How does a hot water recirculation system work?

Why hot water takes so long to arrive

When you open a hot tap, the water that comes out first is the water that was sitting in the pipe and cooled off. You wait until all of it has run down the drain. DOE's Building America Solution Center says that in conventional plumbing, the volume between the water heater and the furthest fixture can exceed 1.5 gallons, and the wait can be more than 90 seconds^[3].

Low-flow fixtures make the wait longer, not shorter. An NREL training for builders explains that at flows under 1.5 GPM, hot water rides on top of the cold water in the pipe instead of pushing it out as a plug, so waits can be up to twice as long as with higher-flow fixtures^[1]. The same training cites a design goal of delivering 105 F water within 10 seconds^[1].

Fix the pipes first, if you can

NREL calls a compact plumbing layout, with no pump at all, the preferred approach: put hot water fixtures close to the water heater, size pipes so they hold less water, and insulate hot lines^[1]. That is easiest in new construction or a major remodel. In an existing home, a recirculation system is the usual retrofit.

How a recirculation system works

A pump moves the cooled water in the hot line back to the water heater, and hot water takes its place. When the line is hot, a valve or a control stops the flow. There are four common setups:

Setup	Return path	Where the pump sits	Typical use
Dedicated return line	A separate pipe from the far end of the loop back to the heater	At the water heater	New homes, remodels, homes that already have a return pipe
Crossover (comfort) valve retrofit	The cold water line, through a thermostatic valve under the furthest sink	At the water heater	Existing homes with no return pipe[6][7]
Under-sink on-demand pump	The cold water line, through a connection at the sink	Under the furthest sink	Retrofits; usually triggered by a button or sensor[1][8]
Built-in pump in a tankless heater	A return pipe or a crossover valve	Inside the water heater	Some Navien, Rinnai and Rheem tankless models

The Grundfos Comfort System and the Watts 500800 are two well-known crossover kits. Both put a pump with a timer at the water heater and a thermostatic valve at the fixture furthest from it, and neither needs a return line^[6][7]. Watts says its 500800 is not designed for tankless water heaters^[7]. See recirculation with a tankless water heater for what works there.

DOE's tankless guide describes the under-sink version: a pump, flexible piping and a remote sensor, installed near a frequently used fixture, that sends the stagnant water in the hot pipe back to the heater through a connection to the cold pipe^[8].

Our guides go deeper on each: dedicated return line systems and under-sink recirculating pumps.

The control matters more than the pump

Every system needs something to decide when the pump runs. That choice drives most of the energy cost.

Control	How it works	Energy use
None (continuous)	Pump runs all the time	Highest. NREL says it "is never recommended"[1]
Timer	Runs during set hours	NREL notes timers often run about 2 hours in the morning and 2 in the afternoon, which wastes energy and can miss off-schedule use[1]
Temperature (aquastat)	Cycles the pump to keep the loop at a set temperature	High; NREL says they often run several hours a day[1]
Learning or "smart"	Learns your usage and runs ahead of it	A form of timer control[1]
On-demand	Runs only when someone presses a button, trips a motion sensor or opens a tap, then stops when the line is hot	Lowest[1][3]

ENERGY STAR puts it plainly: systems that run continuously can use more energy, through pumping and heat lost from the pipes, than they save by reducing hot water waste^[2]. DOE's Solution Center says demand-initiated systems have been found more efficient than timer or temperature controls, because hot water is pulled into the loop only when it is needed^[3]. A typical demand control runs the pump until it senses a small temperature rise, often about 5 F, at the sensor^[3].

Compare controls in our guide to timers, sensors and on-demand buttons.

What the energy code says

The International Energy Conservation Code (IECC), which many states and cities adopt in some form, sets rules for recirculation in new work:

• A pump is required, and gravity loops are out. Heated water circulation systems must have a circulation pump, the return must be a dedicated return pipe or the cold water pipe, and gravity and thermosyphon systems are prohibited^[5].
• The pump must respond to demand. In the 2015 IECC text, controls must start the pump based on a demand for hot water and turn it off when the loop is hot and there is no demand^[4].
• Limits on warm water in the cold line. Demand recirculation controls must limit the water entering the cold water piping to 104 F or less^[4][5].

Your state or city may use a different edition or local amendments. NREL's training, for example, notes that California's 2019 Title 24 rules require demand control where the furthest fixture is more than 50 feet from the water heater^[1]. A licensed plumber or your building department can tell you what applies to your home.

Downsides to know before you buy

• Warm "cold" water. Crossover and under-sink systems send hot line water into the cold line, so the first draw from the cold tap near that valve can be lukewarm. That is part of how they work^[6][7].
• It needs power. Pumps plug into a 120 V outlet. NREL notes that electricians typically provide a non-switched outlet for the pump^[1]. A new outlet is electrical work for a licensed electrician.
• Heat pump water heaters. NREL reports that the major heat pump water heater makers require any recirculation system paired with their units to be demand-controlled^[1].
• Tankless heaters need the right equipment. A tank-style kit may not be designed for tankless units^[7].
• Insulation. NREL recommends insulating recirculation pipes to at least R-3^[1].

Is it worth it?

It depends on your layout and your control. NREL's training cites annual water savings of 3,600 to 12,000 gallons for on-demand systems, depending on setup and use^[1]. Grundfos claims its Comfort System can save up to 12,000 gallons a year in a typical household^[6]. The energy side can go either way: an on-demand system can save energy, while a continuous loop can cost more than it saves^[1][2].

For replacing an existing pump, HomeGuide's 2025 range is $150 to $680^[9]. A new dedicated return line costs more, because it means running pipe through walls or crawlspaces. Our recirculating pump cost and operating cost guide works through the electricity and heat-loss math.

Guides in this section

• Under-sink recirculating pumps
• Recirculation with a tankless water heater
• Dedicated return line systems
• Recirculating pump cost and operating cost
• Timers, sensors and on-demand buttons

When to call a pro

• Adding a return line, a crossover valve, or a pump at the water heater: this is plumbing work, and many areas require a permit.
• No outlet near the water heater or the sink where the pump will go: a licensed electrician should add one.
• Any recirculation project on a tankless or heat pump water heater, so the control settings match the maker's instructions.
• Cold water running warm, a pump that never shuts off, or hot water that still takes a long time after install.