A tankless water heater delivers endless hot water — but it does not change how far that water has to travel. On a long run to a primary bath or kitchen, homeowners still stand at the tap waiting while cooled water drains away. Recirculation solves that problem by keeping hot water moving through the supply line so it is ready at the fixture. With a tankless unit, though, the way the loop is built, the way the pump is controlled, and the specific brand all matter. This guide covers how recirculation works, the two main loop designs, pump control options, and how Rheem, American, Intellihot, and Stiebel Eltron each handle recirculation.
What recirculation does
A recirculation pump moves hot water from the water heater out through the hot water piping and back to the heater in a loop. Instead of sitting still and cooling in the pipes between uses, the water in the hot line is periodically refreshed with heated water — so when someone opens a faucet, hot water reaches the tap much faster and less water goes down the drain while they wait.
The trade-off is energy. Keeping hot water moving means the pump uses electricity and the piping continuously loses heat. That is why the type of loop and the way the pump is controlled are the two decisions that shape how well a recirculation system performs.
Two ways to build the loop
Dedicated return line
A separate return pipe runs from the end of the hot water distribution back to the water heater, carrying cooled water back to be reheated. This is the cleanest design — it does not involve the cold water line at all — and it is the easiest to plan into new construction or a major remodel when walls are open.
Crossover valve (no dedicated return)
In a retrofit where running a new return pipe is not practical, a thermostatic crossover valve is installed at the fixture farthest from the water heater. The valve connects the hot and cold lines, letting cooled water from the hot side flow back to the heater through the existing cold water pipe. Once hot water reaches the valve, it closes.
The advantage is that no new piping is needed. The drawback is that using the cold line as the return path warms the cold water pipe — so the first draw from the cold tap at that fixture may run lukewarm.
Quick rule of thumb: if the walls are open, a dedicated return line is the better long-term system. If they are not, a crossover valve is the practical retrofit — as long as the tankless unit and pump are approved for that configuration. Always confirm in the manufacturer's installation manual.
How the pump is controlled
How often the pump runs has the biggest effect on energy use. The common control strategies are:
- Continuous — the pump runs all the time. Hot water is always at the tap, but this approach uses the most pump energy and has the highest heat loss from the piping.
- Timer or scheduled — the pump runs during set periods, such as mornings and evenings, when the household actually uses hot water.
- Temperature-controlled — a sensor on the return line shuts the pump off once the loop is up to temperature.
- Demand-controlled — the pump runs only when hot water is requested, triggered by a button, sensor, app, or flow detection. This keeps pump runtime and heat loss to a minimum.
Whatever the control method, insulating the hot water piping helps reduce heat loss between cycles.
What is different with a tankless water heater
A tank-type water heater stores hot water, so a recirculation pump simply moves it around. A tankless unit only fires when it detects enough water flowing through it — Rheem's RTG-84, for example, lists a minimum activation flow rate of 0.40 GPM. That means the recirculation loop has to create enough flow to activate the unit, and the unit has to be designed or approved for recirculation.
Depending on the brand and model, recirculation comes in one of three forms: a pump built into the water heater, an approved external pump or kit, or — on some commercial units — a recirculation loop that is required for operation. Here is how each brand at PlumbersCrib.com handles it.
Recirculation by brand
Rheem
- Built-in pump models — the IKONIC SR Series (condensing, with built-in EcoNet Wi-Fi and a dedicated 3/4" NPT recirculation fitting), the Prestige RTGH-RH (condensing), and the RTG-RC (non-condensing).
- Add-on pump — the Rheem AP17920 recirculation pump kit is a timer-based kit with an energy saver mode and built-in check valve. It is compatible with Rheem RTG and RTGH models, excluding the RTG-RC and RTGH-RH variants, which already have a built-in pump. Rheem also lists IKONIC S models as compatible with its external pump accessory.
- Pump modes on IKONIC — Timer (Performance mode cycles every 15 to 30 minutes; Energy Saver every 30 to 60 minutes), Custom Schedule, and On-Demand, set from the unit or, on Wi-Fi models, the EcoNet app.
Rheem estimates its built-in recirculation can save up to 12,000 gallons of water per year compared with average water loss in residential piping. See our Rheem IKONIC Series guide or shop Rheem tankless.
American Water Heaters (Adapt)
The American Adapt models at PlumbersCrib.com do not have a built-in recirculation pump, but recirculation can still be added. American states that its tankless models with X3 Scale Prevention Technology are recirculation pump capable when used with a crossover valve, which is sold separately. That makes the crossover valve approach described above the path for adding recirculation to an Adapt X3 installation.
Learn more in our American Adapt X3 guide or shop American tankless water heaters.
Intellihot
Intellihot's commercial units take a different approach — on some models, recirculation is part of the required installation rather than an add-on.
- iQ251 and iQ251D (wall-hung) — require constant external recirculation. Intellihot specifies an external recirculation pump with 12 feet of head or higher, installed per the pump manufacturer's recommendations, and lists minimum recirculation flow rates in the installation manual.
- Building management — Intellihot's iNTouch BMS offers an External Pump Power feature that powers building recirculation.
Shop Intellihot tankless water heaters.
Stiebel Eltron (Tempra electric tankless)
Tempra whole house electric tankless units are not equipped with a pump, but Stiebel Eltron publishes two ways to use them with recirculation:
- Direct recirculation loop — a Tempra can be plumbed directly into a recirculation loop as long as the loop's flow rate is greater than the unit's minimum activation flow rate. To save energy, Stiebel Eltron suggests a temperature sensor on the return line that cuts power to the pump once the return is within 5° to 10°F of the Tempra's set point, and a timer to disable the loop during low-use hours.
- Loop with an SHC Mini-Tank — pairing the Tempra with a Stiebel Eltron SHC Mini-Tank in the loop minimizes wait time and reduces water usage. Stiebel Eltron recommends keeping the run from the tankless unit to the loop to 5 feet maximum and setting both units to about the same temperature.
For sizing, see our Tempra sizing guide or shop Stiebel Eltron whole house electric tankless.
Installation tips
- Plan the loop early. A dedicated return line is far easier to install before drywall goes up.
- Insulate the hot and return piping to reduce heat loss between pump cycles.
- Choose controls to match the household. Timer, schedule, temperature, or on-demand control usually makes more sense than running a pump around the clock.
- Follow the installation manual for loop length, pipe size, pump compatibility, minimum flow, and approved configurations — and check local code requirements.
- Size the unit first. Start with our guide to sizing the Rheem IKONIC.
- Don't forget water quality. In hard water areas, scale protection matters on every tankless install. See Hard Water and Tankless Water Heaters.
Frequently asked questions
Does a tankless water heater eliminate the wait for hot water?
No. Tankless delivers continuous hot water once it arrives, but the water still has to travel through the pipes to the fixture. Recirculation is what shortens that wait.
Do I need a dedicated return line?
Not always. A crossover valve at the farthest fixture can use the cold water line as the return path. Whether a specific tankless unit or pump can be used in a crossover setup depends on the model — American, for example, states its X3 models are recirculation pump capable with a crossover valve. Check the installation manual before choosing a loop design.
Do any tankless water heaters require recirculation?
Yes. Intellihot's wall-hung iQ251 and iQ251D require constant external recirculation as part of the installation.
Can electric tankless water heaters be used with recirculation?
Some can. Stiebel Eltron's Tempra can be plumbed into a recirculation loop as long as the loop flow exceeds the unit's minimum activation flow rate, or paired with an SHC Mini-Tank.
Does recirculation use more energy?
It can. The pump uses electricity and the hot water piping loses heat. Timer, scheduled, temperature, and on-demand controls — plus pipe insulation — keep that penalty down compared with continuous circulation.
Why order from PlumbersCrib.com?
- Leading brands — Rheem, American, Intellihot, and Stiebel Eltron tankless water heaters and accessories in one place.
- Expert help — our team can help you choose the right unit and recirculation setup for the job.
- Free shipping — on orders over $199 (restrictions apply).
Browse all tankless water heaters and tankless accessories, or contact our team at 877-881-5501 for help choosing a recirculation setup.