Rinnai Not Getting Hot Enough

The Short Answer: A Rinnai that runs lukewarm is not “weak.” It is either undersized for the temperature rise, scaled, mixing-valve limited, crossing over, or starved for gas. Time-to-arrive is a different complaint (takes too long). If water arrives fast and never hits setpoint, measure inlet temperature and flow before you touch the controller.

Search Intent & The Real Problem

Setpoint 120°F. Shower 104°F. Or it is fine until the second shower opens. An R53 at 5.3 GPM in 40°F groundwater cannot make two showers. A scaled RU199 can barely make one. Cranking to 140°F does not add BTUs; it adds scald risk when flow drops.

What other guides get wrong

They start with descaling because it is a product they can sell. Descale fixes scale. It does not fix a 5.3 GPM chassis, a 1/2” gas flex, or a shower cartridge dumping cold into hot. They also confuse wait time with temperature.

Step-by-Step Troubleshooting

Step 1: Split “slow” from “not hot”

• What to look for: Kitchen next to the unit hits 120°F in 10 seconds, master bath takes 45 seconds then hits 120°F = pipe lag, not this article. Every fixture tops out at 105°F = this article.
• Required tools: Thermometer at the closest hot faucet.

Step 2: Temperature rise math (the conversation nobody has)

• What to look for: Inlet water temperature (hose bib or inlet thermistor on the controller). Outlet setpoint minus inlet = rise. BTU availability vs GPM.

Rough field math: GPM × rise × 500 ≈ BTU/h needed. Two 2.0 gpm showers at a 70°F rise need about 140,000 BTU into the water — plus inefficiency. An R53-class unit is done. See R53 problems.

• Required tools: Controller inlet temp or a thermometer, honest GPM (bucket).

Step 3: Mixing valve, crossover, and “eco” settings

• What to look for: Mixing valve at the heater. Controller in a low commercial or “energy save” profile. Failed cartridge: cold riser warms when only hot is open, net flow through the Rinnai collapses, temperature swings.
• Required tools: Contact thermometer on both risers.

120°F at the Rinnai outlet and 108°F after the mixing valve is a valve, not a Rinnai.

Step 4: Scale and gas

• What to look for: Used to hit setpoint, now 10°F low, overheat or LC history, roaring exchanger. Or it only fails on the coldest mornings when the furnace lights — undersized gas line, 11/12 codes. Inlet screen packed so flow is low and the board still cannot hit temp because the exchanger is insulated with scale.
• Required tools: Isolation-valve flush if present; licensed manometer for gas.

Flushing without service valves is a licensed add-on. Vinegar into the house cold is how you kill a mixing valve downstream.

Step 5: Recirc bringing cooled water back

• What to look for: 24/7 recirc with uninsulated returns, mixing a cooled loop into the inlet so the rise just got bigger. Demand recirc should rest. A cheap crossover pump on the cold line also dilutes temperature.
• Required tools: Feel the return line; it should not be hot 24/7 unless designed that way with insulation.

When DIY Ends: Calling a Licensed Pro

Stop for gas-line resize, mixing-valve replacement in a wall, a flush without isolation valves, and an honest “this chassis will never do two showers in January.” You may read inlet temp, clean the inlet screen, and compare faucet vs controller temperature. Undersizing is not a warranty failure. A licensed plumber should say that out loud before quoting a descale subscription.

Frequently Asked Questions

Q: Why is my Rinnai tankless not getting hot enough? A: Undersized for the rise, scale, a mixing valve, crossover, or low gas. Measure inlet temperature and flow. Do not start at 140°F.

Q: Can I just turn my Rinnai up? A: Only if the outlet is actually below a safe 120°F and the unit has BTU left. If it is already at max fire and still short, the unit is too small or scaled.

Q: Why is it hot at the sink and lukewarm in the shower? A: Mixing cartridge, a shower head over the unit’s GPM, or the second fixture. The sink is a low-flow pass. The shower is the test the chassis failed.