Tankless Draining Excessively? Find the Source
The Short Answer: A tankless does not store 50 gallons to empty. Water on the floor is condensate, a T&P that is lifting, freeze-protection drains, open service-valve ports, or a heat exchanger. Cool acidic drip while it fires is condensate. Hot water from the brass relief is pressure. Hot water from inside the jacket with the burner off is a shut-down.
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Search Intent & The Real Problem
The unit is “always draining.” There is no tank. People treat a condensing Rinnai, Navien, Noritz, or Rheem like a 40-gallon that someone left the hose on. Condensing heaters make water on purpose: natural gas at high fire on a ~199,000 BTU unit can yield on the order of 1–2 gallons of condensate per hour if it is actually condensing. That belongs in a sloped drain, not a puddle. A stream from the brass T&P is a different failure. A stream from the cabinet with isolation valves open and the unit off is often the exchanger.
Related: Navien condensate, Navien bottom leak, Rinnai T&P.
What other guides get wrong
They blame “mineral buildup” and “the unit is oversized” as if scale made a storage tank appear. Scale makes error 16 / overheat and T&P drips; it does not siphon 20 gallons onto the slab. They also call condensate “a leak” and tell you to pinch the drain hose. A pinched condensate line overflows into the cabinet and kills the board.
Step-by-Step Troubleshooting
Step 1: Name the wet fitting — not “the tankless”
| Clue | Source | Normal? |
|---|---|---|
| Cool, slightly milky, only while the burner runs, from PVC/trap | Condensate | Flow in a drain is normal; puddling is not |
| From the brass T&P, pulses on a hot-water call | Pressure / overheat | Never — do not cap |
| From outdoor freeze-drain ports below ~36–37°F | Freeze protection dumping the waterways | Designed to dump; piping must go to a drain |
| From isolation-valve hose bibs / missing caps | Service kit left open | Tighten/caps; not a dead unit |
| Hot, from inside the jacket, exchanger, or filter housing | Potable leak | Shut down |
| Only after rain at the vent cap | Wind-driven rain or blocked condensate blowing out the flue | See dead after rain |
- What to look for: One dry paper towel per fitting. Run one hot faucet, then idle 20 minutes.
- Required tools: Paper towel, flashlight, phone photo of the rating plate.
Step 2: Condensate path on condensing units
- What to look for: Trap dry or sludge-packed, drain that runs uphill, 3/8” tubing someone stuffed in a 1/2” fitting, outdoor termination frozen, condensate pump float stuck, neutralizer media rock-solid. Indoor closets without gravity to a standpipe need a pump on GFCI; a dead pump is the “excessive drain.”
- Required tools: Level on the PVC, wet/dry vac on the drain only.
Manufacturer drain size is typically 1/2” PVC, sloped down, trapped. A 20-foot hose looped over the unit is not a drain. Clearing an external trap is DIY. Water on the PCB is not.
Step 3: T&P versus house pressure
- What to look for: Static hose-bib pressure over 80 psi. T&P valves on these units are commonly 150 psi / 210°F. City pressure plus a failed PRV, or a waterlogged expansion tank on a closed system, lifts the valve every heat cycle. That looks like the heater is “draining.”
- Required tools: $12 hose-bib gauge. Tire gauge on the expansion tank Schrader (isolate and drain the tank first; precharge should match static, often 50–60 psi).
Fix the PRV and expansion path. Replacing the T&P without that is a 48-hour callback.
Step 4: Freeze drains and service valves
- What to look for: Outdoor Rinnai/Noritz/Rheem drain-down solenoids or freeze drain ports that opened on a 20°F night and never stopped because a valve stuck. Indoor garage unit that hit 15°F with power off — freeze-split exchanger, not a drain. Isolation kit garden-hose ports missing their caps, weeping 24/7.
- Required tools: Eyes under the cabinet, temperature history.
A freeze valve that will not reseat dumps the waterways until a tech rebuilds it. Do not crimp the copper shut and call it winterized.
Step 5: Heat exchanger versus “just keep using it”
- What to look for: Hot potable water from inside the jacket with the condensate fitting dry and the T&P pipe dry. Isolation valves closed, unit depressurized, still weeping from the exchanger = vessel failure. White scale trails, error history 10/12/16, water at the exhaust after shutdown.
- Required tools: Isolation valves. No epoxy.
When DIY Ends: Calling a Licensed Pro
Stop at a wet PCB, a stream from inside the jacket, a T&P that dumps a hard stream, incoming pressure over 80 psi with no PRV, a stuck freeze valve, or gas smell. You may unkink a condensate line, cap a service port, and read a gauge. Combustion, exchanger replacement, and PRV/expansion work on a closed system are licensed. Running a condensing unit with the condensate hose in a bucket you forget is how boards die and warranties void.
Frequently Asked Questions
Q: Why does my tankless water heater drain excessively? A: Condensate overflowing a blocked or uphill drain, a T&P lifting on high pressure or overheat, freeze-protection ports dumping, open service-valve bibs, or a heat-exchanger leak. It is not a storage tank emptying.
Q: Is condensate from a tankless a leak? A: Condensate is normal while a condensing unit fires — often around 1–2 gallons per hour at high fire on a ~199k BTU heater. Puddling, backup into the cabinet, or hot potable water is not normal.
Q: Can I keep using a tankless that will not stop draining? A: Not if the cabinet or board is wet, the T&P is streaming, or the exchanger is the source. A properly piped condensate drip into a drain can wait for a scheduled repair. Internal potable leaks cannot.