CSLB C-36 Plumbing ContractorFixture and Appliance InstallationMedium
A client reports that their tankless water heater is consistently producing lukewarm water, even when set to its highest temperature. The heater is properly sized for the home's hot water demand. What is the most likely cause of this issue?
- AThere is a significant mineral buildup on the heat exchanger.
- BThe exhaust vent is partially blocked, causing combustion issues.
- CThe gas pressure to the unit is too high.
- DThe incoming cold water temperature is too low, exceeding the unit's temperature rise capability.
Show answer & explanationAnswer & explanation
Correct answer: D. The incoming cold water temperature is too low, exceeding the unit's temperature rise capability.
Tankless water heaters have a maximum temperature rise they can achieve. If the incoming cold water is exceptionally cold, the unit might not be able to heat it to the desired set point, resulting in lukewarm water.
Why the other options are wrong
- A. Mineral buildup reduces efficiency and can cause fluctuating temperatures or reduced flow, but consistently lukewarm water across all uses is less likely to be the primary cause compared to extremely cold inlet water.
- B. A blocked exhaust vent would likely cause the unit to shut down or produce error codes, not consistently lukewarm water.
- C. High gas pressure would typically lead to overheating or lockout, not consistently lukewarm water.
Tankless Water Heater Temperature Rise
The maximum increase in temperature a tankless water heater can achieve between the incoming cold water and the outgoing hot water, measured in degrees Fahrenheit or Celsius.
- Limited by the unit's BTU/kW rating and flow rate.
- Colder incoming water requires a higher temperature rise.
- Exceeding rise capability results in lower-than-set-point output.
Memory trick: Cold inlet water can cool down even the hottest tankless heater's ambitions.