Tankless Recirculation Not Working

What “not working” usually means

Recirculation is a loop: the pump pushes hot water around the plumbing so it is already warm when you open a tap. When it stops doing that you get the wait you had before it was installed — the heater itself is usually fine, and that is the useful starting assumption.

Two failures look identical at the tap and are not the same problem:

• The loop is not moving water. Air, a closed valve, or a clogged filter. This is the common case and most of it is yours to fix.
• The pump has failed. Real, but less common than it is blamed for, and the manufacturers both put it well down their own lists.

The free checks, in the order the manufacturers give them

1. Every valve in the loop is fully open. Navien’s first instruction for an external recirculation fault is exactly this: make sure all water supply and return lines are open. A valve closed during other plumbing work and never reopened is the single most common cause, and it costs nothing to rule out. Walk the loop and look.

2. The inlet water filter is clean. A clogged filter starves the loop. Rinnai names it in its own recirculation checklist, and it appears again in almost every other fault on this site — it is the cheapest maintenance on a tankless heater and the most skipped. How to clean the inlet water filter.

3. There is no air trapped in the loop. Run hot fixtures until you can no longer hear air at them. Rinnai’s procedure goes further — bleeding the system at a valve below the heater — but that is pressurised water and worth leaving to someone who has done it.

4. The pump has power. Rinnai specifies 120 VAC at the pump wiring harness. Checking that a plugged-in pump still has a live outlet is reasonable; opening a wiring harness to measure it is not.

Where the two brands genuinely differ

This is worth knowing before someone quotes you for a part.

Rinnai configures recirculation differently by generation. On the older non-SENSEI units the switches on the control board have to match the type of system installed — crossover or dedicated — while the SENSEI and SENSEI RX generations use a software parameter instead. If your recirculation was recently added, changed, or the board was replaced, a mismatched setting will produce a low-flow code even though every valve is open and the pump is fine. That is a service adjustment, not a part.

Navien splits its guidance by where the pump is. For an external system, check the supply and return lines are open; for an internal one, the direction of the 2-way valve. The first-generation NPE manual says this more clearly than the NPE-2 manual, which reduces the same code to a single line about whether the pump is running.

Crossover systems have one extra trap

If your loop uses a crossover valve at the far fixture rather than a dedicated return line, there is a filter on the recirculation side of the heater and a bypass plug that has to be fitted correctly for the mode you are in. Getting that pairing wrong — plug where the filter should be, or the reverse — produces the same symptom as a failed pump. It is a common after-effect of a service visit where the unit was drained.

When it is not the recirculation at all

If hot water arrives eventually but never gets properly hot, or goes hot then cold, the loop is probably working and something else is wrong. Try water not hot enough or cold water sandwich instead.

If the unit is also showing a code, start there — the codes below are the recirculation faults each manufacturer documents.