Hot Water Recirculation Pumps: Types, Controls and Energy Trade-Offs

Why hot water takes so long

When no one has used hot water for a while, the water in the pipe between the heater and the tap cools to room temperature. Opening the tap pushes that cooled water out first; hot water arrives only after the pipe has been emptied. The volume depends on pipe size and length:

At a 1.5 GPM faucet, a 50-foot run of 3/4-inch pipe takes about 50 seconds to clear, and the pipe walls absorb some heat after that. A household that waits this way a dozen times a day can send several thousand gallons a year down the drain.

Types of recirculation systems

• Dedicated return loop: a separate return pipe runs from the farthest fixture back to the bottom of the water heater, and a small pump keeps water moving around the loop. Common in new construction and the most effective, because cold water lines are untouched.
• Under-sink crossover (comfort valve) systems: a pump at the water heater pushes hot water toward the farthest sink, and a thermostatic valve under that sink lets cooled water cross into the cold line and return to the heater. No new pipe is needed, which makes it the usual retrofit choice. The trade-off is that the cold tap at that sink can run lukewarm for a few seconds.
• Point-of-use pump under the sink: a variation where the pump sits at the fixture and is triggered on demand, pulling hot water forward and sending the cooled water back through the cold line.
• Tankless with built-in recirculation: many gas tankless units include a pump and scheduling. Check that the loop flow exceeds the unit's minimum activation flow, or it will not fire.

Control options, from most to least efficient

1. On-demand: a push button, motion sensor, voice or app trigger runs the pump only until the loop is hot (a temperature sensor stops it). Energy use is close to that of not having a pump at all, and you get the water savings.
2. Thermostatic (aquastat): the pump runs whenever the return water falls below a set temperature, for example 95–105 °F. It is better than continuous but still keeps the pipes warm all day and night.
3. Timer: runs the pump during set windows, such as 6–8 am and 5–10 pm. A reasonable compromise if your routine is regular; many units combine a timer with a thermostat.
4. Continuous: the pump never stops. Instant hot water everywhere, but the largest heat loss and the shortest pump life.

Many energy codes now require recirculating systems to have a control that shuts the pump off when hot water is not needed, and require the loop pipes to be insulated. Check local code before installing a continuous system.

The energy trade-off

The pump itself is small, typically about 5–25 W for under-sink and small circulators and up to around 80 W for larger loops. Even 25 W running continuously is about 220 kWh a year. The bigger cost is heat lost from the pipes. A loop held at 110–120 °F radiates heat into walls, crawlspaces and attics all the time. An uninsulated 3/4-inch copper loop can lose in the region of 20–40 BTU/h per foot in typical conditions; 100 feet of loop at 30 BTU/h per foot running all year is about 26 million BTU, enough to add a noticeable amount to the water heater's fuel bill. The heating cost calculator can price that load for your fuel.

• Insulating the hot and return lines (at least 1/2-inch foam, more in unconditioned space) cuts loss substantially.
• On-demand control reduces the hours the loop is hot to minutes a day.
• In summer, heat lost from pipes inside the house also adds a little to the air conditioning load.

Recirculation with different water heaters

With a tank, recirculation returns cooled water to the bottom of the tank, which slightly increases the heater's run time and stirs the tank. With a heat pump water heater, continuous recirculation can push the unit into resistance backup more often, so on-demand control matters more. With a tankless heater, each recirculation cycle fires the burner, so frequent short cycles add wear; scheduling or on-demand control is again the better choice. Sizing considerations for each are covered in water heater sizing, the tankless size calculator and tankless vs tank.

Installation and safety

• Fit check valves so water does not flow backward through the loop or cold line.
• Use a bronze or stainless pump body for potable water; cast iron is for closed heating loops only.
• Keep water heater storage temperature at or above about 120 °F. Loops that sit lukewarm for long periods can encourage bacterial growth, including Legionella; thermostatic mixing valves at the heater allow hotter storage while delivering safe tap temperatures.

Plug-in under-sink kits are within reach of a handy homeowner, but cutting into supply lines, adding a return loop or wiring a pump to a new circuit is plumbing and electrical work that may require a licensed installer and permit.

Frequently asked questions