25
Rinnai Tankless Water Heater
Severity: ModerateWhat Does This Error Mean?
Rinnai code 25 only appears on condensing models (Sensei series and similar).
It means the unit detected water level above the safe mark inside the condensate trap — usually because the drain hose is blocked, kinked, or routed uphill.
Clearing the path is a 15-minute job in most cases.
Letting condensate back up into the heat exchanger area is a real risk, so do not ignore code 25.
Affected Models
- Rinnai Sensei RU160i
- Rinnai Sensei RU180i
- Rinnai Sensei RU199i
- Rinnai RUC series condensing
- Rinnai RUR series condensing
Common Causes
- Condensate drain hose kinked behind the unit
- Hose routed uphill instead of downhill from the unit
- PVC trap clogged with biofilm or debris
- Outdoor termination iced over in freezing weather
- Floor drain backed up where condensate empties
- Neutralizer cartridge saturated and restricting flow
How to Fix It
-
Power off and find the drain hose.
Switch off the breaker.
Locate the small clear or white hose at the bottom of the unit — that is the condensate drain.
It either runs to a floor drain, a condensate pump, or a neutralizer cartridge. -
Check for kinks or uphill runs.
Trace the hose from the unit to its termination.
The line should slope continuously downward.
Any uphill loop or sharp kink will eventually block flow.
Reroute or shorten the hose so gravity does its job. -
Flush the trap.
Disconnect the hose at the lowest accessible point.
Pour a cup of warm water into the trap from above and watch it drain.
Slow drain = blockage in the trap or hose.
A cup of plain hot water followed by a small drinking-straw to dislodge biofilm clears most clogs. -
Inspect the neutralizer.
If a condensate neutralizer is installed, the limestone or magnesium media inside saturates over 1-2 years and can block flow.
Replace the media (15-40 USD) or replace the cartridge entirely. -
Check outdoor termination in winter.
If the drain runs outside, ice can form at the termination during a freeze.
Pour warm water over the outlet to thaw, then reroute through a heated space if this is a recurring problem. -
Reset and test.
Restore power.
Run hot water for 10 minutes.
Condensate should drip steadily from the drain when the burner is firing.
No drips at all when the heater is firing means the trap is still blocked.
Frequently Asked Questions
Can I just disconnect the drain to clear the code?
No.
The condensate is mildly acidic and damages flooring, drywall, and metal if it leaks freely.
The neutralizer or floor drain is there for a reason.
Fix the blockage rather than bypass the path.