Reducing hot-water waste is one of the rare home energy-efficiency measures that don’t require builders to spend more up front. In fact, says hot-water expert Gary Klein, by focusing on a home’s plumbing design early on, builders can get hot water to the tap faster while saving thousands of dollars in equipment costs.
Klein has been thinking about the design of hot-water systems since 1993, when he was an energy specialist at the California Energy Commission. He went on to be a co-lead on a staff report on the connection between water and energy in California, and he now works as a consultant specializing in high-performance hot-water systems for homes and buildings.
Water heating is the first or second-largest energy use in single-family residential homes and multifamily apartment buildings. Yet despite the increases in efficiency standards for water heaters and the developments in tankless and condensing water heater technologies, little has been done to improve the energy efficiency of the plumbing systems used to deliver that hot water, Klein says. And while Klein’s homeowner clients typically say the priority for their hot-water system is faster hot water at the tap — ideally in five seconds or less — many homeowners are still waiting a minute or two, sometimes much longer, for their shower to heat up.
The problem, Klein says, lies in home designs that spread wet rooms such as kitchens and baths far apart from the water heater and from each other. “The decision is made when you group the wet rooms together or far apart,” he says. “That’s the decision — and it’s made by the architect.”
Reducing the hot-water wait
Klein can diagnose how long it takes to get hot water in your master bath with a few quick questions. Newish three-story, 3,000-square-foot house with the water heater in the basement? That’s probably about 100 feet of 3/4-inch copper pipe, which equates to about 2 1/2 gallons of cold water sitting in the pipe. And since the hot water mixes with the cold water at first, you’ll probably have to dump 5 gallons before the water in the master bath is hot — a wait of about 2 1/2 minutes at 2 gallons per minute.
Smaller pipe diameters will help a bit, but you still have to cover the distance. In addition, pipe sizing rules haven’t been changed since the 1940s, even though washing machines and dishwashers use much less water and the federal government established maximum fixture flow rates in the mid-1990s. And water-efficient fixtures only make the wait even longer. See the problem?
The solution lies in the architecture of the home and reducing the size of what Klein describes in a recent Journal of Light Construction article as the “hot-water system rectangle.” He draws a rectangle around a floor plan’s hot-water heater and fixtures, and then calculates a ratio comparing the size of the rectangle to the total floor area of the home. In one recent success story, George Koertzen, the construction manager for a San Joaquin County, California, affiliate of Habitat for Humanity, shrank the hot-water system rectangle for his affordable houses from about 79 percent of the total footprint down to 15 percent, then 4 percent, eventually settling on a design with a 2.5 percent ratio.
Reduce costs with compact plumbing
While plumbing savings are particularly critical on an affordable project, there’s no reason builders can’t follow the same principles for larger homes, Klein says. A spread-out, one-story ranch, for instance, would typically have a kitchen, laundry room, and half bath on one side of the house and the master bath and second bath on the other. Instead of trying to serve them all with one water heater in the garage, why not create two clusters, each with its own water heater?
“Then all of a sudden you could justify two water heaters, which would mean that the plumbing would get smaller,” Klein says. While the marginal cost of the second water heater is somewhat more than the amount saved on the plumbing, the system would be much more efficient to start with. And builders could enhance the efficiency further with a propane-fired condensing storage or tankless water heater, he says.
Key to this approach is taking the plumbing system into consideration at the beginning of design. Klein says a common pitfall in the pursuit of faster hot water is trying to fix the problem late in the design process. At that point, the location of the water heater and fixtures has usually already been decided. Recirculation loops can still address the problem — at a cost. Depending on the placement of the water heater, Klein will typically design one or two recirculation loops, although he’s used as many as five in the past. But the cost for circulator pumps and smart controls, which help the pumps run only when they’re needed, can add up to as much as $1,000 per pump and control.
Compare that with a solution that makes the plumbing more compact up front. In his analysis, Klein had a plumbing contractor price out a typical mid-market home with a 76 percent area ratio for the hot-water system rectangle versus another at 15.5 percent. The more compact design netted savings of about $2,000, mostly by saving a week’s worth of labor hours, and wouldn’t require an additional recirculation system. The savings come from reductions in the feet of horizontal piping on the hot, cold, drain, and vent pipes.
“So that’s a $3,000–$4,000 swing in costs,” Klein says. “This is a case where better thinking about the architecture will give your customers greater satisfaction at a lower first cost, which means you as a builder could, in principle, make more profit.”