Recirculation pump
The pump that keeps hot water moving around the plumbing so it is already warm at the tap. Built into some units, bolted on outside others — and the part most often blamed for a fault that turns out to be a closed valve.
What it does
A tankless heater heats water on demand, which solves one problem and creates another: the water sitting in the pipe between the heater and your shower is still cold. A recirculation pump moves that water back through the loop so it is already hot when you open the tap.
Two arrangements exist and they fail differently:
- Internal. The pump is inside the heater. Rinnai’s A-suffix and RUR models, Navien’s A models. The unit knows about it and reports on it.
- External. A separate pump on the pipework, sometimes with a crossover valve at the far fixture instead of a dedicated return line.
Which one you have changes the diagnosis, which is why Navien splits its guidance in two: for external systems, check the supply and return lines are open; for internal, check the direction of the 2-way valve.
Why it gets blamed unfairly
Most “the recirculation pump has failed” call-outs are not the pump. They are a valve closed during other work and never reopened, air trapped in the loop, or a clogged inlet filter starving it. All three produce the same symptom — you wait for hot water again — and all three cost nothing to check.
Work through recirculation not working before buying anything.
There is also a configuration failure that looks like a dead pump and is not: on Rinnai’s older non-SENSEI units the control board has to be set for the type of system installed, crossover or dedicated, while the SENSEI generations use a software parameter. A mismatch after a board replacement or a system change produces a low-flow code with a perfectly good pump.
Why it fails, when it does
Bearings and seals. Rinnai documents one failure mode specifically worth knowing about on RUR models: water weeping from the aluminium pump casing means the O-ring on the pump shaft is letting water into the motor windings, and the symptom shows up as the heater tripping a GFCI receptacle or breaker rather than as a recirculation complaint at all. The fix there is the pump-motor assembly, not the electrics.
Why this is not a DIY part
An internal pump means opening a gas appliance and draining it. An external pump is closer to ordinary plumbing, but it is wired to line voltage and its control logic has to match what the heater expects — a pump running continuously where the heater expects to control it also voids part of Navien’s warranty terms, which distinguish controlled from uncontrolled recirculation.
What a replacement usually costs
$120-500 for the pump depending on internal or external. Installed, $350-900. Before agreeing to it, ask whether the valves and the filter were checked — they are the manufacturers’ own first steps.