Volume 4, Number 2
Hot Water on Demand--And No Energy Penalty
Hot Water on Demand—And No Energy Penalty
Running water down the drain while waiting for hot water to get to the tap wastes over 10,000 gallons (38,000 l) each year in an average American household. The most common strategy for reducing this tremendous waste is a hot water recirculation system, which keeps hot water moving through the pipes, so it’s always right there when the faucet is opened. Unfortunately, circulating hot water through the pipes results in an enormous waste of energy because the water must be frequently reheated and because a pump is running constantly. Adding timed controls and temperature sensors to reduce the circulation time can cut down on these energy losses, but they remain substantial. Circulation on demand, however, eliminates the energy penalty, while only slightly reducing the convenience. That’s the principle behind the Metlund Hot Water Demand System from Advanced Conservation Technology (ACT).
The Metlund System consists of a controller box, a zone valve, a pump and a button the user pushes to activate the circulation. If the house is plumbed for recirculation with a hot water return line, that loop is utilized (see figure 2—P-Series).
In a conventionally plumbed house, a connecting loop is added between the hot and cold water lines at the furthest fixture from the water heater (figure 1—S-Series). Once activated, the pump provides hot water to all fixtures in a fraction of the time it would take to get there by running water down the drain. The room-temperature water from the pipes is either circulated back to the water heater (P-Series) or moved into the cold-water supply line. The only other technology we know of with similar advantages is the Divert-It from 21st Century Systems, which stores the room-temperature water in a tank for later use to flush toilets (reviewed in EBN ).
The potential water savings from recirculating systems is so dramatic that many municipalities, particularly in drought-stricken California, now require such systems in new homes. Depending on the assumptions chosen, a full-time recirculating system can use about 900 kWh (more than a full-size refrigerator) and waste 9 million Btu (9,500 megajoules) annually in heat loss from pipes. Temperature and time controls can reduce the electricity use to 59 kWh per year and heat energy wasted to 5 million Btus (5,300 megajoules), according to estimates provided by ACT. While such controls typically are installed, they may not always be working, says Randel Riedel, an engineer with the California Energy Commission. “One or two power failures can throw the timer off, and users will often override it rather than figuring out how to reset it,” Riedel said.
Riedel is convinced that the Metlund System is more foolproof and straightforward than other control systems and provides much greater energy savings. ACT estimates less than 5 kWh per year is used for pumping, and no heat energy losses are attributable to recirculation. After several years of testing and monitoring, ACT has refined the system and identified optimal control temperatures, though these can be adjusted slightly on-site if necessary. The system is designed to stop circulation once water at about 103°F (39°C) reaches the tap, and to prevent reactivation until the water at the zone valve has dropped below about 80°F (27°C).
One possible concern with the S-Series system is that water from the hot water line may be drawn as cold water for drinking or cooking. Hot water is more corrosive than cold water and has been sitting in the water heater, making it more likely to contain lead and other contaminants, so drinking from the hot water supply line is generally discouraged. Acker argues that hot water is almost always mixed with cold when it’s used (especially if it’s really hot), so the circulated water will be drawn off and mixed with the hot anyway. Officials in Massachusetts apparently are not yet convinced, so using the system there may be considered a plumbing code violation.
The Metlund System works for any number of fixtures connected in series to the water heater, with an activator button at each fixture and a controller at the most remote. Any separate lines branching off from the water heater will require their own system. Components for each system cost about $300 to the trades. For installations where running the 24-volt wires from the switches to the controller may be a problem, remote-control buttons are available ($75 for the first one, $25 each thereafter). The payback on these systems is very short when compared with standard recirculating systems. Acker claims that some marginal energy savings are possible even over non-recirculating systems. Their real advantages over non-recirculating systems, however, are water savings (with the resulting savings in sewage treatment costs) and convenience.
For more information:
Advanced Conservation Technology, Inc.
1400 Bristol Street North, Suite 145
Newport Beach, CA 92660