Rinnai Error Code 79

How a Rinnai detects a leak

There are two setups, and they work a little differently.

Many units have a leak sensing element at the bottom of the cabinet, which Rinnai's bulletin describes behind the front panel. When water or heavy condensation reaches it, the controller shows the code.

Rinnai also sells a leak detection kit for non-condensing models. A float switch sits in a drain pan under the heater. When water collects in the pan, the float rises and code 79 shows on the controller. On that setup the switch also closes a water valve inside the heater to stop further leakage, which is why your hot water can quit along with the code.

Common causes of Rinnai tankless code 79

The first thing to rule out is a real leak inside the unit: a loose fitting, a failed seal or a cracked heat exchanger. Rinnai's instruction for code 79 is to shut off the water and call a pro, and this is the reason.

Freeze damage is the next suspect after cold weather. Rinnai points out that tankless heaters are more prone to freezing than tanks because they don't store much heated water, and that their built-in freeze protection only works while the unit has power. A heater that lost power in a cold snap can crack a part, and the leak often shows up once things thaw.

Then there's condensation that only looks like a leak. Rinnai's bulletin TB-101 says that where there's excessive moisture in the air, condensate can form at the bottom of the unit and cause nuisance code 79s. The fix is a modified sensing element installed by a technician.

Last, if you have the drain-pan kit, a dripping valve or fitting on the pipes above the heater can fill the pan even when the heater itself is fine.

What you can check yourself

1. Turn off the cold water supply valve to the heater and switch the unit off.
2. Look under the heater and in the drain pan. Is there standing water, a drip line from a fitting, or just a damp film?
3. Look at the pipes and valves above the heater. Wipe them dry, then check again in a few minutes to see where water reappears.
4. Think about the weather. A recent freeze or power outage points toward freeze damage. A humid crawlspace or garage in summer points toward condensation.
5. If you found a drip on outside plumbing, fixed it and dried the pan, reset the heater. Rinnai's kit instructions say to clear code 79 by resetting power to the tankless water heater.
6. If you can't find an outside source, or code 79 comes back, leave the water off and call.

Don't keep resetting a code 79 to get hot water. If the leak is inside the cabinet, every reset puts more water near the electronics.

When to call a technician

When water is coming from inside the cabinet, when the code returns, or when the unit sat unpowered through a freeze. A tech can pressure-check the unit, find the leak and decide whether a sensor change, a seal or a bigger repair is needed.

HomeGuide puts tankless water heater repairs at $300 to $1,300 on average. A cracked heat exchanger on an older heater is where it's worth comparing the quote against replacement. Our tankless water heater repair page and the general guide to a leaking water heater cover that decision.