HVACopediaRinnai · Tankless water heater
63 Recirculation low flow
What the control is telling you
Flow in the recirculation loop is too low, typically from air in the system, clogged filters, plumbing leaks, incorrect recirculation settings or pump problems. RH180 units flag flow below 1.3 GPM.
Applies to: SENSEI Series, SENSEI RX/CX Series, Non-SENSEI Series (incl. RH180-specific checks)
Likely causes
- Recirculation DIP switches or Parameter 04/05 set incorrectly
- Parameter 04 set to B with no pump connected
- Clogged inlet or bypass filters / bypass plug not removed in crossover mode
- Air trapped in the plumbing or recirculation line
- Leak in building plumbing pulling air into the loop
- Pump not receiving 120 VAC
- Recirculation loop too long for the pipe size
- Closed inlet/outlet valves or restricted return line / faulty check valves
Test and repair steps
- Non-SENSEI Confirm DIP switches match the recirculation type in use (crossover or dedicated); see the owner's manual.
- Crossover valve (COV) mode confirm the bypass plug has been removed and the bypass filter is installed.
- Warning: Shut off the water supply and drain the heater before removing inlet or bypass filters. Clean the filters, reinstall them and bleed all air from the plumbing.
- Confirm the cold inlet and hot outlet valves are open; purge air from the recirculation line by running water until no air is heard at fixtures.
- Open the bleeder valve below the heater to release air. CAUTION: the system is pressurized; catch water in a bucket and be ready to shut the cold feed if the fitting is backed out too far.
- Confirm the recirculation pump receives 120 VAC at its wiring harness.
- Dedicated return check that line length is within the maximum for the pipe size, Economy/Comfort is set correctly, and (water off) the plug is installed in the recirc-side filter housing.
- SENSEI / SENSEI RX/CX Check Parameter 04 (A none, B dedicated, C crossover/long loop, D short loop – SENSEI only) and Parameter 05 (A Economy, B Comfort). With no pump connected, 04 must be A; B causes code 63.
- Isolate house plumbing run a 50–100 ft hose between the valve kit cold and hot connections (adapter needed), open the valve kit, bleed air into a bucket, power up. No code 63 = air or leak in building plumbing.
- Leak test with all fixtures off, put a pressure gauge on an outdoor hose bib, pressurize it, shut the main at the meter/well. Falling pressure = leak; the pump outdraws the leak and sucks in air.
- SENSEI RX/CX If recirculation temperature is inadequate, set pump speed to Max (Parameter 16A); recirculation flow must exceed 0.4 GPM (1.5 L/min).
- Keep loop equivalent length within 400 ft (122 m) for 3/4 in. pipe or 100 ft (30 m) for 1/2 in. pipe.
- RH180 Read flow on the controller (hold UP 3 s, then press ON/OFF; e.g., 5 = 0.5 GPM, 21 = 2.1 GPM). If under 1.3 GPM, do the next step.
- RH180 Bleed air from tank and pump, clean the inlet filter, confirm 6+ GPM supply and 30–50 psi inlet pressure, clean the pump filter (crossover), check return check valves/restrictions (dedicated).
Settings for these units
RSC supply connection sizes · SENSEI
| Connection | Size |
|---|---|
| Gas | 3/4 in. NPT |
| Cold inlet | 3/4 in. NPT |
| Hot outlet | 3/4 in. NPT |
| Condensate drain | 1/2 in. NPT |
| Pressure relief valve | M33 thread |
The PRV and its adapter come in the box; PRV must be rated 150 psi and at least the heater Btu/h.
Condensate drain line must stay at least 1/2 in. over its whole length.
RSC water quality limits · SENSEI
| Contaminant | Maximum level |
|---|---|
| Total hardness | Up to 200 mg/L |
| Aluminum | Up to 0.2 mg/L |
| Chlorides | Up to 250 mg/L |
| Copper | Up to 1.0 mg/L |
| Dissolved CO2 | Up to 15.0 mg/L |
| Iron | Up to 0.3 mg/L |
| Manganese | Up to 0.05 mg/L |
| pH | 6.5 to 8.5 |
| TDS | Up to 500 mg/L |
| Zinc | Up to 5 mg/L |
Damage from water above these limits is not covered by warranty.
Hard water should be treated; use the Service Soon (SS) reminder (parameter 03) for flushing intervals.
- RSC clearances to combustibles · SENSEI
- RSC temperature settings available · SENSEI
- RX supply connection sizes · SENSEI RX/CX
- RX water quality limits · SENSEI RX/CX
- RX water treatment and flushing by hardness · SENSEI RX/CX
- RX clearances to combustibles · SENSEI RX/CX
Models that show this code
89 models in Non-SENSEI, RH180 hybrid, SENSEI RX/CX, SENSEI
Other codes on the same units
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Source: Rinnai Troubleshooting TWH Diagnostic Codes (100000652), p.53–54. Summarized in our own words; check the original manual for your model.
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