After replacing a standard water heater with a Rinnai tankless model, a homeowner in Georgia kept getting an error. The unit would shut down and flash error code 12. 

Early suspicion from commenters centered on two likely trouble spots: the gas supply and the venting.

Here's what to know

The issue was described in a post on r/Plumbing, where the homeowner wrote, "Installed the tankless, not the tank that was already there. It keeps shutting off and showing error code 12. Could it be the venting?" 

This is a frustrating experience for a homeowner just trying to upgrade their water heater. Normally, upgrades such as this one can provide the home with endless hot water, and some models, such as heat pump water heaters, can even save you money on your energy bills thanks to their efficiency.

Much of the response in the Reddit thread focused on whether either the gas setup or the vent path was behind the repeated shutoffs.

The original poster shared that the heater was connected to a one-inch gas line running nearly 60 feet. They also said the vent was short because the unit sat on an exterior wall, and that the previous heater had shown the same code as well.

Some replies asked for more specifics about the vent layout, such as the number of elbows and the overall distance. Others pushed for gas-pressure testing, including one commenter who wrote, "Gonna have to use a manometer to test the inlet and outlet pressure to rule out any gas pressure issues."

Ultimately, the OP identified the problem and shared it in an edit: the recirculation pump was continuing to run, and that was behind the problem.

What can be done?

Commenters pointed toward having a professional check the system methodically: test gas pressure at the meter and at the unit, verify that the piping is properly sized, inspect the venting route, and make sure any recirculation controls are not stuck or set up incorrectly.

The OP summed up the fix: "Update, was not gas pressure or venting. It was the recirculation pump not turning off and now installing an aquastat controller to turn off the pump and everything works fine."

Smart equipment, such as a smart heat pump water heater from Cala, can help regulate a home water supply and ensure that it turns on and off at the right times. That way, you'll have hot water exactly when you need it. Not only does it provide for your comfort and convenience, but a model like Cala's will also help you save money.

Where can I learn more?

Plumbing systems are more complicated than you might expect, and it helps to look deeper than surface problems and simple solutions. 

• Homeowners can save money with recirculation while keeping hot water closer at hand.

• A simple blue-dye toilet leak test can uncover silent water loss that is driving bills higher.

• Off-grid plumbers show how a pump with no electricity can move water reliably.

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