Tankless water heaters are built to deliver hot water on demand, so when one has a problem the symptoms are immediate: a shower that runs cold, hot water that cuts in and out, or an error code on the display.

United Plumbing provides tankless water heater repair and service in Fort Myers, Cape Coral, Estero, Bonita Springs and across Lee County. This guide covers the common problems, what you can safely check, how hard water affects tankless units, and when to call. If you are still weighing the technology, see tank vs tankless water heaters.

Common Tankless Water Heater Problems

What You Can Safely Check First

• Power: confirm the unit has power, the breaker has not tripped and the display is lit.
• The gas shutoff (gas models): make sure the shutoff valve to the unit is open and your other gas appliances are working. Never attempt to repair a gas line.
• Fixture flow: a tankless heater needs a minimum flow to fire, so a very low-flow fixture may not trigger it. Try another tap.
• Temperature setting: confirm the set temperature has not been changed.
• The inlet filter: if your manual shows how, the inlet filter can be cleaned after shutting off the water and power, following the manufacturer's instructions.
• The display: write down any error code and the model number.

Do not open the cabinet, work on gas lines or vent pipes, or bypass safety devices. Those jobs are for a licensed plumber.

Tankless heaters are less forgiving of hard water than most people expect. Many service calls come back to scale, flow or a neglected inlet filter.

Hard Water, Scale and Maintenance

Southwest Florida water is mineral heavy, and that matters more for a tankless unit than for a tank. Scale builds on the heat exchanger and narrows the passages inside it, which lowers efficiency, causes temperature swings and can trigger error codes. Flushing or descaling on the schedule the manufacturer recommends, and more often in hard water, is the best protection. Our guide to why descaling matters in hard water explains how it works, and extending tankless water heater life covers the rest of the routine.

Treating the water before it reaches the heater helps even more. See the link between hard water and failing water heaters and our water softener options.

Gas Models: Safety Comes First

Gas tankless units add combustion and venting to the picture. Treat these as warning signs and stop:

• A gas smell near the unit or anywhere in the home
• A carbon monoxide alarm sounding
• Soot, scorch marks or an unusual exhaust smell around the unit or vent
• Yellow or uneven flames, if you can see them

If you smell gas, leave the building, avoid light switches and anything that could spark, and call your gas utility or 911 from outside. Then call a licensed plumber before the unit is used again. For more, see our guide to gas water heater warning signs and safety.

When To Call A Plumber

Call a licensed plumber for a gas smell or carbon monoxide alarm, any leak, repeated error codes, no hot water after the basic checks, or unusual noises that return after flushing. We diagnose the cause first and give you an upfront quote. If the unit is older and failing repeatedly, our guide to water heater repair vs replacement helps you decide, and we cover tankless installation and water heater financing if replacement makes sense.

Sources and Further Reading

• U.S. Department of Energy, tankless or demand-type water heaters: energy.gov
• U.S. Consumer Product Safety Commission, gas water heater safety and recalls: cpsc.gov
• More guides from United: Fort Myers plumbing articles.

About the author: United Plumbing Inc has been the Fort Myers plumber for residential, commercial and new construction customers since 1980. Licensed technicians repair and service tankless water heaters across Fort Myers, Cape Coral, North Fort Myers, Estero, Bonita Springs, Naples and the barrier islands. Florida License CFC 1433130. Head office: 3560 Teeside Lane, Fort Myers, FL 33916. Reviewed by United Plumbing's service management team. Last updated October 2026.