Case Study - ReVISION House Vegas
Product: Tankless Water Heater
Category: Domestic Hot Water
In a traditionally designed and built home in the US, space heating and cooling are typically the largest energy uses. In a home strategically designed and engineered to be energy-efficient, it is often the domestic hot water that eclipses the space conditioning in energy consumption. This means that to approach a truly low-energy or zero-energy home, hot-water energy use must be aggressively addressed. After analyzing multiple options, the ReVISION House Vegas project team selected a very high performing combination system intended to meet all the projected domestic hot water demands, while providing the performance levels needed for a net zero-energy home: solar-thermal preheat and a Rinnai tankless gas water heater.
The existing water heater at the ReVision House was a standard non-sealed combustion water heating tank. The 60 gallon tank, located in the unconditioned garage was conventionally vented with a combustion chamber at its base, and a flue vent running from the combustion chamber at its base to a draft hood and “B” vent at the top. Since the heat exchange element for the hot gases to the water occurs at the surface of the flue, the heat exchange process works in reverse also allowing the thermal energy stored in the water to escape as convective airflow naturally rises up the flue when the tank is not firing. These thermal losses, and the conductive losses through the shell of the tank, are called “stand-by-losses” and are the most extreme in non sealed-combustion atmospherically vented water heating tanks. By virtue of the stand-by-losses, overall efficiencies for this type equipment is often less than 60%, with over 40% of the input energy escaping.
By contrast the Rinnai® R75LSi natural gas water heater eliminates all these and other disadvantages. By producing hot water only only on demand, stand-by-losses are virtually eliminated. By using sealed combustion technology, air is drawn from the exterior for combustion preventing possible back-drafting through the device resulting in potential indoor-air-quality concerns. The concentric combustion-air/exhaust vent doughnut configuration (exhaust gases in the middle, and combustion air encircling it) heats the combustion gas before it enters the combustion chamber increasing combustion efficiency. The compact dimensions, small clearance requirements to combustibles, sealed combustion, and multiple potential vent configurations (horizontal through wall or vertical through roof ) allow the Rinnai water heater to be installed in a very small space within the home close to the actually hot water use locations, reducing distribution losses and further improving system efficiency.
The variable firing rate (15,000 to 180,000 BTU/hr) inputs only the amount of heat energy that is required by the call for hot water. Variables taken into account for the rate firing include the hot-water flow rate, and the temperature of the input water. This is a critical feature when combined with a solar preheat tank, as the temperature of the water in the tank will vary considerably depending on weather conditions and time of day. Given the complexity of most solar thermal pre-heat systems, the simplicity of the Rinnai and its flexibility of installation make for an ideal application.
With its rated energy-factor of 0.82, the Rinnai R75LSi is nearly 50% more efficient than the water heating tank it replaces, and as an adjunct to the solar preheat, will reduce the amount of natural gas consumed by six-fold, from 166 therms to 28 therms annually.