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Hey, Where's the Hot Water?

In many ways, new homes may be experiencing the best of times. But for their hot water distribution systems it can be the worst of times.

September 01, 2004
September/October 2004
This article originally appeared in the September/October 2004 issue of Home Energy Magazine.
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        It is 6 am on a workday. Picture yourself standing before the master bath lavatory in your brand-new home with the faucet on full getting ready to shave. Minutes pass as you await the arrival of hot water to help you awake and ease the shave.This scene is being played out throughout the nation in new homes, even energyefficient ones, as a combination of factors converge to give the homeowner longer waits for hot water and higher energy,water, and sewer bills (see “The Worst of Times for New Hot Water Distribution System Efficiency,” p. 38).
        The Oak Ridge National Laboratory (ORNL) studied hot water distribution systems for the California Energy Commission using a newly developed model to simulate the performance impact of a number of hot water distribution system parameters (see “Are You Getting into Hot Water?” HE Sept/Oct ’03, p. 33).This article looks at the results from three distinctly different residences and evaluates changes to conventional trunk and branch distribution systems —a large-diameter trunk from the water heater serving small-diameter branches to individual fixtures. Our evaluation included
        • comparing alternative piping materials used in conventional trunk and branch systems;
        • relocating the water heater to a more central location (for single-family homes); and
        • adding insulation to the various piping materials in standard system configurations.
        In addition,we evaluated three alternative hot water distribution systems. These were
        • a demand-actuated recirculating pump and controls in a conventional trunk and branch system using the cold water line for the return;
        • a continuous recirculation system with a dedicated return line for larger residences; and
        • a parallel-pipe system with a manifold located near the water heater and 1/2-inch piping from the manifold to each individual fixture.
        The demand-actuated recirculation system typically interconnects the hot and cold water lines at the point furthest from the water heater (frequently the master bath) with a relatively high-velocity pump.When hot water is desired, the occupant actuates a pump that transfers the ambient-temperature water in the hot line to the cold line, thus drawing hot water from the water heater.
        A continuous recirculation system uses a small low-velocity pump to circulate hot water continuously through the supply and return loop.Therefore, hot water is available throughout the house at all times.The only portions of the piping that cool to ambient temperature between draws are the branches off the loop connected to individual fixtures.
        Finally, a parallel-pipe system minimizes the trunk between the water heater and the manifold, maximizing the small-diameter branches from the manifold to the fixture. Many systems use 3/8-inch crosslinked polyethylene (PEX) tubing for the branches.However, the ORNL study used the code-permitted minimum 1/2-inch tubing.

Simulation Results

        We simulated hot water distribution systems for three residences. A cold-start use pattern assumes that hot water is drawn periodically from the hot water system, so that the water in the piping has cooled completely to ambient temperature between uses.A more efficient clustered-use pattern assumes that the draws on hot water are clustered in the early morning and late evening hours, so that hot water in the pipes does not cool completely between uses.We considered only the less wasteful clustereduse pattern in our simulations.The energy use figures would be much higher if the more wasteful cold-start use pattern were simulated.
        The waiting time for hot water (105ºF) to arrive at the faucet or shower is a primary factor in the evaluation of system performance.According to Larry Ackers of ACT, Incorporated, Metlund Systems in Costa Mesa, California, a producer of hot water distribution systems, after 30 seconds of waiting for hot water, most homeowners note the delay and actively consider alternatives to reduce waiting time—or the impact of waiting time on their lifestyle.We considered reasonable waits to be less than 30 seconds.
        We did not indicate the cost of the water wasted in the results of our simulations for all of the systems and parameters, since the cost of water is rather small compared to the cost of heating the water. In a 2,010 ft2 house, and also in a 580 ft2 apartment, our simulations show that the cost of wasted water ranges from $1 to $6 per year, depending on the distribution
system employed. For the 3,080 ft2 house, the cost of wasted water ranges from $2 to $15 per year.

2,010 ft2 Single-Family House
        This unit represents a typical singlestory house. It contains a laundry room, one bath with a combined tub and shower and two lavatories, and another full bath with a tub/shower and one lavatory (see Figure 1 and Table 1, p. 37).The kitchen includes a sink and dishwasher. The water heater is in the garage.The layout spreads hot-water-consuming devices throughout the house.
        Based on our simulations, the best systems for this house, based on waiting period for hot water at the fixtures and total costs, are, in order of preference,
        • a conventional trunk and branch system, located in the attic, made of chlorinated polyvinyl chloride (CPVC), with a centrally located water heater;
        • a conventional trunk and branch system, located in the attic, made of CPVC;
        • a demand recirculation system, located in the attic, made of CPVC; and
        • a parallel-pipe system, located in the attic, made of PEX.


3,080 ft2 Single-Family House

        This unit represents a typical large single-story house. It contains a laundry room, one bath with a separate tub and a shower stall along with two lavatories, a half bath (lavatory only), and another full bath with a tub/shower and two lavatories (see Figure 2, p. 38 and Table 2, p. 39).The large kitchen includes a sink and dishwasher. The water heater is in the garage adjacent to the master bath.The layout spreads the hot-water-consuming devices to the four corners of the house.The best systems for this house, based on waiting period for hot water and total costs are, in order of preference,
        • a parallel-pipe system, located in the attic, made of PEX;
        • a demand recirculation system, located in the attic, made of CPVC;
        • a conventional trunk and branch system, located in the attic, made of CPVC, with a centrally located water heater; and
        • a conventional trunk and branch system, located in the attic, made of CPVC.

580 ft2 Apartment
        This unit represents typical small apartments and housing for the elderly (see Figure 3 and Table 3). It contains a single bath with a shower stall (no tub).The small kitchen includes a sink and dishwasher.The water heater is located in a closet off the balcony/patio, and there is no provision for a clothes washer.While the layout is compact, the external location of the water heater significantly increases the overall length of the system.
        The best systems for this apartment, in order of preference, are
        • a conventional trunk and branch system, located in the attic, made of CPVC; and
        • a parallel-pipe system, located in the attic, made of PEX.

Some Recommendations


        Continuous recirculation systems cost substantially more to install and operate, and waste more energy than any other system.Although they minimize water waste and wait times for hot water, continuous recirculation systems should not be installed for these reasons.
        Adding a demand recirculation pump and controls increases the cost of a conventional system by about $600 but reduces operating cost,water and energy waste, and wait times.Wait times can be similar to those for continuous recirculation systems, while water and energy waste is significantly less. Demand recirculation systems can be installed both in new construction and in retrofits.
        For the segment of the new construction market that is sensitive to first cost (many production homes), centrally locating the water heater cuts wait times and waste for a modest additional cost.
        Parallel-pipe distribution systems may be an attractive alternative for some house designs, distribution system layouts, and usage patterns. These systems are less costly to install than conventional systems and can reduce wait times to acceptable levels. However, the energy and water savings of parallel-pipe systems are sensitive to hot water use patterns.When the clustered- use pattern is modeled, parallelpipe systems use about the same amount of water and energy as conventional systems, and offer no advantage with regard to waste.When the cold-start use pattern is modeled, parallel-pipe systems perform better than conventional systems.
        Adding insulation to piping has a significant impact on the energy waste of underslab piping systems, but much less impact for systems in other locations, when modeled using the clustered-use pattern.The payback of installing pipe insulation in underslab locations ranged from 4.2 years for the 2,010 ft2 house to over 40 years for the apartment. A cold-start water use pattern, by definition, reduces the benefit of insulation.
        Making these changes in hot water distribution systems can have a dramatic impact on energy and water use.Take California as an example (see “Projected Impact of Improved Hot Water Distribution on California”). The potential annual savings in energy use realized by moving to more efficient hot water distribution systems are equivalent to the annual energy use of several thousand new homes.

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