Symptoms
- Water is warm but not properly hot
- Problem worsens when multiple fixtures run at once
- Temperature seems to drop further in cold weather
- Unit doesn't display an error code
Likely causes
- CommonExceeding the unit's flow rate capacityRunning multiple showers or a shower plus a dishwasher can exceed what a single tankless unit can fully heat.
- CommonDirty inlet water filter restricting flowA clogged filter reduces flow enough that the unit undershoots the target temperature.
- PossibleIncoming water temperature much colder in winterA larger temperature rise required in cold months can exceed the unit's heating capacity at high flow.
- PossibleScale buildup on the heat exchangerMineral scale reduces heat transfer efficiency, especially in hard water areas.
What to do
- Check the set temperature on the unit's control panel and confirm it's set high enough for your needs, typically 120°F or higher.
- Reduce the number of fixtures running hot water at the same time to see if temperature improves.
- Locate and clean the inlet water filter screen if your model has one, following the manual's instructions.
- Note whether the problem is worse in winter, which points to incoming water temperature exceeding the unit's capacity at high flow.
- If you're in a hard water area and the unit hasn't been descaled, plan a descaling service, which usually requires isolation valves and a pump kit.
- If none of these explain it, the heating element or gas modulation may not be reaching full output.
Tools and parts
- Manufacturer's manual
- Descaling kit (if applicable)
When to call a pro
If cleaning the filter and reducing demand don't help, a plumber can descale the unit and test whether it's reaching full heating capacity.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.