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Tankless water heater repair usually comes down to a handful of causes. Work through these checks first, and know when a pro is the safer choice.
Most tankless hot water heater repair calls start with these checks, and many end with them. If you are about to repair tankless water heater problems yourself, stop at anything involving gas, venting or electrical parts.
If you smell gas, leave the building and call your gas utility or 911 from outside.
| Problem | Likely causes | What you can do | Call a pro if |
|---|---|---|---|
| No hot water | No power, gas off, valves closed, low flow, ignition failure | Check power, gas, valves, open tap fully | Code 11 or ignition failure persists |
| Lukewarm water | Undersized unit, scale, low gas pressure, crossover | Reduce simultaneous use, flush, check mixing valves | Gas pressure or sizing issue |
| Temperature swings | Low flow, scale, cold-water sandwich | Increase flow slightly, flush unit | Persists after flush |
| Low pressure | Clogged inlet filter, scale, partly closed valves | Clean filter, open valves, flush | Pressure still low |
| Unit shuts off mid-use | Blocked vent, flame loss, overheating | Clear vent terminal of snow or debris | Code 10/12 or overheat codes repeat |
| Water leaking | Loose fittings, relief valve, cracked heat exchanger | Tighten accessible fittings, shut off water | Leak from inside the cabinet |
| Loud noises | Scale, fan issue, gas pressure | Flush unit | Fan or combustion noise continues |
| Display shows LC or maintenance code | Scale buildup | Flush the unit | Code returns quickly |
Tankless water heater not heating water or tankless water heater no hot water at all are the most common complaints. If the water pressure lower after switching to new tankless water heater, check the inlet filter and isolation valves first.
The flow sensor tells the unit when water is moving. If it fails or is clogged, the unit may not fire or may cycle on and off. Debris and scale are common culprits; cleaning the inlet filter and flushing often help. Flow sensor replacement, such as on Navien and Rinnai units, is a part swap best done by a technician.
For Noritz tankless water heater repair and Navien tankless water heater repair, have the model number and code ready; a failed Navien tankless water heater flow sensor is a common, fixable part. Typical Rinnai tankless water heater problems are covered in our Rinnai codes guide. Richmond tankless water heater repair follows the same steps as Rheem-built units.
When you search for a plumber tankless water heater repair specialist or tankless water heater repair services, these questions help you compare quotes.
We don’t offer repair services, but here’s how to choose a technician:
If the heat exchanger is leaking or the unit is over 15–20 years old and needs an expensive part, replacement may be better value. See how long tankless water heaters last.
A flush kit fixes many tankless problems and prevents repeat visits.
Common causes are no power, a closed gas valve or empty propane tank, flow below the minimum activation rate, a clogged inlet filter, heavy scale, a blocked vent or a failed igniter or flame sensor. Check the error code first.
Usually too much simultaneous demand for the unit’s size, flow dropping below the activation rate, scale buildup, low gas pressure, or a crossover in the plumbing such as a faulty mixing valve.
Tankless units restrict flow slightly to maintain temperature, and a clogged inlet filter or undersized piping can reduce pressure further. Clean the inlet filter, check that isolation valves are fully open, and make sure the unit is sized for your demand.
Simple repairs such as sensor replacement or cleaning typically cost $150–$600 including the service call. Fans, gas valves and control boards cost more, and heat exchanger replacement can exceed $1,000 if not under warranty.
Yes for anything involving gas, venting, burners, electrical components or the control board, and whenever you smell gas. Simple checks like power, valves, inlet filters and flushing can be done by homeowners.
This guide is general information for U.S. homeowners, not professional advice. Gas, electrical and venting work must follow the manufacturer’s instructions and local codes, and in most areas requires a licensed plumber, electrician or gas fitter and a permit. Specifications and prices change; always confirm details on the product page and in the installation manual.
