Reduce the Waiting Time for Hot Water
Bird's eye view
Eliminate wait, eliminate waste
People spend a lot of time waiting for hot water. It can take minutes for hot water to make its way from the basement or mechanical room to a distant bathroom, and all that water and energy goes down the drain.
Hot water circulation systems are a way of getting hot water to the point of use with virtually no wait, saving thousands of gallons of water each year and sometimes lowering energy bills.
Check valve and pump for circulating water
Depending on the manufacturer of the equipment being used, a hot water circulation system pump is installed near the water heater or under the sink of the most distant bathroom. Systems that use the cold water supply as the return leg of the loop must include a normally closed check valve under the sink between the hot and cold water supply pipes. When the pump is activated, the check valve opens, allowing water from the hot supply pipes to cross over into the cold pipe.
Water and energy savings depends on design and behavior
All hot water circulation systems conserve water, but to also be energy efficient, they have to be designed and used efficiently. A timed system that automatically runs on a programmed schedule can save energy only if the home's occupants have predictable water use habits. An on-demand system is not as instantaneous, but it allows for more random behavior. If the water heater is below all of the hot water taps, a continuous, gravity fed loop can eliminate the need for an electric pump, but such systems are notorious energy wasters. Keeping the points of use as close to the water heater as possible will save energy for any of these systems.
Hot water pipes should always be insulated, but this is more important in certain situations. If not wrapped with foam or fiberglass, hot water pipes will introduce some extra heat to living spaces. This may not be a problem in a heating dominated climate, but it is still an unintended use of energy. In a gravity fed recirculation system, the pipes are essentially an extension of the hot water storage tank so they should be as resistant to heat loss as the tank itself.
Insulate hot water lines
All hot water supply pipes from the water heater to the farthest fixture should be insulated.
R-2 minimum insulation
The International Residential Code covers circulating hot water systems in Section N1103.4. Among the requirements: hot water piping should be insulated to at least R-2, and all circulating hot water systems with a pump shall have a readily accessible switch to turn off the pump.
ABOUT HOT-WATER CIRCULATION
Distant water heaters lead to waste
Hot water circulation systems use a pump to circulate hot water from the water heater to the tap, reducing the waiting time for hot water to nearly nothing.
All hot water circulation systems save water. But not all systems save energy; some systems actually result in higher energy bills.
Choose an on-demand system. There are three types of hot water circulation systems: thermosyphon systems, time and temperature systems, and on-demand systems.
On-demand systems make the most sense in a green home because hot water is pumped to the point of use only when it is needed, minimizing standby losses. They're activated manually or by a wireless remote or motion detector.
Such a system is inexpensive to add when plumbing is being roughed in, and can be affordably retrofitted to an existing plumbing system. Installed as a retrofit, on-demand systems use existing cold water lines as the return with the help of a special valve.
Time and temperature systems. Hot water is circulated automatically to anticipate demand with the help of a timer and thermostat. An override allows homeowners to activate the pump at non-scheduled times.
While time and temperature systems cut waiting time, they increase standby heat losses. Hot water lines are essentially turned into extensions of the hot water storage tank. They save water but increase net energy use.
Thermosyphon systems. These work only in a home with a tank-type water heater located below the hot water taps — for example, in a basement. A continuous piping loop is installed between the water heater and the fixtures, with a return line coming back to the heater after the farthest fixture is served.
Because the returning water is cooler than the water in the top of the water heater, a thermosyphon is established, and water circulates through the loop, 24 hours a day, without the need of a pump.
Such systems waste large amounts of energy; according to one calculation, more energy is lost in the loop than the energy required to heat the water in the first place.
See Hot-Water Distribution Systems, Part III by Gary Klein .
- Don Mannes/Fine Homebuilding
- Krysta Doerfler / Fine Homebuilding
Feb 7, 2011 6:42 AM ET