Rheem Tankless Leaking Inside the Cabinet
The Short Answer: Water inside a Rheem tankless cabinet is not automatically a dead exchanger. Cool acidic drip is condensate on Prestige-class condensing units. Hot water from the brass PRV is pressure. A weep at the filter cap or a service-valve union is an O-ring. Water on the fins or the PCB with those dry is the exchanger — shut it down. Do not run a wet board.
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Search Intent & The Real Problem
You pulled the front cover on an RTG, RUTG, RTGH, or Prestige condensing Rheem and found water on the exchanger, the fan, or the PCB — not just a neat puddle on the slab. Gravity still lies. A condensate union, a T&P tube, or a filter cap tracks down and looks like “the heater is shot.” Treating every internal drip as a $1,800 exchanger is how people replace a Paloma-class box because a 3/4” isolation union was hand-tight.
This page is water inside the jacket. Puddle under the case with the cover on is the same physics as Rinnai from the bottom, Navien from the bottom, Noritz from the bottom, Takagi leaking. A storage tank wet in the foam is dripping inside, not this appliance.
What other guides get wrong
They call condensate “a leak” and tape the drain. They skip temperature: condensate is cool; PRV discharge is house-temp to hot; an exchanger leak is usually hot and from the copper or stainless fins. They acid-flush through the house plumbing. And they Flex Seal a freeze-split outdoor RTG and call it a repair. Paloma-based non-condensing RTGs should not be raining condensate onto the board — water there is potable, rain, or freeze until proven otherwise.
Step-by-Step Troubleshooting
Step 1: Identify the fluid, inside the cabinet
| Clue | Source |
|---|---|
| Cool, slightly acidic, only while the burner runs, from the PVC/PP drain or trap | Condensate — Prestige / condensing. Normal if piped; a leak if it dumps on the PCB |
| From the brass PRV, pulses on a call for heat | Pressure / expansion — do not cap |
| Filter-cap threads, isolation-union, polymer water-valve body, hex drain plug | Service fittings — often a save |
| Hot, from the exchanger fins, collector, or bottom pan with fittings dry | Potable leak — shut down |
| Only after rain, outdoor RTG | Wind-driven rain or a missing cap — after rain |
| After a freeze night, then a thaw | Split exchanger or freeze-relief port |
- What to look for: Dry it. Run one hot faucet. Wipe again. Temperature of the drip, and whether it is the drain, the PRV tube, the filter, or the fins. Water beading on the PCB is a shutdown even if the source is “just condensate.”
- Required tools: Paper towel, flashlight. Optional pH strip.
Step 2: Condensate path (Prestige / condensing only)
- 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 Rheems will wet the pan and the board and look like an exchanger failure. A blocked collector can also shove droplets out the flue on post-purge — same physics as Rinnai leaking by exhaust. Sister drain: Navien condensate.
- Required tools: Eyes on the drain path, wet/dry vac on the condensate drain only.
Non-condensing outdoor RTG-64/84/95 should not have a condensate hose raining into the case. If they do, you are looking at potable water, rain, or a cracked collector. Clearing an external trap is DIY. Water behind the cover on the PCB is not.
Step 3: PRV versus house pressure
- What to look for: Relief on the hot outlet with a discharge pipe. Static pressure over ~80 psi at a hose bib. Waterlogged expansion tank (Schrader spits water) — expansion tank full. PRV that dribbles after someone lifted the test lever. Same story as Rinnai T&P. Excessive dump from freeze ports or open service bibs: tankless drains excessively.
- Required tools: Hose-bib gauge, tire gauge on the expansion tank.
If that pipe is dry and the cabinet is wet, you are past “just a valve.” Never plug a relief.
Step 4: Filter housing, isolation valves, drain plugs
- What to look for: Cold-inlet screen cap not seated after a flush, cracked polymer housing, isolation-valve stem packing, heat-exchanger drain plugs left weeping. Rheem/Paloma water-flow servo bodies leak at O-rings and drip down the front onto the pan — looks like “leaking inside.” Recirc-pump models (some Prestige / built-in pump kits) weep at pump unions into the same cavity.
- Required tools: Isolation valves. Inlet-filter wrench or taped channel locks. Water off.
A new cap O-ring is a parts-store save. Close cold and hot isolation. If it still weeps from inside after the unit depressurizes, that water is the exchanger or a cracked internal waterway.
Step 5: Heat exchanger and freeze
- What to look for: White scale trails down the copper (non-condensing Paloma) or the stainless secondary (condensing Prestige). Isolation valves closed, residual drained, drip continues from the fins. Outdoor RTG that iced with power off — freeze-split exchangers leak on the first thaw, same as Rinnai after a power outage. Error history 14 / 16 next to a wet jacket is scale plus a leak, not a reset loop. Code 11 after a flood is a drowned igniter, not the first problem — Rheem code 11.
- Required tools: Isolation valves, freeze history. No epoxy, no RTV on the exchanger.
Internal exchanger leaks are replacement of the exchanger or the unit. Do not silicone the cabinet floor. Do not keep firing a unit with water in the burner — that is a carbon monoxide path.
When DIY Ends: Calling a Licensed Pro
Stop at a wet PCB, a stream from inside the jacket, gas smell, a freeze-split exchanger, or a PRV that is streaming. You may snug a condensate union, rinse the inlet filter with water isolated, and photograph the drip. Heat-exchanger diagnosis, combustion, and replacement are licensed. Rheem will not cover an exchanger that ran with a blocked condensate line or a unit that froze with the switch off.
Frequently Asked Questions
Q: Why is my Rheem tankless water heater leaking inside? A: Condensate overflow on condensing models, a loose drain or filter fitting, PRV discharge, or a heat exchanger. Identify whether the water is cool condensate or hot potable before you buy parts.
Q: Can I keep using a Rheem tankless that drips inside the cabinet? A: Not if the board, fan, or burner 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 Rheem tankless heat-exchanger leak repairable? A: Internal exchanger leaks are replacement of the exchanger or the unit. External fittings, filter caps, and the condensate trap are repairable. Do not epoxy the exchanger.