Domestic Hot Water Efficiency: Page 2 of 2

In new construction, different pipe diameters can be used for different fixtures and appliances (see “Home-Run Plumbing Systems”), but in existing houses, there is usually no easy way to match pipe diameter to water flow requirements.

To eliminate the delay in hot water at a source, recirculation (“recirc”) systems are sometimes installed. Hot-water piping is installed in a loop, and a small pump circulates hot water through these pipes so that hot water will be available almost instantly—even at fixtures farthest from the water heater. Even with pipe insulation, recirc systems use a lot of energy, both electricity to operate the pumps and heat lost from the pumps. Continuous-circulation systems should not be installed.

Instead, on-demand, button-controlled systems (see Access) that deliver hot water only when needed can be used. A small pump delivers hot water to the tap and cycles the cooled-off water standing in the hot water pipes back to the water heater. The homeowner gets hot water in the bathroom or kitchen very quickly via the small pump, and water is no longer wasted waiting for hot water to reach the user. This practice saves water, and can save energy depending on usage patterns (if homeowners forget that they’ve turned on the shower to wait for the hot water, for example).

Home-Run Plumbing Systems & PEX Tubing

Small-diameter, cross-linked polyethylene (PEX) tubing has gained popularity for water supply plumbing (cold and hot) in the past two decades and can provide significant water and energy savings. PEX tubing is most commonly used in “home-run” plumbing configurations, in which continuous lengths of tubing extend from a central manifold by the water heater to distributed fixtures and appliances—instead of using branching copper pipe systems with soldered joints. By having each PEX line run to its own fixture or appliance, fittings and elbows are eliminated, thus reducing friction losses, and the tubing diameter is matched to the fixture or appliance. For example, 3/8-inch-diameter tubing may be used to bring hot water to a bathroom faucet, while 3/4-inch-diameter is used for a bathtub.

By using smaller-diameter PEX tubing instead of copper, hot water is delivered more quickly, less water is wasted waiting for the hot water to reach the tap, and less energy is wasted as the hot water in the tubing cools. With a long history of use in radiant-floor heating systems, PEX tubing has proven itself to hold up very well.

Capturing Wastewater Heat

A huge amount of energy is wasted in hot water going down the drain. Even if hot water generation and distribution were 100% efficient, more than 90% of the energy in that water is still lost when it drains out of our showers, sinks, bathtubs, and clothes washers. “Drain water may be one of our largest untapped resources,” says Cautley.

Fortunately, some of this energy can be recovered. There are several manufacturers of heat exchangers that have copper piping tightly coiled around a section of copper drainpipe. Water on its way to the water heater flows through the coiled pipe and is preheated when hot water flows through the drainpipe (beneath a shower, for example). These are sometimes called gravity film exchange (GFX) systems, reflecting the fact that hot water going down a drainpipe will form a film on the inner wall of the pipe, allowing heat to be transferred across the pipe wall. These heat-recovery units are made by at least four companies (see Access).

A properly installed GFX system capturing waste heat from showers should reduce overall water heating energy use by 12% to 15%, according to Environmental Building News. When installed during new construction, the cost typically runs between $500 and $800. These systems are most cost-effective where hot water consumption is very high—for example, laundromats, commercial kitchens, and health clubs.

Access

Alex Wilson is the founder of BuildingGreen in Brattleboro, Vermont, and executive editor of Environmental Building News. He recently founded the Resilient Design Institute (resilientdesign.org).

On-Demand Recirc Systems:

ACT D’mand • gothotwater.com

Taco D’mand • taco-hvac.com

GFX Units:

EcoGFX • EcoInnovation Technologies Inc • ecoinnovation.ca

PowerPipe • RenewABILITY Energy Inc. • renewability.com

ReTherm • ReTherm Energy Systems Inc • retherm.com

WaterFilm Energy Inc• gfxtechnology.com