Can you add a recirculation pump to a tankless water heater?

Why tankless and recirculation need care

A tank water heater keeps hot water ready, so a small pump can pull from it at any time. A tankless unit only fires when water flows through it fast enough. DOE's tankless guide says each unit has a minimum flow, typically 0.4 to 0.9 GPM, that turns it on^[1]. A pump that moves less than that just circulates cool water.

Every recirculation cycle also lights the burner. Pumps that run all day, or controls that cycle often, mean many short burner runs and more heat lost from the pipes. ENERGY STAR warns that continuous recirculation can use more energy than it saves^[10].

DOE's tankless guide recommends on-demand recirculation as one way to cut wait times in a retrofit: a user triggers a sensor, the pump sends the stagnant water back to the heater, and hot water reaches the fixture before the tap opens^[1].

If you smell gas at any point, stop. Leave the house, avoid anything that can spark, and call the gas utility or 911 from outside^[11].

Option 1: a tankless model with a built-in pump

Several makers sell tankless units with the pump and controls inside. Your installer still needs a return path: a dedicated return line or a crossover valve.

Maker	What it offers	Key points from the maker
Navien NPE-A2	ComfortFlow: buffer tank, recirculation pump and controls built in	Recirculates through an external return line, or through existing supply lines using the NaviCirc valve[3]
Rinnai RUR models (RUR98, RUR160, RUR199)	Internal pump and crossover valve included	Runs Circ-Logic recirculation patterns[6]
Rheem IKONIC SR series	Built-in pump with Wi-Fi	Timer, custom schedule or on-demand modes[8]

Navien lists five ways to run the NPE-A2 pump: 24/7 continuous, a self-learning mode that studies seven days of use, an external control, HotButton on-demand (up to 10 buttons), and a seven-day schedule from the NaviLink app^[5]. With HotButton, a press starts the pump and heats the loop, and the pump shuts down once the loop is at temperature until the next press^[5].

Rheem describes its timer modes as cycling every 15 to 30 minutes in Performance mode or every 30 to 60 minutes in Energy Saver mode^[8].

Option 2: an external pump on a tankless that supports it

Rinnai lists models that include Circ-Logic controls but need an external pump bought separately. Examples include the RL75 and RL94 (VC series), RU80, RU98, and the SENSEI RU130, RU160, RU180 and RU199^[6]. Rinnai's GTK15 kit uses a Grundfos pump on a dedicated return line with these heaters^[7].

Circ-Logic has two modes, chosen by settings inside the heater^[7]:

• Comfort mode cycles the pump more often and keeps the loop hotter, but uses more energy.
• Economy mode cycles less often and uses less energy.

On a dedicated return line, the heater and pump turn off when the return water reaches about 15 F below the temperature setting^[7]. Rinnai's internal pump on RUR models with a crossover valve runs on time intervals only, not water temperature^[6].

Option 3: on-demand buttons and sensors

On-demand is the lowest-energy way to recirculate with any heater, and tankless makers support it:

• Rinnai. Push buttons, motion sensors, the Rinnai app, and voice assistants can send a request. Rinnai stresses that it is a request, not a command: the heater decides whether the pump can run. If it accepts, the pump runs for five minutes or until the water reaches temperature, whichever is first, then enters a mandatory rest period^[6].
• Navien. HotButton activates the internal pump and burner on the NPE-A2^[5].
• Rheem. On-demand runs from the app or the control panel on IKONIC SR models^[8].

See timers, sensors and on-demand buttons for how each control type compares.

No return line? The crossover valve

Most existing homes have no return pipe. A crossover valve under the sink furthest from the heater lets the hot line drain back through the cold line. Navien's NaviCirc works best under the furthest accessible fixture, is designed for NPE-A series heaters, and blocks circulation when the water in the valve reaches about 95 F^[4]. Rinnai includes a crossover valve with its RUR internal-pump models^[6].

The tradeoff is warmer water in the cold line near that fixture. Ask your installer how the valve and controls limit it.

Kits that are not made for tankless

Tank-style crossover kits are built around a heater that always has hot water ready. Watts states that its 500800 recirculating system is not designed for tankless water heaters^[9]. Before buying any kit, check that the pump maker or the water heater maker lists your tankless model as compatible.

Built-in pump is not the same as whole-house recirculation

NREL's 2025 builder training points out that some tankless heaters with onboard pumps have modes that only circulate water between the heat exchanger and a small onboard buffer tank^[2]. That mode helps with the cold water sandwich, but it does not serve as a whole-house recirculation pump^[2]. The settings have to be programmed correctly for the house loop^[2].

What it costs

See recirculating pump cost and operating cost for pump electricity and heat-loss math. For the heater itself, see tankless water heater cost.

When to call a pro

• Adding a return line, crossover valve or external pump: plumbing work, and gas appliance settings must match the manufacturer's instructions.
• Changing settings inside the heater (DIP switches or parameters): these sit behind the front cover, which is a job for a licensed plumber or the maker's service network.
• No outlet for an external pump: a licensed electrician should add one.
• Ignition or flame codes that start after recirculation is added. See no hot water from a tankless.