Hot Water on Demand: Natural Gas Tankless Hot Water Heaters Fit your Energy Budget  

When the water heater and the furnace both failed, green designer John Beeson, LEED AP, of Quinn Evans Architects, convinced his wife to let him try something new. He incorporated a gas-fired, low voltage hot water tankless heater into his solar hot water and radiant floor system, providing his family with continuous hot water for his Ann Arbor, MI residence. He also continues to save money on his monthly energy budget. Designing a 21st century green building can challenge design professionals to rethink the mechanical systems of the 20th century. On-demand technology is a tool often used by businesses to streamline product flow in order to increase efficiency. When products are closer to the source, and when inventory declines at the same rate that it increases, then retailers no longer have to store aging inventory in warehouses. In comparison, the design of a smart building hot water system responds to demand for heating or cooling, without requiring excess storage of hot water to maintain comfort. Hot water on demand is one way to reduce energy costs using a space saving, on-demand technology without storing water, which needs to travel across the length of a building in order to deliver hot water for washing, cooking and heating.

In 2007, on the occasion of its 150th anniversary, the American Institute of Architects challenged its members to find ways to reduce the use of carbon fossil fuels in buildings. The AIA encourages good energy stewardship to preserve natural resources. They recommend that by the year 2030, architects reduce the consumption of fossil fuel by one third of an average energy budget for all buildings. Each component of a building contributes to its energy budget. Recent studies by the U. S. Department of Energy (DOE) revealed that "water heating currently represents up to 17 percent of national residential energy consumption."¹ The water heater by itself, is the second highest single energy user in the home, behind heating. It comes in third only if measured against all the homes appliances and lighting combined. The DOE is implementing new criteria for ENERGY STAR^® Water Heaters which they expect will save Americans "$780 million in utility costs, avoid 42 million tons of carbon dioxide emissions, and achieve cumulative energy savings of more than 3.9 billion kilowatt-hours and 270 million therms of natural gas."² As of January 1, 2009, five types of residential water heaters will be eligible for the Energy Star label, including gas tankless water heating systems.

 

These images show the operation of gas flow and water flow through a tankless hot water heater, installed inside (left) or outside (right). Photo courtesy of GE Appliances

 

 

This article will review the components of on-demand water heating through tankless water heaters and the criteria which make these systems a good choice for energy and space savings in residential buildings. Understanding the design requirements of tankless hot water heaters to meet the needs of even the largest residential homes will maximize efficiency and reduce maintenance costs. The influence of the temperature of local ground water, the number of bathrooms, and the importance of locating units close to the point of use, are important considerations when specifying tankless hot water heaters in both new and renovated projects.

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.

Ground Water Temperature and Higher Demand Temperatures

Architects who design sustainable buildings know that climate and environment can strongly influence design decisions. This is true for specifying tankless hot water systems. These units heat water from the local water source to a required hot water temperature and the difference between these temperatures influences the size of a unit specified by the design professional. The average ground water temperature varies by climate. Colder climates have colder ground water than a warmer climate. The U.S. Geological Survey publishes ground water temperature data and most manufacturers will provide online or phone support to assist the architect or builder who will need to know the ground water temperature of the building site, before choosing the right tankless hot water heater.

 

Actual usage conditions may require the next size unit or multiple units to meet demand. Colder ground water temperatures or higher set point temperatures may require higher gallons per minute (gpm) capacity or multiple units to properly meet household hot water requirements. Photo courtesy of GE Appliances

 

 

If there are multiple demand fixtures in a residence and if a higher set point temperature is desired, the design will also require more GPM capacity to properly meet household hot water requirements. The common residential temperature average set point is 120 °F. Tankless hot water heaters are the most efficient when there is no greater than 30 degrees differential required for heating water.

Manufacturers will assist the designer with determining the right tankless gas water heater for the project, climate, and design temperature as shown in the chart below.

Cascading Demand

The most common concern by users is whether the volume of water can meet the demand, and if the home user will have enough flow for simultaneous hot water needs such as showers, laundry or running the dishwasher. A typical showerhead uses about 2.5 gallons per minute of total water, hot and cold mixed. Depending on the time of year, the mix could be 50 percent hot and 50 percent cold, making the hot water need about 1.25-1.5 GPM for each shower. This means a home with three showers could need about 3.75-4.75 gallons per minute of hot water, if all showers are used simultaneously. Proper sizing of a tankless heater will match the GPM to the flow rate and the ground water temperature and account for even large temperature differences of as much as 77 degrees.

 

This whole house unit is mounted on an exterior wall and landscaping should be designed to shield the view of the unit, while providing adequate clearance for maintenance. Photo courtesy of GE Appliances

 

By sizing the tankless hot water heater by the number of shower heads and the number of bathrooms, a tankless hot water heater provides a continuous flow of water to as many fixtures, appliances and shower heads as required. Units are designed in tandem to cascade and fire additional heating burners as designed by the professional for each project type. When the unit senses a water flow of about ? gallon, burners in the first demand sequence fire to provide hot water, engaging only a portion of the 180,000 BTU's available for heating. A continuous supply of hot water is available to the homeowner as they turn on the dishwasher, start the laundry, or use additional showers, engaging additional burners to heat the additional demand for hot water. Units can also be clustered to fire even more heating burners as needed for larger homes or commercial installations.

Ventilation

Tankless gas water heaters come in two types. Outdoor models can be hung directly on the outside of a house and self vented with no additional ventilation required. Indoor models require venting and placement adjacent to an exterior wall. Venting must be 36 inches above grade and use the manufacturer's specified pipe. Many utilize direct vent, concentric pipe so only one vent hole is needed. Exhaust gases pass through the center of the pipe while the air needed for combustion is pulled in from the outer ring. This method does not require the use of "make up" air from the homes ventilation system.

Venting should be as direct as possible and for maximum efficiency, ventilation runs should be no longer than 21continuous feet. Each elbow reduces the length of the ventilation piping. For example, a 90 degree elbow equals six feet of ventilation run and a 45 degree elbow equals three feet of ventilation run. Ventilation piping can go as long as 41 feet but will longer runs will lessen the efficiency of the heater.

Designers should consider the following intake/exhaust guidelines for ventilation:

• Do not combine vent components from different manufacturers' since the tankless hot water heater is certified and listed with the vent system.
• The vent system must vent directly to the outside of the building and use outside air for combustion.
• Every vent connection must be accessible for inspection, cleaning and replacement.
• Support horizontal vent runs every four feet and support all vertical vent runs every six feet or in accordance to local codes.
• Do not reduce the vent diameter.
• Venting should be as direct as possible avoiding long runs and a minimum number of pipe fittings.
• Do not connect the venting system with an existing vent or chimney or any other vent pipe of other appliances or water heaters.
• Provide an inlet screen to minimize blockage from debris.

 

Installed in an unheated attic space, the professional will include an automatic emergency drain valve in case of power failures. Photo courtesy of GE Appliances

 

 

Because a concentric, direct vent system is used, condensation will occur. Some manufacturers include a condensate collar and drain attached to the top of the tankless heater where the vent pipe will be attached. A flexible tube with a loop will hang from the heater and drop into either a floor drain or stand pipe provided by the plumber.

Proper placement of the ventilation piping is a key to the location and installation of tankless hot water heaters. In new installations, the designer will locate the heater along an outside wall or near a cluster of appliances to reduce plumbing runs as well as the delivery time for the hot water. However, when used in renovations, the designer must consider the plumbing already in the house to determine where the unit should be placed. Should the designer locate the unit where the original hot water storage tank was installed? Will this reduce the amount of plumbing required to add the unit? How close can the unit be placed near an exterior wall in order to reduce ventilation piping? Is it better to provide multiple units for several areas of a home? Can long plumbing runs be avoided by separating baths on the second floor of a residence from those on the first floor and by providing separate units to those areas? Professionals should also take into consideration any local codes which apply to installations in different parts of a residence.

Efficient Delivery - System Comparisons

The basic components of a traditional hot water storage system are a burner, a thermostat and a storage tank that heats water to a constant temperature and which is attached to plumbing connected to the appliance or faucet. Cold water replaces hot water as needed and the entire tank is heated to an average temperature - between 120 °F to 140 °F. The higher temperature is recommended for optimal dishwasher use and laundry use and to prevent bacterial growth in the tank. For use in a shower this hot water is tempered by cold at the faucet in the ratio of one part hot water to four parts cold water. In a hot water storage tank, as hot water is drawn out by the user, at a typical flow rate of 2.5 gallons/minute, approximately 40 percent of the tank is filled with cold water which must be over-heated to maintain an average temperature in the hot water storage tank. The tank must be constantly heated to maintain an even temperature. Since the tank holds both cold and hot water, this temperature in the tank must be set to a higher rating than required at the faucet.

As a water storage tank ages, given that 85 percent of the United States has hard water, mineral deposits can reduce the area of the tank used for water storage. In fact, an older tank may not be able to supply the amount of hot water as originally designed as its capacity is reduced by deposits.

A natural gas tankless water heater system is very different in its construction and ability to deliver hot water. Only the water used is heated, and to the exact temperature requested - no water is stored and the heating burners are off when not in use. The components of a tankless hot water system are a powerful burner, a blower for exhaust and sophisticated electronics to ensure it delivers a continous flow of hot water.to existing or new plumbing. A tankless water heater system can provide water for the whole house with a single unit or the professional can cascade a couple of units together to prevent flow rates from diminshing. They can even be zoned placing two units in different locations to take advantage of shorter plumbing lines. Tankless hot water heaters can provide hot water simultaneously to many faucets, fixtures and appliances because multiple units can be cascaded to prevent flow rates from diminishing. Demand-activated technology turns the heating system on when a hot-water tap is opened and off when the tap is closed.

Tankless hot water heaters are not instant hot water, and the time the hot water takes to reach the faucet can be similar to the time it would take for hot water to travel from a traditional storage system. The location of the units can reduce the amount of cold water that is run through the plumbing lines before hot water is delivered to the source. Most fixtures drain cold water while waiting for hot water to be delivered to the source. To save water, tankless hot water heaters can be placed close to the demand for the hot water, near tubs, showers, laundry areas and kitchens because they can be located in small areas throughout the residence.

Location

Gas tankless hot water units can be designed to provide hot water to the whole house or as described previously, the designer can divide the fixtures and appliances which require hot water and locate the units in several locations throughout a house. The design professional should analyze the location of each faucet, fixture and appliance to determine which strategy would be the best for saving plumbing runs, saving water, and maximizing the efficiency of the tankless hot water heater system.

Indoor units can be placed in closets, under sinks and as well as in attics as long as there is access to an outside wall for ventilation. Units can also be placed outdoors. In a cold climate, manufacturers recommend that heat trace be placed on the exterior piping. Outdoor units or units placed in seldom-used vacation homes can be protected from freezing at temperatures as low as minus 30 °F. In extreme climates an automatic drain valve should be installed to eliminate freezing of the water supply in the event of a power failure. Specify testing of the drain valves as part of the installation sequence in cold climates.

Water Quality and Maintenance

Water quality is an important consideration that can be overlooked for both tank and tankless water heaters. Mineral deposits or scaling with all water heaters can lead to inefficient performance and shorten the life of the unit. As previously noted, about 85 percent of the United States has hard water, and designers can consult with the local water company for hardness ratings.

 

About 85 percent of the United States has hard water. This designer specified a water softener and water filter in conjunction with the tankless hot water heaters in this garage installation. Photo courtesy of GE Appliances

 

 

All manufacturers require the use of a water softener if the water is 11 grains per gallon or higher. Many manufacturers even make this a warranty requirement. According to the Water Research Council and Water Quality Association, soft water results in less scale build up in water heaters and up to 25 percent less energy consumption.

If a water softener is not installed, most units can also be "flushed" out with a vinegar wash which will extend the life of the system particularly in homes with hard water. This will usually require the help of a service technician since a pump is needed. The control components, compartments, burners and circulating air passageways should always be kept clean. The water inlet filter can be cleaned by first removing power from the unit and then brushing the filter with a small brush. If scale is visible, it can be cleaned in a white vinegar solution. Keeping the unit free from dust will maximize its life span and efficiency. All manufacturers will provide maintenance information to the homeowner.

Integration With Passive Solar

Tankless hot water heaters can be used in conjunction with solar hot water heating to increase the supply of hot water at higher temperatures. By supplying the units with water already pre-heated by the sun, fewer BTU's are required to raise the temperature for residential use. This combination allows for even greater efficiencies and energy cost savings. Some designers are also using tankless hot waters as part of a whole house radiant heating system tied in with solar hot water panels. In these installations, the designer connects the water supply to the solar panels which heats the water to the solar temperature and then stores that water in a supply tank. The tank is compartmentalized to create a closed loop system for radiant floor heating and there is a burner which maintains the heating source to a set temperature. In cold climates, the design professional can vary this temperature depending upon the demand for heating in the winter, lowering it in the summer.

The gas tankless hot water heater is then installed as a component above the closed loop system, to raise the temperature of the preheated hot water for use in the domestic hot water supply. This increases the efficiency of the entire system, since only enough energy is required to raise the temperature an additional 10 degrees to 30 degrees as needed by the homeowner.

Conclusions

Energy use in a home is divided into many components and according to the U.S. Department of Energy water heating is a large portion of the energy pie. The homeowner often asks the designer whether the initial cost of a unit will provide long term savings. With rising fuel costs, any appliance which reduces energy use will save money. The DOE provides guidelines as to how to assess the payback period of energy-efficient water heating systems. The design professional can also compare the cost of reduced plumbing and square footage savings to guide the homeowner to the best choice for their project.

There are many checklists for determining which green solutions are right for a project. Sustainable design-build consultant, Kathleen O'Brien, lists tankless hot water heaters as a top-ten green home tool pick in her book, the Northwest Green Home Primer. She recommends that homeowners reduce water consumption and save energy by installing these efficient cost saving devices. She points out that although the storage tank water heater has been the standard for many years in American homes, that there is an alternative which is now available to designers and to homeowners. "One danger with on-demand water heaters is that the supply of hot water is endless. If you have teens, you may not want to go this route!"⁶ Designers continue to find ways to make homes more energy efficient. Organizations such as the Energy Efficient Codes Coalition (EECC) have been formed to mount a comprehensive, integrated campaign to achieve a 30 percent increase in residential energy efficiency. Nationally, mayors of many cities are encouraging code officials to support broad-based changes in the model residential energy code. Energy savings result from both tightening the building envelope, but also from reducing the energy required to maintain comfort. ENERGY STAR^® appliances are considered one of the best ways to guarantee that a product will save energy dollars. The inclusion of gas tankless hot water heaters as a qualified ENERGY STAR-labeled product will recognize this product as a means to conserve water and energy.

The American Institute of Architects provides a list of 50 ways to "green" a building. This list was developed by numerous green designers to provide guidance on simple means to achieve energy savings and it is part of the extensive 2030 toolkit created by the AIA to show architects how to "Walk the Walk" of sustainable design. "The strategies were selected to provide a list of readily effective tools and techniques that will have an effective and immediate impact on the architects' ability to achieve significant carbon reduction.⁷ When combined with low flow water fixtures and shower head adapters, tankless hot water systems, control the use of potable water and are energy saving appliances, using existing technology to reduce the use of energy in homes.

Tankless hot water heaters save water, space and energy. They can be used in commercial installations with limited warranties as well as in the residential applications. They can be placed inside or outside. Tankless hot water heaters can be designed to provide hot water for the whole house and located in a central equipment area, but they can also reduce extensive plumbing runs by locating them near clusters of fixtures. Designers, seeking to maximize efficiency through design will welcome this technology as an effective means to achieve sustainable design goals as well as complete their energy saving, hot showers.

 

Celeste Novak, AIA, LEED AP, is a principal at RizzoloBrown + Novak in Ann Arbor, Michigan and author of articles focusing on sustainable building materials and products.

 

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