Tankless Water Heater: No hot water
No hot water on a tankless water heater: Water runs but never gets hot, and the unit may or may not show an error code.
Applies to: Tankless water heaters
Common causes of no hot water
- Flow below the unit's minimum activation flow
- Gas supply off or undersized gas line
- Clogged inlet water filter
- Plumbing crossover
- Unit in error or lockout
How to fix no hot water: step by step
- Check the remote or display for an error code and look it up under Error Codes
- Open a hot tap fully and confirm flow is above the minimum activation flow
- Clean the inlet water filter
- Confirm gas is on and check inlet gas pressure at full fire
- Check for a crossover: shut the cold inlet and see if water still flows from hot taps
- Verify power to the unit
If the unit shows an error code, look it up: Bosch Tankless Water Heater codes Navien Tankless Water Heater codes Noritz Tankless Water Heater codes Rinnai Tankless Water Heater codes
Source: General HVAC service practice — verify per model. Written in our own words as a summary; always check the original manual.
General guidance for qualified technicians. The manufacturer's installation and service manual and local codes always take priority. Aurora Star Tech is not affiliated with, endorsed by or sponsored by any equipment manufacturer; brand and model names are used only to identify equipment and belong to their owners.