What It Is
A tankless water heater (also called an on-demand or instantaneous water heater) heats water directly as it flows through the unit rather than storing a supply of hot water in a tank. When a hot water tap opens, a flow sensor activates the heat source - a gas burner or electric heating element - and cold water passes through a heat exchanger where it is heated to the set temperature before continuing to the fixture. When the tap closes, the heater shuts off completely.
What It Does
The fundamental advantage of a tankless heater is that it does not maintain a standing reserve of hot water. A conventional tank heater keeps 40 to 80 gallons at temperature all day and night, continuously adding small amounts of heat to offset standby heat loss. A tankless unit eliminates that standby loss - it uses energy only when hot water is actually flowing.
The tradeoff is that tankless heaters must heat water very quickly on demand, requiring either a large gas burner (typically 150,000 to 200,000 BTU for a whole-house gas unit) or substantial electrical capacity (often two 30-amp, 240-volt circuits for an electric unit). This means installation may require upgrading the gas line or electrical service in older homes.
Tankless heaters also have a flow rate capacity measured in gallons per minute (GPM). A whole-house gas unit typically delivers 7 to 10 GPM at a 35-degree temperature rise. If multiple high-demand fixtures run simultaneously - two showers and a dishwasher, for example - demand can exceed the unit's capacity, resulting in water that does not reach the full set temperature.
Tankless units have a longer service life than tank heaters - typically 20 years or more versus 10 to 12 for a conventional tank - and require annual descaling (flushing with a vinegar or citric acid solution) to prevent mineral buildup in the heat exchanger, particularly important in areas with hard water.
Where It Is Located
Tankless water heaters are compact - roughly the size of a large suitcase - and mount on a wall. Gas units require a dedicated flue and combustion air supply, which in condensing units can be PVC pipes run horizontally through an exterior wall rather than a traditional metal flue through the roof. This gives more flexibility in placement than a conventional tank heater.
How It Works
When water flow is detected (typically at about 0.5 GPM or more), the unit ignites the burner or activates the element. A computer module monitors incoming water temperature and outgoing temperature and modulates the burner or element to deliver water at the target temperature. The modulation is rapid and continuous. When flow stops, the heat source shuts off within seconds.
Common Problems or Signs of Failure
- Cold water sandwich - a brief burst of cold water between hot water draws, because residual hot water in the pipes is used first, then cool water in the unit before it heats up
- Error code on the display - most tankless units have diagnostic codes; common errors involve ignition failure, flow sensor issues, or overheating
- Reduced hot water output over time - scale buildup in the heat exchanger reducing efficiency and flow
- Unit does not ignite - gas supply issue, igniter failure, or venting blockage
- Lukewarm water when multiple fixtures run simultaneously - demand is exceeding unit capacity
Can a Homeowner Check It?
You can check whether the unit is displaying an error code, confirm gas or electrical supply is active, and note whether the problem occurs at low demand or high demand. Annual descaling can be done by a homeowner with the right equipment, though most prefer to have a plumber handle it to ensure the procedure is done correctly.
- Check the display panel for error codes and look up the code in the owner's manual
- Confirm the gas shutoff valve to the unit is open
- Note whether the issue occurs only when multiple fixtures are used - this suggests capacity rather than equipment failure
- Check when the unit was last descaled - annual flushing is recommended in hard water areas
When to Call a Plumber
Call for error codes you cannot resolve by resetting the unit, for descaling service, for no-hot-water situations that are not obviously a supply issue, or for any gas component concerns. Tankless units have more sophisticated electronics than tank heaters and generally benefit from service by someone familiar with that brand's diagnostic system.
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