Navien Leaking From the Bottom
The Short Answer: Water under a Navien is not one failure. Cool, slightly acidic drip from the drain fitting is condensate. Hot potable water from the brass PRV is pressure. A drip from the cabinet bottom while those two are dry is often a heat exchanger or an internal fitting — shut the unit down. Do not run a Navien with a wet board.
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Search Intent & The Real Problem
You pulled the cover or looked under an NPE and found a puddle. Indoor units hide three wet systems in one jacket: potable water, condensate, and the T&P. Outdoor units add rain. Treating all of them as “the tankless is shot” is how people replace a $1,800 exchanger because a 1/2” condensate union was hand-tight.
Related: condensate drain leaking, Rinnai T&P.
What other guides get wrong
They call condensate “a leak” and tell you to seal the drain with tape. They also skip temperature: condensate is cool; T&P discharge is house-temp to hot; an exchanger leak is usually hot and from inside the jacket. One paper towel test saves a wrong part.
Step-by-Step Troubleshooting
Step 1: Identify the fluid
| Clue | Source |
|---|---|
| Cool, only while the burner runs, from the PVC drain | Condensate — normal if piped, a leak if the joint failed |
| From the brass T&P, pulses on a call for heat | Pressure/expansion — do not cap |
| Hot, from inside the cabinet, filter housing, or exchanger | Potable leak — shut down |
| Only after rain, outdoor unit | Wind-driven rain or a missing cap — not a “bottom leak” |
- Required tools: Paper towel, flashlight, dry the area, run one hot faucet.
Step 2: Condensate path
- What to look for: Trap full, drain line uphill, frozen termination, loose union at the cabinet, neutralized media blocking flow so it overflows inside. Indoor NPE units will wet the pan and look like a bottom leak.
- Required tools: Wet/dry vac on the drain only.
Clearing an external trap is DIY. Water behind the cover on the PCB is not.
Step 3: PRV versus exchanger
- What to look for: T&P on the hot outlet with a discharge pipe. If that pipe is dry and the cabinet is wet, you are past “just a valve.” White scale trails down the exchanger, or a drip from the bottom pan with isolation valves closed (after residual drains), points at the exchanger or an internal waterway.
- Required tools: Isolation valves.
Close cold and hot isolation. If it still weeps from inside after the unit depressurizes, that water is from the exchanger or a cracked internal fitting.
Step 4: Filter housing and isolation-valve packing
- What to look for: Inlet filter cap not seated, cracked polymer housing, isolation-valve stem packing. These mimic “leaking from the bottom” because they drip down the front.
- Required tools: Eyes on the cold-side service assembly.
A new filter cap O-ring is a parts-store save. A cracked heat exchanger is a licensed replacement — often the whole unit if the exchanger is not sold separately for that year.
Step 5: Freeze and overpressure
- What to look for: Outdoor NPN that iced, indoor unit in a garage that hit 15°F with no freeze kit. Freeze-split exchangers leak from the bottom on the first thaw. High incoming psi with no expansion path lifts the T&P every heat cycle.
- Required tools: Hose-bib gauge; freeze history.
When DIY Ends: Calling a Licensed Pro
Stop at a wet PCB, a stream from inside the jacket, gas smell, or a freeze-split exchanger. You may snug a condensate union, clean a trap, and rinse the inlet filter. Heat-exchanger diagnosis, combustion, and replacement are licensed. Navien will not cover an exchanger that ran with a blocked condensate line or a unit that froze with power off.
Frequently Asked Questions
Q: Why is my Navien leaking from the bottom? A: Condensate overflow, a loose drain fitting, T&P discharge, a filter housing, or a heat exchanger. Identify whether the water is cool condensate or hot potable before you buy parts.
Q: Can I keep using a Navien that drips from the bottom? A: Not if the cabinet or board is wet, and not if the exchanger is the source. Condensate at a proper drain can wait a day. Internal potable leaks cannot.
Q: Is a Navien heat-exchanger leak repairable? A: Internal exchanger leaks are replacement of the exchanger or the unit. External fittings and the condensate trap are repairable. Do not epoxy the exchanger.