Condensate trap and condensate pump
A condensing heater makes water as a by-product of burning gas, and that water is acidic. The trap holds it, the drain carries it away, and a pump lifts it when gravity will not. Freeze any part of that and the unit stops.
What it does
A condensing tankless heater extracts so much heat from its exhaust that water vapour in the flue gas condenses back to liquid. That liquid is mildly acidic, and it has to leave the building.
The trap is a small reservoir that holds a water seal so exhaust gas cannot travel up the drain line. The drain line takes the condensate away. Where the drain point is above the heater — a basement installation, typically — a condensate pump lifts it, and a float switch inside that pump tells the heater to stop if the level rises too far.
Why it stops the whole heater
Because it is a safety interlock, not an inconvenience. If condensate backs up, the unit shuts down rather than let it overflow inside the cabinet or push exhaust gas out through a dry trap.
Rinnai gives a blocked condensate trap its own place in the code 10 family on its SENSEI and SENSEI RX generations — and notably not on the older non-SENSEI units, where code 10 is about the vent path alone. A backed-up condensate pump or a triggered float switch is a separate number again, code 25.
Why it fails
Overwhelmingly: freezing. A condensate line run through an unheated space or terminated outside will freeze solid in the first hard cold snap, back up into the trap, and stop the heater — on the coldest morning of the year, which is exactly when you notice. The second cause is a drain line that sags, holds water, and eventually blocks. The third is a pump that has simply lost power, because these are often plugged into an outlet shared with something else.
What you can do yourself
This is one of the few genuinely homeowner-side items on this list, and it is mostly looking rather than working:
- Follow the condensate line from the unit to where it drains. Look for a kink, a sag holding water, or ice where it passes somewhere cold.
- Check the pump still has power — a tripped shared outlet is common.
- Empty and check the pump reservoir if it has one.
- Confirm the drain line has the air gap the manufacturer asks for.
Thawing a frozen line and rerouting it out of the cold is a real fix, not a workaround. What is not yours is the trap inside the cabinet.
What a replacement usually costs
A condensate pump is $40-220 and many are a plug-in swap once you can reach them. The trap itself is inside the unit and comes with a service call: budget $150-350 all-in.