Rinnai Leaking From the Bottom
The Short Answer: Water under a Rinnai is not one failure. Cool, slightly acidic drip from the drain fitting is condensate. Hot potable from the brass PRV is pressure. A drip from the cabinet bottom while those two are dry is a heat exchanger or an internal fitting — shut it down. Do not run a Rinnai with a wet board.
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Search Intent & The Real Problem
You pulled the cover or looked under an RU/RUR/SENSEI/R53 and found a puddle. Indoor units hide three wet systems in one jacket: potable water, condensate, and the T&P. Outdoor units add rain and freeze-split fittings. 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: Rinnai T&P, leaking by exhaust, Navien from the bottom, code 25.
What other guides get wrong
They call condensate “a leak” and tell you to seal the drain with tape. They 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. They also Flex Seal a freeze-split outdoor R53 and call it a repair.
Step-by-Step Troubleshooting
Step 1: Identify the fluid
| Clue | Source |
|---|---|
| Cool, only while the burner runs, from the PVC/vinyl 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” |
| After a freeze/outage thaw | Split fitting or exchanger — after a power outage |
- What to look for: Dry it. Wait two minutes. Open one hot faucet. See which bead returns.
- Required tools: Paper towel, flashlight.
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 condensing units will wet the pan and look like a bottom leak. Code 10 or 25 often rides along.
- 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. Static over ~80 psi at a hose bib. Waterlogged expansion tank (Schrader spits water). 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, hose-bib gauge.
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. Service-valve union that was never tightened after a flush.
- 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 unit that iced with power off. Indoor 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. Code 79 on some firmware is a leak sensor — treat it as wet until proven otherwise.
- Required tools: Freeze history. Do not epoxy the exchanger.
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. Rinnai 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 Rinnai 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 Rinnai 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 Rinnai 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.