Recirculation is the most-requested Rinnai feature — and the area where Rinnai has the deepest engineering lead over Navien, Rheem, and Noritz. Two architectures, three configurations.
Why recirculate?
Without recirculation, a tankless heats water on demand — but the cold pipe between the unit and the farthest fixture still has to drain before hot water arrives. In a large house this can mean 30-90 seconds of wasted cold water and a perceptible "hot, then cold, then hot" cycle (the cold-sandwich effect). Recirculation keeps the loop primed.
Option 1: Sensei RX or Sensei+ with built-in pump
The premium option. The Sensei RX (RX130, RX180, RX199) and Sensei+ (RUR199, RUR98) have a recirculation pump inside the cabinet plus a dedicated return-line port. You wire it once, hook up the return line, and the unit's Circ-Logic firmware handles scheduling.
Required: a dedicated return line from the farthest fixture back to the unit. In new construction this is trivial; in retrofits it's expensive ($1,500-3,000 for a plumber to fish a 3rd line).
Option 2: external pump + dedicated return line
Standard Sensei (RU) or any tankless can recirculate with an external Grundfos UP15 or Taco 006 pump installed on a return line. Pump mounts inline. Schedule via timer or building automation. Roughly $200-400 in parts plus install.
Option 3: external pump + crossover valve (retrofit-friendly)
If you don't have a return line, install a thermostatic crossover valve at the farthest fixture (under sink, behind shower wall). The crossover routes hot-line water back through the cold line when the loop temp drops below 95°F, creating a virtual loop on the existing two-pipe plumbing. Pump still required at the heater. $300-500 in parts.
Tradeoff: the cold line carries warm water periodically, so the "instant cold" tap experience suffers slightly. Most homeowners don't notice.
Scheduling: timer vs learning vs always-on
- Always-on: simplest, highest energy cost. Burner cycles every 5-15 min to maintain loop temp.
- Timer-based: set windows (6-8 AM, 5-7 PM). Saves significant energy. Easy via ControlR app.
- Learning (Circ-Logic): on Sensei RX / Sensei+, the firmware learns your usage pattern over 2 weeks and runs the pump only when it predicts a draw. Best efficiency.
- On-demand button: push a wireless button under each sink before you turn the tap. Maximum efficiency, minimum convenience. Some homeowners run this for outlier fixtures.
Recirculation and warranty
External pumps don't affect the Rinnai warranty. Improper return-line plumbing (no isolation valves, no check valve, undersized line) can void warranty if it causes back-pressure damage.
Bottom line
If you're spec'ing new construction, run the return line and buy a Sensei RX — cheapest and best long-term. If retrofitting and budget allows the return line, install a Grundfos with timer. If retrofitting on a budget, crossover valve. Avoid always-on schedules — they double your gas bill on the recirc side.