cost is major hurdle at current energy prices.
There are also outstanding questions about qualitative aspects of IWH performance, such as increased time required for the water heater to produce hot water, minimum flow rate required to activate the burner, and "cold water sandwiches," which occur when hot water remains in the pipes from a previous draw when a new draw is initiated.
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