What causes Rinnai Tankless Water Heater 63?

Air trapped in the recirculation line, the pump or the heater's plumbing, often after installation or after the water was shut off. A clogged inlet water filter or bypass filter. The cold water inlet or hot water outlet valve not fully open. Recirculation set up wrong for the plumbing: DIP switches (non-SENSEI) or Parameter 04 (SENSEI) not matching crossover or dedicated recirculation, or the bypass plug and bypass filter in the wrong place for the mode. On SENSEI models, Parameter 04 set to B (dedicated recirculation) when no circulation pump is connected. The pump not getting its 120 VAC supply, or a loose or damaged pump harness to the PC board. A dedicated return line that's too long for its pipe size, has a stuck check valve or a restriction. A leak in the home's plumbing, which Rinnai says lets the pump pull air into the system. Low inlet water supply or pressure; Rinnai's RH180 checks call for 30 to 50 psi and a supply of six gallons per minute or more.

How do I fix Rinnai Tankless Water Heater 63?

Write down the code and your model number. Code 63 checks differ between SENSEI and non-SENSEI models and between crossover and dedicated-return setups. Make sure the cold water inlet and hot water outlet valves under the heater are fully open. Purge air from the loop: run hot water at the fixtures served by the recirculation line until you no longer hear air, as Rinnai's manual describes. If you're comfortable doing it and your manual covers it, clean the inlet water filter (and the bypass filter in crossover mode). Rinnai says to turn off the water supply and drain the heater before removing these filters, and to bleed air from the plumbing afterward. If you don't actually have a recirculation pump or loop, check the recirculation setting. On SENSEI models, Rinnai says Parameter 04 should be A (no recirculation) when no pump is connected; your owner's manual shows how to change parameters. If you hear running water when nothing is on or your water meter moves with every fixture off, you may have a leak, which Rinnai lists as a cause of code 63. Call a plumber. If 63 keeps coming back, contact a licensed plumber or Rinnai. If the heater is under warranty, talk to Rinnai or an authorized technician before anyone replaces parts.

When should I call a pro for Rinnai Tankless Water Heater 63?

Confirming the recirculation mode matches the plumbing: DIP switch positions on non-SENSEI models, Parameter 04 (A no recirculation, B dedicated, C crossover long loop, D crossover short loop) and Parameter 05 (A Economy, B Comfort) on SENSEI models. In crossover (COV) mode, making sure the bypass plug is removed and the bypass filter installed; in dedicated mode, making sure the plug is installed in the filter housing on the recirculation side. Clearing the inlet and bypass filters, then bleeding air through the bleeder valve below the heater and from the recirculation line. Checking for 120 VAC at the pump wiring harness and inspecting the pump harness back to the PC board. On dedicated returns, checking that the line doesn't exceed Rinnai's maximum length for the pipe size and that check valves work and the line isn't restricted. Isolating the house plumbing by looping a 50 to 100 ft hose between the valve kit's hot and cold connections. If the heater runs the pump without code 63 on the hose loop, Rinnai says the problem is air or a leak in the home's plumbing. Pressure-testing the house: with every fixture off, putting a gauge on an outdoor hose connection, shutting off the main at the meter or well, and watching for pressure bleed-down that points to a leak. On RH180 models, reading the flow rate on the temperature controller (UP for 3 seconds, then ON/OFF; 5 means 0.5 GPM) and, below 1.3 GPM, checking air, filters, water supply, 30 to 50 psi inlet pressure, the pump filter in crossover mode and the return line.

Code: 63🚿 Rinnai Tankless Water Heater

Rinnai Tankless Error Code 63: Recirculation Low Flow (Meaning, Causes, Fixes)

Q: What does Rinnai Tankless Water Heater 63 mean?

Code 63 on a Rinnai tankless water heater means Recirculation Low Flow: the heater's recirculation pump loop isn't moving enough water. It only shows up on heaters set up for recirculation, and the usual culprits are air in the lines, a clogged inlet or bypass filter, a closed valve, the wrong recirculation setting, or no power to the pump. A homeowner can check valves, settings and filters; leak testing and pump wiring checks are best left to a plumber. Rinnai's Troubleshooting Tankless Water Heater Diagnostic Codes manual names code 63 Recirculation Low Flow and applies it to both SENSEI and non-SENSEI models. The heater's built-in recirculation pump (or Rinnai's ThermaCirc360 setup on pump-equipped models) pushes water around a loop so hot water reaches the taps faster, either through a crossover valve under a sink or a dedicated return line. If the pump runs but the heater doesn't see enough flow through that loop, it sets code 63. For RH180 models Rinnai's manual gives the threshold as a circulation flow rate below 1.3 GPM. Rinnai's steps cover the recirculation mode settings (DIP switches on non-SENSEI models, Parameter 04 on SENSEI models), the bypass plug and bypass filter, the inlet water filter, air in the plumbing, the 120 VAC pump supply and, finally, leaks in the house plumbing. Rinnai explains that a leak can cause code 63 because when the pump comes on it draws a higher flow rate than the leak, which sucks air into the system. On SENSEI models, Parameter 04 set to B (dedicated recirculation) with no pump connected will also trigger code 63.

Reviewed 2026-10-23 · Based on 3 manufacturer & service sources

What the code means

Rinnai's Troubleshooting Tankless Water Heater Diagnostic Codes manual names code 63 Recirculation Low Flow and applies it to both SENSEI and non-SENSEI models. The heater's built-in recirculation pump (or Rinnai's ThermaCirc360 setup on pump-equipped models) pushes water around a loop so hot water reaches the taps faster, either through a crossover valve under a sink or a dedicated return line. If the pump runs but the heater doesn't see enough flow through that loop, it sets code 63. For RH180 models Rinnai's manual gives the threshold as a circulation flow rate below 1.3 GPM. Rinnai's steps cover the recirculation mode settings (DIP switches on non-SENSEI models, Parameter 04 on SENSEI models), the bypass plug and bypass filter, the inlet water filter, air in the plumbing, the 120 VAC pump supply and, finally, leaks in the house plumbing. Rinnai explains that a leak can cause code 63 because when the pump comes on it draws a higher flow rate than the leak, which sucks air into the system. On SENSEI models, Parameter 04 set to B (dedicated recirculation) with no pump connected will also trigger code 63.

Likely causes

Air trapped in the recirculation line, the pump or the heater's plumbing, often after installation or after the water was shut off
A clogged inlet water filter or bypass filter
The cold water inlet or hot water outlet valve not fully open
Recirculation set up wrong for the plumbing: DIP switches (non-SENSEI) or Parameter 04 (SENSEI) not matching crossover or dedicated recirculation, or the bypass plug and bypass filter in the wrong place for the mode
On SENSEI models, Parameter 04 set to B (dedicated recirculation) when no circulation pump is connected
The pump not getting its 120 VAC supply, or a loose or damaged pump harness to the PC board
A dedicated return line that's too long for its pipe size, has a stuck check valve or a restriction
A leak in the home's plumbing, which Rinnai says lets the pump pull air into the system
Low inlet water supply or pressure; Rinnai's RH180 checks call for 30 to 50 psi and a supply of six gallons per minute or more

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Write down the code and your model number. Code 63 checks differ between SENSEI and non-SENSEI models and between crossover and dedicated-return setups.
Make sure the cold water inlet and hot water outlet valves under the heater are fully open.
Purge air from the loop: run hot water at the fixtures served by the recirculation line until you no longer hear air, as Rinnai's manual describes.
If you're comfortable doing it and your manual covers it, clean the inlet water filter (and the bypass filter in crossover mode). Rinnai says to turn off the water supply and drain the heater before removing these filters, and to bleed air from the plumbing afterward.
If you don't actually have a recirculation pump or loop, check the recirculation setting. On SENSEI models, Rinnai says Parameter 04 should be A (no recirculation) when no pump is connected; your owner's manual shows how to change parameters.
If you hear running water when nothing is on or your water meter moves with every fixture off, you may have a leak, which Rinnai lists as a cause of code 63. Call a plumber.
If 63 keeps coming back, contact a licensed plumber or Rinnai. If the heater is under warranty, talk to Rinnai or an authorized technician before anyone replaces parts.

🛠 When to call a pro

Confirming the recirculation mode matches the plumbing: DIP switch positions on non-SENSEI models, Parameter 04 (A no recirculation, B dedicated, C crossover long loop, D crossover short loop) and Parameter 05 (A Economy, B Comfort) on SENSEI models.
In crossover (COV) mode, making sure the bypass plug is removed and the bypass filter installed; in dedicated mode, making sure the plug is installed in the filter housing on the recirculation side.
Clearing the inlet and bypass filters, then bleeding air through the bleeder valve below the heater and from the recirculation line.
Checking for 120 VAC at the pump wiring harness and inspecting the pump harness back to the PC board.
On dedicated returns, checking that the line doesn't exceed Rinnai's maximum length for the pipe size and that check valves work and the line isn't restricted.
Isolating the house plumbing by looping a 50 to 100 ft hose between the valve kit's hot and cold connections. If the heater runs the pump without code 63 on the hose loop, Rinnai says the problem is air or a leak in the home's plumbing.
Pressure-testing the house: with every fixture off, putting a gauge on an outdoor hose connection, shutting off the main at the meter or well, and watching for pressure bleed-down that points to a leak.
On RH180 models, reading the flow rate on the temperature controller (UP for 3 seconds, then ON/OFF; 5 means 0.5 GPM) and, below 1.3 GPM, checking air, filters, water supply, 30 to 50 psi inlet pressure, the pump filter in crossover mode and the return line.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Water pressure test gauge (hose thread)

What Rinnai's leak test uses on an outdoor hose connection to see whether the house holds pressure with the main shut off.

[affiliate link slot: AFFILIATE_SLOT_WATER_PRESSURE_GAUGE]

⚙

Tankless water heater flush kit

Useful for maintenance on the same isolation valves; not a fix for code 63 by itself.

[affiliate link slot: AFFILIATE_SLOT_TANKLESS_FLUSH_KIT]

Sources

Last reviewed 2026-10-23. Always confirm against the manual for your exact model number.

Rinnai Tankless Error Code 63: Recirculation Low Flow (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Water pressure test gauge (hose thread)

Tankless water heater flush kit

Related codes

Sources