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On-demand pumps are activated by the user — either manually or via motion sensors. As a result, hot water is delivered only when it is actually needed. Eliminating the timer significantly reduces the actual runtime of the pump compared with a whole-house circulation system. It also minimizes standby heat loss from the hot-water pipes and keeps the water heater from firing unnecessarily.

Manual switches. The typical manual system uses a hard-wired low-voltage push button — just like a doorbell switch — to trigger the pump. After pushing the button, which is generally located on the countertop or cabinet face, the user waits 10 to 20 seconds (or until the low whirring sound of the pump stops) before turning on the hot tap. If the wall framing is reasonably accessible, we can hard-wire several low-voltage switches together — one at each tap on the branch — to control the pump. If the framing isn’t particularly accessible, we’ll use wireless remote-control switches along with a small receiver connected to the master controller.

Motion-detector switches. Some of our customers prefer that the system be triggered automatically, by motion detectors. Like on/off switches, these wireless motion sensors can be placed in several bathrooms and configured to communicate with a single pump, which, depending on the system, is located either downstream at the farthest fixture or back at the water heater. When the motion detector in any bathroom is activated, hot water moves along the main line supplying all the branches. The pump runs for only about 20 seconds at a time, so if it is activated but the faucet isn’t turned on, the energy penalty is minimal.

We typically install any necessary low-voltage wiring but leave the AC work to the electrician. Because even the largest on-demand pump draws a mere 1 or 2 amps, the only electrical requirement is an outlet located near the water heater or in the vanity cabinet. In some code jurisdictions, these outlets may need to be GFI-protected.

Sizing the Pump

The single-speed pumps used in on-demand systems are sized according to the length and diameter of the pipe in the branch and the flow resistance through the piping and the water heater, known as “head loss.” The higher the head capacity of the pump, the more resistance it can overcome. For example, the smallest of the three Taco pumps used in Metlund and Taco systems produces 1/40 hp at 3,250 rpm, is rated at 8 gallons per minute, and has a total head of 10 feet. The largest pump produces 1/8 hp, is rated at 28 gallons per minute, and has a total head of 30 feet. Matching the pump to the system is important. If we oversize the pump, we risk pipe corrosion, which happens when water pumped too rapidly erodes the copper pipe wall. (This is more of a problem with whole-house recirculation pumps, especially ones that operate almost continuously.) On the other hand, if the pump has a lower head than the piping system, it won’t circulate any water at all.

On-demand pumps can be used with tankless water heaters, but because these heaters require a minimum flow of water before they activate, we always spec the largest pump size. The pump also has to overcome friction loss, caused by the turbulence of water moving through the piping within the heater itself.