Questions

Scale in the heat exchanger tops the list, then ignition faults, then flow sensor and inlet screen problems. Each usually announces itself with an error code or with water that runs hot and then fades.

A short burst of cool water right after you reopen a tap is the heater relighting. If the temperature swings while the water keeps running, suspect flow under the minimum the heater needs, scale in the exchanger or a weak flow sensor.

First make sure the outlet has power and the gas valve is open. If both check out, look at low flow, a plugged inlet screen, a bad flow sensor or trouble in the ignition circuit.

Rinnai, Noritz and Rheem use 11 for a failed ignition. Usual causes are the gas supply or its pressure, the vent, the flame rod or the igniter. Navien shows the same fault as E003, and Takagi as 111.

On Rinnai and Noritz heaters, 10 signals a combustion-air or exhaust problem, such as a nest or snow at the termination, a blocked vent, or an overly long vent run. Navien flags venting faults as E110.

Most reset by cycling the power or through the control panel. Resetting clears the code but not the underlying problem; ignition or exhaust codes that return call for a repair visit.

Most manufacturers recommend once a year, or every six months where the water is very hard. Soft-water homes can go longer, but years without a flush lead to scale faults.

Plenty of homeowners do it, using a small pump, two hoses and a bucket connected to the service valves. If your heater has no service valves, those get installed first, and flushing won't cure a part that has failed.

Leave the heater powered so its freeze protection keeps working, shut the heater's water valves if a pipe has split, and thaw exposed lines slowly. Have it inspected before you put heavy demand on it.

Usually, yes. Flow sensors, igniters, thermistors, flame rods, blowers, gas valves and control boards can all be swapped, and mineral scale can be flushed out. The main exception is an older unit whose heat exchanger leaks.

Gas models often run 20 years or longer with regular flushing. Electric tankless heaters tend to give 7 to 10 years before a replacement makes more sense.

Part repairs generally run an estimated $150–$800. A descale is estimated at $150–$350; replacing a heat exchanger is estimated at $1,000–$2,500.

A new exchanger usually makes sense on a younger heater or one still covered by its parts warranty. Past 15 years, a new unit may be the smarter spend, and we'll tell you which way we'd lean.