By Dale Stroud, Uponor North America

Because on-demand, hot water recirculation systems can deliver hot water quickly to every faucet and shower head in a house — regardless of their distance from the hot-water source — and help home owners conserve water and save on energy, they are growing in popularity and can provide opportunities for remodelers, builders and contractors.

Most early hot-water recirculation systems extended the main trunk beyond the last fixture, returned the main trunk back to the water heater inlet and slowly pumped hot water continuously through the trunk line — not a cost-effective system for the home owner.

Alternatively, some designs eliminate the pump in favor of the siphoning effect, which causes the temperature differential to force water to circulate slowly within the plumbing system. This system is always charged with hot water and more efficient, but it has its deficiencies.

When a user opens a hot-water tap, only the cooled water in the branch line that connects the trunk to the faucet is flushed out before hot water flows from the tap. Users waste less water waiting for the system to heat up compared to a system without recirculation.

The D'MAND pump operates only when there is a call for hot water, which can save energy, water and wear and tear on the system.

But because the pump continuously operates in these systems, the system consumes energy, causes undue wear and tear on plumbing components and forces the water heater to cycle — turn on and off — more frequently.

A less energy-intensive system uses a timer to restrict the pump’s operating time, limiting hot-water recirculation to peak-demand periods. With this system, the pump operates only during the “timer-on” period — for example, during the morning or evening shower hour.

During non-timed periods, however, the pump does not operate — which negates the benefit of rapid hot-water delivery.

Today’s lifestyle patterns do not necessarily coincide with such restrictive, pre-programmed, theoretical peak-demand periods. Hot-water use today can be unpredictable or irregular — so it’s not unusual for a home owner to override the timer and convert the system to a continuous circulation mode.

On-Demand: A Greener Alternative

The only bona fide green alternative to the continuous and timed systems is an on-demand recirculation system, which allows the home owner to select when to circulate hot water.

The system features water pumping through a looped plumbing design in which the main trunk returns to the cold-water inlet connected to the water heater. Typically, a pump is installed near the end of the return line just before it tees into the cold-water supply to the water heater.

With an on-demand system, the pump operates only when there is a demand for hot water. By controlling when the pump runs, home owners can save on electricity and reduce wear and tear on the plumbing system.

How On-Demand Operates

Mount an on-demand pump near the end of the return line near where it tees into the cold-water supply to the water heater.

An on-demand system uses a higher capacity pump than other systems to rapidly fill the trunk line with hot water. However, unlike continuous or timed systems, the on-demand pump operates only when activated by the occupant.

User-controlled transmitters — usually motion detectors or buttons that are either hard-wired or wireless — send a signal that activates the pump. To limit its running time, the system features an on-board thermistor that senses when hot water fills the trunk line. The thermistor registers a temperature rise in the water and shuts off the pump as soon as it detects a preset temperature rise.

The thermistor can also detect if hot water is already in the system and prevents the pump from turning on even if an activation device sends a signal to the pump.

This system is not instantaneous. Once activated, the pump does take several seconds to fill the trunk line, so the home owner should wait a few seconds while the pump completely fills the trunk line before opening the hot-water tap. Waiting those few seconds optimizes the system’s performance and maximizes water and energy savings.

When fitting such a system in existing construction, an on-demand pump with a wireless receiver can be installed under the sink that is farthest in piping distance from the hot water heater. The receiver eliminates the need for special wiring and makes the job easier.

On-Demand Earns Green Points

The on-demand method is the only system recognized by national residential green building programs, including the ANSI-approved National Green Building Standard, which provides six points for these systems in Section 801.1.

In addition, under the Environmental Protection Agency’s WaterSense Single-Family New Home Specification, the trunk line in on-demand recirculation systems is categorized as the “hot-water source,” giving builders and remodelers greater flexibility when creating systems because they are no longer limited by the distance between a water heater and faucet.

Multi-port tees supply fixture groupings from a single inline fitting.

Remodelers should insulate hot water lines, which will help maintain water temperature at a usable level — for example, above 105°F for a longer period of time — to further enhance an on-demand system’s performance. Both the green building standard and LEED-H grant additional points when the lines are fully insulated.

Studies conservatively estimate that an on-demand system can save home owners at least 20 gallons of water a day, or more than 7,300 gallons a year, in a typical home — or energy and water savings of between $140 and $250 a year, depending upon local energy and water rates and whether the water heater is gas, electric or propane.

In addition, on-demand systems reduce the amount of potable water utilities have to prepare and the waste water they have to treat, saving them money as well.  

Tips for Installing an On-Demand Recirculation System

To install an on-demand system, plumb the system in a loop so the main trunk line supplies all the hot-water fixtures in the home. Route the hot-water trunk line as closely as possible to fixtures, so the branches that supply faucets, showerheads, etc., are as short as possible.

• Do not downsize the return portion of the trunk line. A return line should have at least a 3/4-inch nominal diameter.
  
  
• Choose an on-demand pump that is appropriate for the overall length of the trunk line. A longer trunk requires a larger pump. The manufacturer can recommend the best pump for an application.
  
  
• The on-demand system works equally well with tank and tankless water heaters. Tankless models that require a pressure differential for activation may require a larger on-demand pump.

To enhance the performance of an on-demand recirculation system, use cross-linked polyethylene (PEX) tubing. Its flexibility allows for smooth, gradual transitions when changing tubing direction, which is preferable to an abrupt, 90-degree directional change with rigid pipe. Also, use multi-port tees when practical to supply fixture groupings from a single inline fitting.

When installing a system:

• Plan the type and location of activation devices — such as motion sensors and wireless or hard-wired buttons. To minimize unnecessary pump activation, do not install motion sensors in a high-traffic area, such as a kitchen.
  
  
• Install a grounded, 110-volt outlet within six feet of the pump — usually near the cold-water supply to the water heater — to easily provide electric power to the pump.
  
  
• Install an inline shutoff valve in the trunk line between the pump outlet and the juncture of the return line with the cold-water supply to the water heater to isolate the return and make it easier to service the plumbing system.
  
  
• Insulate all hot water lines with tubular foam wrap to enhance the system’s performance.
  
  
• For homes that require two or more piping loops to supply all the hot-water fixtures, install a separate on-demand pump in each loop.
  
  
• For homes with a pressure-regulator valve that acts as a backflow preventer, incorporate an expansion tank before installing an on-demand system.

Dale Stroud is a senior manager of new business development and market research for Uponor North America, a leading manufacturer and supplier of residential and commercial plumbing and heating systems based in Mississauga, Ontario, Canada. For more information, visit the Uponor website at www.uponor.ca; or call 888-994-7726.

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