Hot water recirculation pumps and the wait at the tap

A hot water recirculation pump is a small circulator that moves the cooled water sitting in your hot pipes back toward the water heater, so that hot water is already at or near the faucet when you open it. It does not make more hot water. It changes where the hot water is waiting.

That sounds simple, and the hardware is simple. The part that decides whether a pump saves water and energy or quietly wastes both is the control: what tells the pump to run, and what tells it to stop. This guide covers why the wait happens, the ways a recirculation system is plumbed, the control types, what the energy code now says about them, how they behave with tankless heaters, and the side effects to expect in an existing house.

Why the wait happens

Between draws, the hot water left in the pipe cools to the temperature of the wall, crawlspace or slab around it. When you open the tap, every ounce of that cooled water has to come out before hot water from the heater can reach you. The wait is set by two things: how much water the pipe holds between the heater and the faucet, and how fast the faucet lets it out.

The Building America Solution Center, run for the Department of Energy, notes that in a conventional trunk-and-branch layout, even a relatively small 1,200-square-foot home can hold more than 1.5 gallons between the heater and the farthest fixture, and the time to tap can be more than 90 seconds. EPA’s WaterSense program puts the household cost at 6.35 gallons a day sent down the drain while people wait for showers and faucets to run hot, about 3.7 percent of average indoor daily water use, citing Lawrence Berkeley National Laboratory research.

Pipe volume is easy to estimate. EPA’s 2026 guide to efficient hot water delivery reproduces a table, modified from the International Plumbing Code, of how many ounces of water each foot of pipe holds. A few common sizes:

A hypothetical example shows how fast it adds up. Say the far bathroom is fed by 50 feet of 3/4-inch type L copper and then 10 feet of 1/2-inch type L. That is about 161 ounces plus 15.5 ounces, roughly 176 ounces, or close to 1.4 gallons.

A WaterSense labeled bathroom faucet uses no more than 1.5 gallons per minute, so it would take close to a minute just to push that volume out, and longer still while the cold pipe walls take heat from the first hot water to arrive. A WaterSense showerhead, at no more than 2.0 gallons per minute, clears it a little faster, but the same water still goes down the drain.

EPA recommends that new homes be designed so that no more than 0.5 gallons is stored between the hot water source and any fixture. That figure was once a requirement of the WaterSense homes specification and is now a recommended benchmark. Most existing houses were not built to it, which is where a pump comes in.

The ways a recirculation system is plumbed

Every recirculation system needs a path for the cooled water to get back to the heater. There are three common ways to give it one.

A dedicated return line

The classic system is a loop. Hot water leaves the heater, runs past the fixtures, and a separate return pipe brings it back to the heater, with a pump somewhere on the loop. EPA describes the pump drawing hot water through the loop and returning the ambient-temperature water to the heater, and notes that the loop is typically kept as short as possible and within about 10 feet of every fixture.

The short branches from the loop to each faucet are what make the wait short. This is the cleanest design, and it is easiest to build in new construction or a major remodel, when walls are open.

The cold line as the return

Most retrofits do not have a return pipe, so they borrow the cold water line. A pump at the water heater pushes water toward the house, and a thermostatic valve under the sink farthest from the heater connects the hot and cold supplies. While the hot line is cool, the valve lets water cross into the cold line and back toward the heater. When warm water reaches the valve, it closes.

The Watts installation manual for its Model 500800 system, for example, puts the pump on the water heater’s hot outlet, pumping away from the heater toward the house, with a sensor valve at the sink with the longest wait. Armstrong describes its Astro Express 2 valve the same way: when the water at the hot faucet cools below a low set point, the valve opens and injects the cooled water into the cold line, and it closes again at its high set point.

EPA calls the cold-line return a convenient solution for existing systems that are being retrofitted, but notes that it may not achieve the efficiency or performance of a dedicated return.

A pump built into the water heater

Some water heaters include a recirculation pump. EPA notes that these integrated pumps can either circulate water within the heater, to keep it ready for the first draw, or circulate it through the house. Which one a given model does, and what piping or valve it needs to do it, is in that model’s installation manual.

Controls decide the energy bill

A recirculation pump can be told to run in four broad ways. The difference between them is the difference between a pump that runs a few minutes a day and one that keeps a loop of pipe hot around the clock.

• Continuous. The pump never stops. The ENERGY STAR technical bulletin on demand water heaters says continuous circulating systems should be avoided because they keep the heater firing, and recommends a push-button demand circulating pump with a temperature-sensing shutoff instead.
• Timer. The pump runs during set hours. The Watts manual suggests setting the timer around peak use, for example from 30 minutes before the first shower until 15 minutes after the last one. EPA notes that the efficiency of timer schedules is highly variable, depending on the settings and on how consistent the household’s routine is.
• Temperature. The pump or valve cycles to keep the line within a temperature band. EPA groups timer- and temperature-based systems together and says research indicates they can use a large amount of energy keeping the loop warm, and are not considered energy-efficient.
• Demand. The pump starts only when someone asks for hot water, by pushing a button or tripping a motion sensor near the fixture, and stops when the loop warms. The Building America Solution Center says the controls typically stop the pump after a small rise, about 5°F, and describes demand-initiated systems as more efficient than timer or temperature systems because hot water is drawn into the loop only when it is needed.

The older Department of Energy water heating guide describes the demand version in one sentence: on-demand systems recirculate water when a user trips a switch at the faucet, and once water of the desired temperature reaches the faucet, the pump shuts off. In practice, you press the button, wait a short while with the tap closed, and then open it. The cooled water went back toward the heater instead of down the drain.

EPA lists two reasons a demand system can save energy even though the pump uses some. The water returned to the heater is usually warmer than the water entering the house, so the heater has less to do.

And because the loop is primed at a high flow rate, less heat is lost along the way and less hot water is needed to prime it. EPA adds that the operating savings from not heating and losing that water typically far outweigh the energy the pump uses. That statement is about demand-initiated systems. It does not cover a pump left on a 24-hour timer.

What the energy code says

The residential energy code has a section on hot water circulation systems, and the 2021 and 2024 editions spell out the controls. The International Code Council’s summary of the 2021 International Residential Code and International Energy Conservation Code, and the 2024-based residential text on UpCodes, say in substance:

• A heated water circulation system must have a circulation pump. Gravity and thermosyphon circulation are prohibited.
• The return pipe must be a dedicated return pipe or a cold water supply pipe.
• The controls must turn the pump off automatically when the water in the loop is at the desired temperature and when there is no demand for hot water.
• The controls must limit the water entering the cold piping to no more than 104°F. The 2024-based text also places a temperature sensor on the hot supply within 2 feet of the connection to the cold pipe when the cold line is used as the return.

For demand recirculation systems, the 2024-based text requires controls that start the pump from a user’s action, a presence sensor, or sensed flow to a fixture. The controls must shut the pump off at a rise of no more than 10°F above the starting pipe temperature or when the pipe reaches 104°F, run it no more than 5 minutes after activation, and wait at least 5 minutes before running it again.

Two caveats. Codes are adopted, and sometimes amended, by states and cities, so the edition in force where you live may differ. And they are applied to new installations through the permit process. Ask the building department which edition applies before you buy, because a timer-only pump may not pass where the newer text is in force.

Recirculation with a tankless heater

Tankless heaters and recirculation have an awkward relationship. A tankless heater fires whenever enough water flows through it, so a pump that keeps moving water keeps the burner or elements cycling. The ENERGY STAR bulletin says continuous circulation negates the energy-saving benefit of a tankless unit and shortens the heater’s life, and points to a push-button demand pump with a temperature-sensing shutoff instead.

Not every retrofit kit is meant for tankless heaters at all. The Watts 500800 manual states plainly that the system does not work with tankless water heaters. EPA notes that some water heaters come with a built-in recirculation pump. Whether a particular tankless model supports recirculation, and with which pumps, valves and settings, is a question for that model’s installation manual, not for a general retrofit kit.

If a tankless quote includes recirculation, ask which control mode it will run in. The page on how a tankless heater works explains minimum activation flow, and a recirculation crossover valve stuck open is one of the causes listed under tankless symptoms.

Side effects to expect in an existing house

A cold-line retrofit changes how the cold tap behaves at the sink with the valve. The Watts manual warns that you may have some warm water in the cold line under that sink, and that it clears quickly once the cold tap runs. It also says the system is not an anti-scald device. If the house has young children or older adults, the water heater setting still matters, and the Consumer Product Safety Commission’s recommendation of 120°F is covered in temperature, scalds, and Legionella.

Other points from the manufacturer instructions:

• The pump needs power at the water heater. The Watts unit plugs into a grounded 120-volt receptacle, and the manual says the pump must not touch the vent of a gas or oil water heater.
• One valve serves one branch. Watts and Armstrong both place the valve at the fixture with the longest pipe run, and Watts notes that a house with a branched hot line may need more than one valve.
• Long or uninsulated runs lose heat. Watts suggests a second valve in parallel where piping is uninsulated or runs on a concrete foundation, to increase flow and reduce the temperature drop.
• Adjustments take time to show. Armstrong advises allowing at least several hours after changing the valve or timer before judging the result.
• Installing the pump means shutting off and draining the heater’s hot side, and for a gas heater, turning the gas off first. That is plumbing work, so ask the building department whether it needs a permit where you live.

Pump, point-of-use heater, or new piping

A recirculation pump is one of three ways to shorten the wait, and the right one depends on the house. A point-of-use heater fits one remote fixture that uses little hot water. A pump fits when several fixtures share a long branch, because one loop or one crossover valve can serve all of them.

New piping fits a remodel where walls are open and the hot line can be shortened, downsized, or given a dedicated return. EPA also notes that where extended pipe runs would be long and large in diameter, a second water heater can avoid the extra piping and distribution losses.

A pump does not fix a heater that runs out of hot water. If every fixture is lukewarm, or hot water fades partway through a shower, start with the hot water runs out too fast.

Recirculation only changes how long you wait for the first hot water.

The short version

The wait at the tap is the volume of cooled water in the pipe divided by the faucet’s flow. A recirculation pump moves that water back toward the heater instead of down the drain. Use a dedicated return where you can build one, and a cold-line crossover valve where you cannot, and expect warm water at that sink’s cold tap.

Choose demand control, a button or sensor that runs the pump only when someone is about to use hot water, over continuous, timer, or temperature control. Check the energy code edition your building department enforces, and with a tankless heater, confirm in the model’s manual that the pump and control mode are supported.

Sources

U.S. EPA WaterSense, “Best Practices for Efficient Hot Water Delivery in Homes” (epa.gov PDF), for the 6.35 gallons a day and 3.7 percent figures, the 0.5-gallon benchmark and its history, the pipe volume table, dedicated and cold-line returns, integrated pumps, the 10-foot loop distance, the comparison of timer, temperature and demand controls, the reasons demand systems save energy, and the second-heater option. U.S. EPA, “WaterSense Labeled Homes Technical Sheet: Hot Water Delivery” (epa.gov PDF), for the 0.5-gallon design target.

U.S. Department of Energy, Building America Solution Center, “Demand Plumbing” (basc.pnnl.gov), for the 1.5-gallon and 90-second trunk-and-branch figures and the typical 5°F shutoff rise. U.S. Department of Energy, “Water Heating: Energy-Efficient Strategies for Supplying Hot Water in the Home,” DOE/GO-102001-0785 (energy.gov PDF), for how on-demand recirculation starts and stops. ENERGY STAR, “Technical Bulletin: Demand Water Heaters” (energystar.gov PDF), for the warning against continuous circulation with tankless heaters and the push-button demand pump recommendation.

International Code Council, “Week three Building Safety Month event covers ‘hot’ water safety” (iccsafe.org), for the 2021 IRC N1103.5.1.1 and IECC R403.5.1.1 circulation system text. UpCodes, “N1103.5.1.1 (R403.5.1.1) Circulation Systems” (up.codes), for the 2024-based residential text, including the sensor location and the demand control limits.

U.S. EPA WaterSense, “Bathroom Sink Faucets” (epa.gov) and “Showerheads” (epa.gov), for the 1.5 and 2.0 gallon-per-minute maximums. Watts, “Installation and Operation Manual, Hot Water Recirculation System Model 500800” (watts.com PDF), for pump and sensor valve placement, the tankless and anti-scald warnings, warm water in the cold line, the timer guidance, the vent clearance, and parallel valves.

Armstrong Fluid Technology, “Astro Express 2 Installation and Operating Instructions” (armstrongfluidtechnology.com PDF), for how a crossover valve opens and closes and the advice to wait several hours after adjustment. U.S. Consumer Product Safety Commission, “Avoiding Tap Water Scalds” (cpsc.gov PDF), for the 120°F recommendation.