Tankless water heaters have been used for over 40 years in Europe. When architect Fred Thurston, AIA, of Four Front Design in Rapid City, South Dakota was designing his own solar home in 1982 he chose a tankless water heater as a backup, a technology he had learned while a student in Holland in 1972. Today, he is installing tankless hot water units as a means to gain credits towards his new LEED^® Gold Certified office building.

 

Hot Water Heating for an Arizona Residence

Tucson architect Susan Schafer Kliman, PhD, AIA of Klimatic Architecture, has a new project on her drawing board which will incorporate tankless hot water heaters combined with an extensive solar panel system. The house will be composed of a series of small cottages which make up the main house, an office and guest suites attached by trellises. Instead of running the plumbing throughout the entire complex, each cottage will have a separate tankless hot water system. The main house will include a combination of solar panels which will preheat hot water which is then used by the tankless heater for the domestic hot water supply. The house is located in southern Arizona, about 50 miles south of Tucson on a working ranch that has been subdivided into 40 acre parcels. The house has several ?green' features. Exterior walls are constructed of aerated autoclaved concrete (AAC), which is manufactured about 60 miles north of Tucson ? and the sand comes from mine tailings which go through a cleaning process. The mechanical system is a radiant system that will handle both the heating and the cooling. The heated water for this system will come from solar panels which will be supplemented with tankless hot water heaters. There are also a lot of passive features designed into the house. The architect has designed a grey water system for the house, and will also be harvesting the rainwater on the site. Since the guest suites and office are not going to be used for much of the time, the tankless hot water heating system is a very economical and energy efficient way to provide hot water to those areas of the complex.

This unique design will use gas tankless water heaters to provide hot water to each cottage component of this Arizona residence. Drawing courtesy of Klimatic Architecture

 

 

The higher cost of a typical tankless hot water system is offset by the energy savings, which are estimated by the DOE to achieve a 30 percent reduction in energy use, per unit.³ Current legislation may expand the tax credits for energy saving appliances, which will also add to offset the initial purchase cost. One tankless hot water heater can provide more hot water to a home than a typical 75 gallon hot water storage tank. Most units are the size of a large backpack and can be placed in closets, in the attic and even outdoors. The designer can reduce the amount of space needed for mechanical equipment by using a gas tankless hot water heater.

In the past, architects have specified tankless hot water heaters in conjunction with solar panels, and in vacation houses. Today, they can be specified to provide a continuous flow of precisely heated hot water in almost every residential application, as well as for some commercial installations. Some architects are including tankless hot water heaters in every LEED project.

Residential Water Usage

Showers, baths, dishwashers, washing machines, toilets, the coffee pot or cappuccino maker: homeowners use water in many ways. Some appliances demand a burst of hot water over a short period of time like a dishwasher or a clothes washer. But the demand for a hot shower is a demand for a continuous flow of water heated to a constant temperature provided as quickly as possible to the user.

Showers demand the most water of any fixture over the longest period of time. Some "whole body" showers provide several shower heads in one shower stall. These systems require an even greater output of hot water. Designers need to understand the water use of a facility and consider all of the appliances which may be used by the occupants. Many green designers specify fixtures, which will have low flow water systems to preserve water usage, but these will not affect the supply of hot water from the tankless hot water heaters. When designing a hot water system, the designer considers the flow rate of all of the plumbing fixtures primarily based on the flow rate for each bathroom. When designing a tankless hot water heater the designer should consider the following:

• Is the fuel source natural gas or liquid propane?
• How many full baths?
• Are there specialty bath fixtures such as a hot tub or a body shower?
• What is the average ground water temperature in the winter?
• Will the unit be located indoors or outdoors?

With proper sizing, tankless hot water heaters can provide on demand hot water, for all users in a residence, even when multiple showers, sinks, and appliances are all in use at the same time.

Energy Consumption

The DOE ENERGY STAR^® rating will apply to natural gas or liquid propane tankless hot water heaters. Sizing the gas service is critical to proper installations. A gas line into a residential home should be sized to provide gas for the entire gas load of a home, calculated by adding the BTU rating of each gas appliance in the home. State and local codes must be met, as well as utility requirements, to

ensure gas supply to the unit is adequate to meet the rated demands. Tankless hot water heaters may require a gas service which ranges from 180,000 BTU's to 199,000 BTU's. This may require that the professional will specify a larger gas service into the home than normally provided.

Tankless gas hot water heaters, unlike traditional electric water heaters, or electric tankless hot water heaters only need 120 VAC, 60Hz power from a properly grounded circuit, which simplifies installations in most homes. The electric service must be grounded in accordance to local codes or to the most recent edition of the National Electrical Code, ANSI/NFPA 70. Do not rely on the gas or water piping to ground a tankless water heater. Low voltage power supply requirements make these units ideal for replacement hot water heating systems in existing homes.

Electric digital controls display the set temperature and allows for diagnostic maintenance if required. The heating system only activates when the hot water tap is opened and closed when the tap is closed. If a hot water faucet is dripping, the heater will not sense a demand for hot water unless the flow rate can be detected at a rate over .6/gallons/minute. The temperature setting is precise to the exact amount of hot water needed at the appliance or shower head and does not have to compensate to heat replacement water as part of a storage system. Remote controllers are available to adjust temperature or provide diagnostic codes as needed for indoor installations.

In order to meet the new ENERGY STAR^® label, a wholehome tankless water heater must have a minimum energy efficiency rating (EF) of 0.82, minimum gallons-per-minute flow (GPM), of 2.5 at a 77 °F rise, or be 41.4 percent more efficient than the current Federal standard.⁴ The DOE estimates that these units will be expected to reduce energy use by approximately 30 percent and save the home owner approximately $108 in annual energy costs in comparison to a typical gas storage water heater.

An energy factor (EF) indicates a water heater's overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. This rating evaluates the following:

• "Recovery efficiency - how efficiently the heat from the energy source is transferred to the water
• Standby losses - the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)
• Cycling losses - the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes."⁵

Manufacturers provide this information in their product literature and can discuss with the designer how to specify the right system which will provide the highest energy rating for a home installation. Higher efficiencies of other types of hot water systems do not necessarily mean that they provide equal energy savings as the gas tankless hot water heater is only used when there is a demand for hot water.