Quick answer
Rinnai error code 25 means a condensate pump error or a clogged drain: condensate has backed up and tripped the condensate safety (float) switch. The heater stops so acidic condensate cannot overflow, and it will not run until the water can drain. Confirm the condensate pump is plugged in and running and that the drain line is not frozen or kinked. If the code stays, call a technician.
A condensing tankless heater makes water as well as heating it, and Rinnai error code 25 means that water has nowhere to go: the manufacturer lists it as a condensate pump error or clogged drain. Rinnai's guide adds that the code appears only when the condensate safety switch contacts close, which happens when condensate backs up and trips the float switch or when the accessory pump stops working. The pump is a third-party add-on, so read both the pump's own label and the label inside the panel of the heater; where they disagree with a general guide, they win.
Can the heater keep running with code 25?
It will not, and you should not try to make it. The heater stops to prevent condensate overflowing into the cabinet or onto the floor. Condensate from a gas appliance is acidic: left to spill, it marks concrete, corrodes the metal it sits on and damages flooring. Mop up any that has escaped and sort out the drainage before you reset; the control holds the burner off for as long as the float switch stays tripped.
The safe check: pump power and the drain line
- Find the condensate pump: a small plastic box on the floor or wall near the heater with a thin tube leaving it. Not every installation has one; some drain by gravity to a floor drain.
- Confirm the pump is plugged in and its outlet is live. A tripped GFCI outlet or a plug knocked loose is enough to set the code.
- Look at the pump reservoir. Full of water and silent means the pump is not running.
- Trace the discharge tube and the drain line for kinks, sags and slime plugs, and for ice wherever the line passes through an unheated garage, a crawl space or an outside wall.
- Reset the heater once the water has drained away. If the reservoir refills and the code returns, stop there.
What trips the condensate safety switch
- An accessory condensate pump that has failed or lost power. These pumps have a small motor and a float of their own, and both wear out.
- A clogged or frozen drain line. Rinnai's checklist includes confirming the line is neither clogged nor frozen and that it has an air gap.
- A restricted condensate trap, packed with debris.
- A loose, damaged or broken wire between the pump and the control board, or between the trap and the board, which can set the code with no water present.
What a technician checks when the pump runs and the line is clear
If the pump runs, the line is clear and code 25 stays, the fault is in the trap, the switch or the wiring. The technician confirms power at the pump and tests that it lifts water, checks the drain for an air gap, opens and cleans the condensate trap, and follows the safety-switch wiring back to the board looking for a damaged connector.
Where a line freezes every winter, the lasting fix is rerouting it through heated space or to an indoor drain, not thawing it each January. Any opening of the heater cabinet is gas-appliance work for a TSSA-licensed technician, even when the fault turns out to be water.
When you book, say whether the heater drains by gravity or through a pump, and whether that pump also serves a furnace or air conditioner. A shared pump changes where the technician starts.
Cost to fix a Rinnai code 25
- Condensate drain cleaning or trap replacement: $150–$350.
- New condensate pump, installed: $250–$450.
- Diagnostic visit: $90–$150, deducted from the repair if you proceed.
Rerouting a drain line that freezes is quoted in writing once we have seen the route, since the length and the walls it crosses set the price.
Both codes can involve condensate, which is why they get confused. Code 10 covers blocked air supply or exhaust and, on SENSEI models, a full condensate trap inside the heater. Code 25 is specifically the accessory pump and the float switch circuit outside it.
If the pump itself is finished, condensate pump replacement covers the swap, and why a condensate pump overflows deals with pumps shared with a furnace. For anything past the pump, book tankless water heater repair through the contact page or at (647) 801-1252. The remaining codes are listed in the Rinnai error code guide.