What causes Rinnai Tankless Water Heater 79?

A leak from the heat exchanger or the water piping inside the cabinet. Water pooled in the bottom pan of the cabinet from an earlier leak. Condensation forming at the bottom of the unit in a very humid location (nuisance code 79, per Rinnai Technical Bulletin 101). Loose, broken or damaged wiring or connections at the leak detector. A faulty leak sensor, or a short circuit at the leak sensor's connector on the PC board.

How do I fix Rinnai Tankless Water Heater 79?

Look under and around the heater for dripping water, and check the water connections below the unit for leaks you can see from outside. If you see water coming from inside the cabinet, turn off the gas, power and incoming water and call a licensed technician. Rinnai's troubleshooting steps for 79 are written for qualified service people, and they involve removing the front panel. Note whether the heater is in a damp spot, like a humid garage or basement or a spot exposed to steam. Rinnai says high humidity can cause nuisance 79s from condensation, which a technician can fix with a shorter sensor screw. Write down your model number from the rating label and whether 79 keeps coming back after the heater dries out. That helps the technician tell a real leak from condensation. Keep a working carbon monoxide alarm near any gas water heater, and if you smell gas, leave and call your gas utility.

When should I call a pro for Rinnai Tankless Water Heater 79?

Checking the wiring harness at the leak detector for loose, broken or damaged wiring or connections. Checking the heat exchanger and all piping inside the water heater for leaks. If there's no leak, wiping or blowing out water in the bottom pan, drying the leak sensor, and restarting the heater to see if 79 comes back. If no water is found, unplugging the leak sensor and restarting: if 79 persists, checking the sensor's connector on the PC board for a short; if the heater runs with the sensor unplugged, shortening the sensor screw by three threads (or installing Rinnai's shorter modified sensing element from Technical Bulletin 101) and reinstalling it.

Code: 79🚿 Rinnai Tankless Water Heater

Rinnai Error Code 79: Water Leakage Detected (Meaning, Causes, Fixes)

Q: What does Rinnai Tankless Water Heater 79 mean?

Code 79 on a Rinnai tankless water heater means Water Leakage Detected: the leak sensor in the bottom of the cabinet is touching water. That can be a real leak from the heat exchanger or internal piping, but Rinnai also says condensation in very humid spots can set off nuisance 79s. Rinnai's diagnostic guide lists code 79 for its non-SENSEI models, and its fix for nuisance codes is a shorter sensor screw. Rinnai lists code 79 as Water Leakage Detected. Non-SENSEI Rinnai tankless heaters (such as the V, RL, RU and RUC lines and the KA, KB, KBD, KBP and KCM model families) have a leak sensor at the bottom of the unit, made up of a sensing element (a screw held in a bracket) and a wiring harness to the PC board. When water bridges the sensor, the heater stops and shows 79. Rinnai's current troubleshooting guide says the code applies to its non-SENSEI series. The water can come from a leak in the heat exchanger or the piping inside the cabinet, or from condensation. Rinnai's Technical Bulletin 101 explains that where there's a lot of moisture in the air, condensate can form at the bottom of the unit and cause nuisance code 79s, and that a shorter, modified sensing element may be needed to stop them. Rinnai's newer guide does the same thing by having the technician cut the original sensor screw shorter by three threads. If the code stays even with the sensor unplugged, the guide says to check the sensor's connector on the PC board for a short circuit.

Reviewed 2026-10-24 · Based on 3 manufacturer & service sources

What the code means

Rinnai lists code 79 as Water Leakage Detected. Non-SENSEI Rinnai tankless heaters (such as the V, RL, RU and RUC lines and the KA, KB, KBD, KBP and KCM model families) have a leak sensor at the bottom of the unit, made up of a sensing element (a screw held in a bracket) and a wiring harness to the PC board. When water bridges the sensor, the heater stops and shows 79. Rinnai's current troubleshooting guide says the code applies to its non-SENSEI series. The water can come from a leak in the heat exchanger or the piping inside the cabinet, or from condensation. Rinnai's Technical Bulletin 101 explains that where there's a lot of moisture in the air, condensate can form at the bottom of the unit and cause nuisance code 79s, and that a shorter, modified sensing element may be needed to stop them. Rinnai's newer guide does the same thing by having the technician cut the original sensor screw shorter by three threads. If the code stays even with the sensor unplugged, the guide says to check the sensor's connector on the PC board for a short circuit.

Likely causes

A leak from the heat exchanger or the water piping inside the cabinet
Water pooled in the bottom pan of the cabinet from an earlier leak
Condensation forming at the bottom of the unit in a very humid location (nuisance code 79, per Rinnai Technical Bulletin 101)
Loose, broken or damaged wiring or connections at the leak detector
A faulty leak sensor, or a short circuit at the leak sensor's connector on the PC board

Safe DIY checks

⚠ Before you start

Unplug the appliance (or switch off its breaker) and shut off the water supply valves before removing panels or disconnecting hoses. Keep towels handy for residual water.

Look under and around the heater for dripping water, and check the water connections below the unit for leaks you can see from outside.
If you see water coming from inside the cabinet, turn off the gas, power and incoming water and call a licensed technician. Rinnai's troubleshooting steps for 79 are written for qualified service people, and they involve removing the front panel.
Note whether the heater is in a damp spot, like a humid garage or basement or a spot exposed to steam. Rinnai says high humidity can cause nuisance 79s from condensation, which a technician can fix with a shorter sensor screw.
Write down your model number from the rating label and whether 79 keeps coming back after the heater dries out. That helps the technician tell a real leak from condensation.
Keep a working carbon monoxide alarm near any gas water heater, and if you smell gas, leave and call your gas utility.

🛠 When to call a pro

Checking the wiring harness at the leak detector for loose, broken or damaged wiring or connections.
Checking the heat exchanger and all piping inside the water heater for leaks.
If there's no leak, wiping or blowing out water in the bottom pan, drying the leak sensor, and restarting the heater to see if 79 comes back.
If no water is found, unplugging the leak sensor and restarting: if 79 persists, checking the sensor's connector on the PC board for a short; if the heater runs with the sensor unplugged, shortening the sensor screw by three threads (or installing Rinnai's shorter modified sensing element from Technical Bulletin 101) and reinstalling it.

Parts that commonly fix it

Always match the part number to your exact model.

⚙

Water leak detector

A floor leak alarm under the water heater warns you about drips before they do damage.

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⚙

Carbon monoxide alarm

Keep a working CO alarm near any gas water heater.

Search eBay →

⚙

Tankless water heater flush kit

Regular descaling and inspection is part of normal tankless maintenance.

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Sources

Last reviewed 2026-10-24. Always confirm against the manual for your exact model number.

Rinnai Error Code 79: Water Leakage Detected (Meaning, Causes, Fixes)

What the code means

Likely causes

Safe DIY checks

🛠 When to call a pro

Parts that commonly fix it

Water leak detector

Carbon monoxide alarm

Tankless water heater flush kit

Related codes

Sources