ECOBROWNSTONE: The Art of Brownstone Greenovation -
Water Heating - Understanding Water-Saving and Energy-Saving Measures
Noreen Adler
Founder and President, Ecobrownstone

Noreen is Founder and President of Ecobrownstone.  She has been a resident of brownstone Brooklyn (Park Slope, Brooklyn Heights and Carroll Gardens) for over two decades and has planned, designed and managed a wide range of renovation and real estate development projects in Brooklyn and elsewhere. As a developer she is a member of the NYC Committee evaluating the LEED for Homes Guidelines for application in New York.  She also has a personal passion for sculptural relief ceramic tiles and murals which she has designed and fabricated at her studio on the Gowanus Canal. 

By Noreen Adler
Published on 04/24/2008
Comprehensive overview of hot water heating energy efficiency and conservation measures, from working with your existing heater to designing and installing an entirely new plumbing and hot water system, including an analysis of solar and in-line, tankless hot water systems.

One of the largest energy consuming devices in the home is the hot water heater. On average in the United States, heating water accounts for 15 to 19 percent of energy costs, second only to space heating and cooling. In addition, in a typical brownstone or urban row house, hot water must travel up several stories from the basement to reach fixtures on the upper floors and, without a well designed hot water system, thousand of gallons of potable water each year swirl down the drain as the homeowner runs the tap waiting for the hot water to reach the fixture.  See our Article Water Conservation -- A Holistic Approach that Saves Water and Energy.  For example, at 168 Clinton St., our case study green brownstone renovation (see Noreen's 168 Clinton St. blog) we measured that in order for the hot water being heated in the basement to reach the shower on the top/5th floor, we ran through 4 gallons of water (when the outdoor temperature was 45 degrees) before the hot water reached the tap. If this occurs twice a day, once in the morning for adults’ showers and once at night for children’s baths, then 8 gallons daily, 56 gallons weekly, and 2,912 gallons annually of potable water are swirling down the drain -- that’s enough drinking water to serve the needs of a family of four, each of whom drinks 8 glasses of water a day, for 4 years.

This Article will instruct you how to increase the efficiency of your existing hot water heater, how to evaluate and choose a replacement hot water heater, and if you are renovating, how to design a better hot water delivery system.

Making Your Current Hot Water Heater More Efficient
Standard Heater With Storage Tank
Tankless, aka In-Line, aka Demand Systems
Solar Hot Water Heating Systems
Designing or Retro-Fitting a High-Efficiency Hot Water Delivery System
Drain Water Heat Recovery
Hot Water Recirculation System
Central Manifold Distribution System
Compact Design Conventional System
Integrated Water Heating and Home Heating

Making Your Current Hot Water Heater More Efficient
The Rocky Mountain Institute (RMI) and the Department of Energy recommend the following procedures for making your current water heater more efficient:

1. Add insulation to your current tank. An R-value of 24 or higher is desirable, and most tanks come with less insulation. Wrap your tank with an insulation jacket rated above R-10 but do not cover the thermostat or drain, and on a gas heater do not obstruct the flue or airflow to the gas burner. The Rocky Mountain Institute estimates that the jacket should reduce heat loss through the walls of the tank by 25–45 percent, saving about 4–9 percent of your water heating costs. Jackets are inexpensive and can pay for themselves through energy cost savings within a few months.

2. Insulate hot water pipes wherever you can reach them, and in particular within three feet of the heater, but on gas heaters do not place insulation within 6 inches of the flue. Use the split foam insulation (looks like a hollow tube or hose made out of dense foam) found in hardware and building supply stores at a size that will completely encircle your pipe with no gaps, and tape the seam closed with acrylic tape. The more hot water pipes you can insulate, the less heat will be lost when the water is not running so you will wait less time for hot water to reach the faucet, and therefore waste less water as well as save energy.

3. Turn off the heater when you will be away for long periods of time. An electric heater is easily switched off, but a gas unit may require special procedures for shutting off and relighting the pilot so check the instructions for your unit.

4. If you have an electric hot water heater, installing a rigid board of insulation under the heater keeps heat from escaping into the floor, which the RMI estimates can save 4-9 percent of water heating energy. Gas heaters have drains and pilots on the underside which cannot be blocked by insulation.

5. If you have an electric hot water heater, install a timer on your heater to turn it off during certain times, like during the night when you are sleeping and during peak load times, saving and estimated 5 to 12 percent of water heating energy.

6. Retrofit anti-convection valves and loops to prevent heat loss through the inlet and outlet pipes. These cost under $30 and can pay for themselves within a year.

7. Reduce the temperature of your hot water. Follow the instructions for your particular unit regarding how to reach and reset the thermostat which may be located behind a control panel, necessitating for safety reasons that power to the unit be shut off at the breaker before starting. 120 to 130 degrees is sufficient to provide enough hot water for comfortable showers without the risk of scalding. For each 10-degree reduction, you can save up to 5 percent on your water heating costs. Lower temperatures will also increase the life of your heater.

Standard Heater with Storage Tank - Fuel Choice and Sizing
Most homes have heaters with storage tanks holding 40-80 gallons of hot water and the size should be determined based on providing enough hot water for the busiest part of the day.

Gas systems are more efficient than electric.  The efficiency of a water heater is noted by its energy factor (EF), a rating provided by the manufacturer. Since 2004 federal regulations require that electric heaters have a minimum EF of 0.90-0.93, and gas heaters have a EF of 0.56 to 0.61 but you should specify an EF of 63 or better (up to 67).  High efficiency water heaters are not as readily available as moderate and low efficiency units, and may cost a bit more, but they can pay for themselves through energy cost savings in about 3 years and usually last up to 20 years, substantially longer than the customary 10-year life span of a typical hot water heater. Smaller tanks are generally more efficient because they have less surface area exposed to the air through which heat can escape. 

You may wonder why gas heaters are considered more efficient if they have a lower EF than electric heaters.  The reason is that electricity is a very expensive way to produce heat and the heat produced is only as clean and efficient as the fuel that is burned to produce the electricity, usually coal; it may be possible, however, to purchase electricity generated by renewable sources from your utility, worth looking into if gas water heaters is not an option for you.  Check that your gas heater has en electric pilot.

To ensure that your household air does not get contaminated by the combustion of fuel-burning water heaters, and to maximize efficiency, install them with a form of sealed combustion ("direct-vented" or "power-vented), which means that outside air, not room air, is brought directly to the water heater, and exhaust gases are vented directly to the outside.

Check the Gama Association of Appliance & Equipment Manufacturers website to find water heaters by EF and tank size. 

Tankless, aka In-line, aka Demand, Flash Systems
Tankless hot water systems – also known as “in-line”, “demand” and “flash” hot water heaters -- do not store hot water; rather, they heat the water intensely as it passes through a heating coil whenever the hot water tap attached to the heater is turned on.

Tankless hot water heaters are rated by the maximum temperature rise they deliver for a given flow rate, so the key to getting the amount of hot water that satisfies you is to very carefully calculate your flow rate. The desired flow rate is calculated by adding up the flow rates – gallons per minute—of every fixture that you want to be attached to a particular heater. Next, determine the temperature rise you need by subtracting the temperature of the water that would flow into the tankless heater (use 50 degrees unless you know the temperature to be something different) from the desired hot water output temperature -- 120 degrees should be adequate for sinks and showers, 140 degree for dishwashers that do not internally heat the water. The typical temperature rise capability of heaters currently on the market is about 70 degrees, but gas units will render this temperature for higher flow rates (5 gallons per minute) than electric units (2 gallons per minute). Some units have thermostats and can vary the temperature rise according to the temperature of the incoming water and the flow rate.

Tankless heaters also have minimum flow rates, important because the heater only produces heat/hot water when water is flowing through it. If the fixture to which the heater is attached is only used sporadically and thus does not meet minimum flow requirements for the unit, the unit won’t have time to operate long enough to heat the water, and the user’s experience will be that the heater does not work or does not deliver water when needed. Minimum flow rates can range from ½ gallon to 25 gallons per minute.

Because water must be running through the tank in order to produce hot water, it is inevitable that some cold water must run through the heater to activate the thermostats to start heating, thus resulting in cold water flowing through the tap for some period of time and attendant water waste, and the user’s experience being that there is a delay in getting hot water. This glitch, however, can be addressed in a few ways, such as preheating the water through a solar hot water system so that the water flowing through the tankless heater is always somewhat heated, or by keeping a small (5-10 gallon) storage tank of hot water (obtained from the tankless heater) to dilute the cold water coming out of the heater before it gets to the faucet during the period between when the faucet is turned on and when the heater produces hot water, or installing a drain water heat recovery system ,or a hot water recirculation system.

Typically gas units have greater capacity and, as of the time of this writing, are cheaper to run – that, of course, is a function of your relative electric and gas utility rates – but they must be vented to the outside which could pose an issue for retrofitting into bathrooms located in the center of a brownstone as it would require long runs of ducting and the opening of walls. They also require a large supply of fresh air, so the heater may not be able to be installed in a tight space posing perhaps another barrier in sometimes tight brownstone bathrooms. Electric units, however, require large electrical circuits because it takes a lot of electrical current to heat water quickly, and it can be expensive to run large-capacity electrical cables long distances; installing smaller units at each fixture or on a per-room basis may alleviate the drain on the capacity of your household electrical supply and may result in less water waste, because the unit is closer to its use point and will not lose as much heat during the journey, and less water sits in pipes that will eventually cool.

Check the ACEEE (American Council for an Energy-Efficient Economy) website for a list of tankless water heater manufacturers.

Solar Hot Water Heating Systems (Solar Thermal)
According to the National Renewable Energy Laboratory, replacing an electric water heater with a solar heater can offset the equivalent of 40% to 100% of the carbon dioxide emissions of a modern passenger car. The DOE estimates that solar hot water can reduce hot water heating bills by 50%-80%.

A prerequisite for installing a system for a brownstone is to have an unshaded roof from which you can orient south-facing solar panels – note I did not say a “south-facing roof” because, as brownstone roofs are often relatively flat or only slightly sloped, it’s not really relevant whether the roof slope towards the south as long as the roof has a large enough unshaded area to position panels in a southerly orientation. Consult the DOE Consumer Guide to Siting Your Solar Water Heating System's Collector for guidelines on orientation and tilt.

The volume of hot water that a solar thermal system produces depends on the size of the unit. A general rule of thumb is that 20 SF of solar collector area (say panels stretching over a 3-foot by 6-to-7 foot area) will provide enough hot water for 2 people, and in the northeast for each additional person you would need an additional 12 to 14 SF of collector area. Given the limited size of brownstone roofs it may not be possible to have a system large enough to supply all your hot water needs, which vary seasonally, but it can certainly make a dent and lighten the load on a conventional or tankless back-up heater.

Solar thermal basic components:
Basically the systems consist of solar heat collection plates and a storage tank. The collection plates come in two types: flat plate panels and evacuated tube solar collectors. Flat plate panels are weatherproofed boxes holding a dark heat absorber plate which can typically heat water to up to 160 degrees. Evacuated tube solar collectors contain rows of solar heat absorber tubes housed within outer tubes and the air is removed (“evacuated”) from the space between the tubes which eliminates conductive and convective heat loss. These systems can heat water from 170 – 350 degrees but are more expensive than flat plate panels and are typically used in very cold climates or commercial applications, although they are cropping up more and more in residential projects so discuss this with your solar congtractor.

Either kind of heating system can be “active” which relies on pumps to move the water through the system, or “passive” which relies on gravity and physics, in particular the natural tendency of hot water to rise, to move the water through the system.

Active, or Forced circulation, solar systems
This type of system uses a pump to circulate water or other fluid from the solar collector point where it is heated, to a storage tank. There are two types of active systems: Direct circulation systems pump the actual household water through the heat collectors then into the home plumbing system for use, but can only be used in mild climates where there is no risk of the water pipes freezing; thus this system would not work in NYC.  Indirect circulation systems circulate either a non-freezing, heat-transfer fluid, or air, through the collectors and a heat exchanger. This heats the water that then flows into the home. The indirect circulation system would work in the northeast climate zone.

A liquid-to-liquid active system passes the heat transfer fluid through the solar collector, absorbing heat in the process and then passing the heated liquid through a heat exchanger to transfer this heat to the domestic hot water. The hot water and the heat transfer fluid never mix because the heat transfer liquid may be toxic; the two are separated by single- or double-wall piping. Special care should be taken when choosing the heat transfer fluid depending on your climate zone, but for NYC and anywhere where the temperature dips below 42ºF the one viable choice is to use an anti-freeze liquid, otherwise you will need to drain the pipes whenever there is a risk of the temperature dropping below 42ºF, a high-maintenance non-viable approach as it decommissions your hot water heater during the winter season. Certain anti-freezes, such as ethylene glycol, require double walled piping because they are more toxic than other alternatives, such as a food-grade propylene glycol/water mixture that is certified as non-toxic and therefore can be used with single-walled piping. Anti-freeze fluids need to be checked annually for pH value, freezing point and adjustments made to maintain their stability. Silicones may be another option because they have a very low freezing point and a high boiling point but they are viscous and require more energy to pump and they leak easily even through microscopic holes.

U.S. Department of Energy

Passive Solar Systems
Passive solar systems do not use pumps or any mechanical means to circulate water or heat transfer fluid. There are three basic types of passive systems: integral collector-storage or “batch” system, thermosiphon system, and self-pumping system.

An integral collector-storage (ICS) system or “batch” system combines the collector and storage tank into one unit by, essentially, placing a storage tank inside an insulated box with its dark, heat-absorbing side facing the sun. These systems are also used to pre-heat water that is then passed through a conventional water heater which consequently needs to use less energy to further heat the already-warmed water; however, the tank must be drained in cold climates so a batch system is not viable for NYC.

A thermosiphon system has a separate storage tank located above the solar collector. As water heats up in the collector tank, thermodynamics will cause this heated water to rise into the storage tank. You likely realize that this means all the water storage is on the roof, so careful attention must be paid to structural support for heavy water tanks.

U.S. Department of Energy.

A self-pumping system uses a “phase change”, such as the change of a liquid to vapor or other passive means, to cause the water or fluid in the collector to circulate and transport heat from the collector to the storage unit.

The need for a back-up system.
Keep in mind that the sun isn’t always shining, so it is necessary to design any solar hot water system with a back-up hot water heater, either a conventional storage tank or an in-line/tankless heater(s).

Understanding Solar Water Heater Energy Efficiency
For a solar water heating system, the solar energy factor (SEF) and solar fraction (SF) are used to determine a system’s thermal performance. The SEF is analogous to the Energy Factor (EF) rating given to conventional hot water heaters. The Solar Rating and Certification Corporation (SRCC), a non–profit organization that tests and rates solar heating systems, includes on its website a Directory of SRCC Certified Solar Water Heating Systems Ratings February 17, 2008.   In the SEF calculation, the energy delivered by the system is divided by the electrical or gas energy put into the system; consequently, the type of back-up hot water heating system that is being used will have a large impact on the performance of the system as a whole.

The SF calculation involves the comparison of the SEF and EF; however, in the marketplace the term is used to refer to two different calculations: one calculation/definition yields a result that describes the portion of the total conventional hot water heating load (delivered energy and tank standby losses) provided by solar energy (this is the definition and calculation used by the SRCC); the alternate definition/calculation yields a result that states the portion of the total water heating load (losses are NOT included) provided by solar energy, and this alternate method of calculating solar fraction will yield higher solar fractions. Consequently, ensure when comparing system results that the same calculation was used for each unit or you will be comparing apples to oranges. The higher the solar fraction, the greater the solar contribution to water heating, which reduces the energy required by the backup water heater. The solar fraction varies from 0 to 1.0. Typical solar factors are 0.5–0.75.

Now the big question: is solar hot water heating worth it economically? Consult the Department of Energy’s Consumer’s Guide Estimating a Solar Water Heater System's Cost for an excellent analysis and explanation of how to determine the annual operating costs of a solar thermal system for your home, and how to calculate the pay-back period.  The SRCCs ratings document in the link above also includes information on how to compare the costs of systems along with different back-up systems.  Also check back to our 168 Clinton St. blog where we will apply the analysis for brownstone use. 

Federal, state, local and utility incentive programs exist to help homeowners afford the costs of increasing hot water energy efficiency. For a complete discussion of incentive programs see our Article on Incentive Programs for Green Renovation

Designing or Retro-Fitting a High-Efficiency Hot Water Delivery System
If you are embarking on a major house renovation with at least partial opening of walls and floors then it may be worth your while to take a hard look at the entire hot water delivery system and plumbing layout in your brownstone to determine if you can introduce a new high efficiency approach. 

The LEED® for Homes Guidelines include LEED® credits for instituting various kinds of water saving and energy saving measures, including such things as high efficiency hot water heaters, hot water pipe insulation, hot water recovery systems and efficient distribution systems that meets certain prescribed standards. The points vary depending on the kind of plumbing system installed (conventional, central manifold or hot water recirculation systems, see below). Separate LEED® credits can be earned for installing solar hot water heating systems.

The following options are currently available on the market:

Drain-Water Heat Recovery
Hot Water Recirculation System
Central Manifold Distribution System
Integrated Water Heating and Home Heating

Drain Water Heat Recovery
Drain water heat recovery is very simply a conductive coil (such as copper) wrapped around a pipe to remove the heat from drain water for use in pre-heating water destined for the hot water heater, or a hot water storage tank, or hot water fixture such as a shower. The DOE estimates that 80–90% of the energy used to heat water in a home goes down the drain, and a drain water heat recovery system can reduce energy needed to heat water.

There are two basic types of systems, either with or without storage tanks. In a system with no storage tank, essentially the cold water input pipe is wrapped around the drain and as that coil extracts heat from the drain pipe it pre-heats the cold water inside which is then fed to an in-line (tankless) heater or a running fixture – this works well for showers as the water you use that is going down the drain immediately has the heat extracted from it to pre-heat water going either to the tankless heater that services that shower or to further heat the hot water coming from a conventional. In a system with a storage tank, the heated drain water flows through a heat exchanger coil at the bottom of a cold-water storage tank and heats that water; the heated water rises to the top of the tank and the “cold” water input for a hot water heater draws this pre-heated water from the top of the storage tank. Pre-heating water helps increase water heating capacity and may enable you to get a smaller unit.

U.S. Department of Energy

The DOE estimates that DWHR systems have paybacks ranging from 2.5 to 7 years, depending on how often the system is used.

Hot Water Recirculation System

These systems apply the simple principle that if the water in the hot water feed line is kept hot, it will reduce the amount of hot water that must travel to get to the fixture. 

The standard plumbing configuration involves one long hot water feed line coming off the hot water heater and branching to fixtures on each floor. This is the least efficient configuration for several reasons: it causes long waits for the hot water to reach fixtures further down the line, resulting in excess wasted water flowing into the sewer system, and energy is wasted by heating water that will remain, and cool, in the feed lines after the user is finished; heating all this wasted water results in higher hot water heating costs. Heating excess water also taxes the heating unit. It is possible to retrofit onto this system a hot water recirculation system.

The basic function of a hot water recirculation system is twofold: to draw cooled (but still warm) water from the hot water feed lines leading to fixtures and to send it back to the hot water heater for re-heating, and to pump hot water to the fixtures faster so that the user does not experience a long wait for hot water and wastes less water. Essentially the system circulates the cooling (but still warm) water in the hot water feed lines back to the hot water heater (not down the drain) to be re-heated, and draws into those feed lines new hot water from the water heater so that the hot water feed lines to the fixtures always contain hot water.  There is no wait on the using end. The result is that hot water does not sit in the feed lines getting cold and consequently resulting in energy waste (the energy that was used to heat it in the first place), and does not result in water waste (as the user would otherwise allow all the cold water that flows out when the hot tap is first turned on, to flow down the drain while waiting for hot water to reach the fixture).

There are passive and active/pump systems. Passive systems operate by placing a pump on the hot water heater outlet pipe, or at each fixture, to increase the outlet water pressure. This increased pressure allows for the installation of a bypass valve at the point of use (sink, shower). The bypass valve allows small amounts of hot water to periodically flow into the cold water system and be replaced by new hot water from the hot water heater thereby maintaining a constant hot temperature level so that the user does not waste water waiting for it to get hot. The systems that place a pump near the hot water heater require an outlet near the hot water heater and can be retrofitted after the walls are closed. One inconvenience is that the user, from time to time, may experience the cold water running a little warm when the tap is first turned on. Statistics show that this system can save over 15,000 gallons of water per year.

Active/pump loop systems use a separate hot water line that sends hot water throughout the house and then loops back to the hot water heater. They can operate automatically through the use of thermostats that monitor the temperature of the water in the hot water feed line and automatically circulate the cooling water back to the heater while circulating new hot water from the heater, or operate on a timer or motion sensor that turns on whenever someone enters the space where hot water may be used. Note, however, that the return loop must be insulated or else the hot water will cool and not result in energy savings.  This system can also work on a timer, or a manual switch. These systems are more expensive to buy and install than passive systems. Be careful how you set up the trigger for turning on the pump because if it runs all the time then the energy costs in running the pump can outweigh the savings in hot water heating. Timers will need to be reset if you experience a power outage. Only one pump is needed for a household. 

Structured Plumbing™  Under the auspices of the “The Department of Energy’s Inventions and Innovation Program”, ACT, Inc. developed the Metlund® Hot Water D’MAND® System  to address both water savings and energy savings in the distribution of hot water. This system is designed to circulate the dormant water in the hot water feed lines back to the hot water heater so that it does not have a chance to cool and get lost down the drain. The company represents that it gets hot water to the fixture four to five times faster than a conventional system. The system can be retrofitted into existing hot water systems without the need to open walls to run new plumbing lines because the pump can be located either at the location of the fixture furthest from the hot water heater or at the hot water heater, and can use the cold water feed lines as the return line rather than using a dedicated return pipe. The pumps are operated by a door-bell like button at one or all the locations where hot water is used; the button is either hard-wired to an electronic controller (feasible only if you are opening your walls and can run the wiring) or wirelessly (up to 100 feet), or via automatic activation devices through door or motion sensors. The system operates on demand, 24 hours a day. The system can work with both storage tank and tankless systems.

Central Manifold Distribution System
The Central Manifold Distribution System is a hot water system in which hot water enters a kind of small holding tank near the hot water heater from which many small lines go out directly to each hot water fixture in the home. This type of system eliminates some water waste because the run from the hot water storage tank to an individual fixture is typically smaller than in a conventional system, but like a conventional system can result in water and energy waste if measures to recapture or recirculate the heat from the hot water feed lines are not put into place. Given the layout of brownstones, with utilities typically located in the basement, this system may not be practical.

Compact Design Conventional System

In a compact design conventional system the hot water line runs as a main line (no holding tank, unlike a central manifold distribution system) from the hot water heater then branches to the fixtures.  Typically the branches are short and the main hot water line is long; however, this system, if not designed well it can lead to long waits for hot water the further the fixture is from the hot water heater, and a lot of wasted cold water flows down the drain as the user waits for the hot water -- a common problem in brownstones where water has to travel vertically several floors from a single heater in the basement.

Integratad Home and Water Heating

The latest energy saving furnace and boiler technologies can also combine high efficiency house heating with hot water heating. If you use oil to heat your home, or if you use a gas-fired high-efficiency condensation boiler, you can also consider an indirect tank water heater which uses the boiler to heat your domestic water by circulating the hot water from the boiler through a heat exchanger in an insulated water storage tank. These heaters eliminate the energy lost up the flue from venting a gas-fired hot water heater. There is general agreement in the industry that energy savings are yielded even in the summer, particularly with a high-efficiency condensation boiler which effectively makes the hot water heater a zone that allows only that zoned area of the boiler to fire up. See our Article Heating Systems Explored

It is also possible to integrate water heating with hot air house heating through systems referred to as hydro air.   These systems essentially take the heated water from the water heater and pass it through a heat exchanger coil in an air handler through which air is blown and heated, and this heated air is then channeled into the duct system.   This system can also double as a cooling system by passing the air through a loop of chilled water in the air handler. 

The efficiency of a combination water and space heating system is indicated by it combined appliance efficiency rating (CAE). The higher the number, the more energy efficient. The CAE ratings vary from 0.59 to 0.90 and specify CAE 0.85 or higher.

Useful Links
Department of Energy Distributed Energy Program Solar Hot Water

Energy Star guide to High Efficiency Water Heaters

Department of Energy Guide to Solar Hot Water 

National Renewable Energy Laboratory Consumer Guide Heat Your Water with the Sun
Solar Rating and Certification Corporation 

Department of Energy Consumer's Guide Estimating a Solar Water Heater System's Cost
DOE Consumer Guide Drain Water Heat Recovery

Gama Association of Appliance & Equipment Manufacturers website to find water heaters by EF and tank size.