The short answer
A tankless heater senses flow and only heats when water moves fast enough. Stiebel Eltron's DHX chart marks its activation flow at 0.26 GPM; other makers state their own minimums. A faucet barely cracked open, or a low-flow aerator below the minimum, can run cold even though the heater is fine.
Fixes
- Open the tap further, or use an aerator the heater's maker recommends.
- Check and clean the inlet filter.
- Flush a scaled heat exchanger on the maker's schedule.
Products mentioned
Questions readers ask
Why does my tankless water heater not work at low flow?
It needs a minimum flow to switch on. Below it the burner or elements don't fire. Check the minimum in your model's manual.