Can an electric tankless water heater fill a bathtub?
Can an electric tankless water heater fill a bathtub? Yes, an electric tankless water heater can fill a bathtub, but only if it is powerful enough and correctly matched to the required flow rate and temperature rise.
In UK terms, an electric tankless water heater is often called an instantaneous electric water heater. It heats cold mains water as it passes through the unit, rather than storing hot water in a cylinder. This means that the water heater’s electrical output limits the amount of hot water, not the tank size.
Bath tap flow rates
A bath normally requires a large volume of hot water in a fairly short time. This scenario is where many small electric tankless water heaters struggle. Smaller units may be suitable for a hand basin, kitchen sink or single low-flow shower, but they may not deliver enough hot water quickly enough to fill a bath comfortably.
Modern bath taps have an approximate demand rate of 12 litres per minute. This means that to achieve this level of flow rate in wintertime conditions, whereby the incoming cold water main’s temperature is 10°C (the lowest we can expect to see in the majority of UK locations – although it can get as low as 5°C in Scotland and Northern Ireland). You would need a 24 kW or 27 kW electric tankless water heater to match the bath taps’ full flow rate. At these kW outputs, these units would be 3-phase powered.
The key issue is electrical power. To produce a sufficient hot water flow rate, especially in winter when incoming mains water is colder, a high-output electric heater is required. Larger units may need a significant electrical supply and require three-phase power, which many UK homes do not have.
For this reason, an electric tankless water heater can fill a bathtub, but the flow rate may be slower than a gas combi boiler or stored hot water cylinder. In many homes, an unvented hot water cylinder, a direct electric cylinder, a thermal store, or an electric system boiler with an indirect cylinder may be a better option for regularly filling a bath.
In simple terms, yes, it is possible, but for a bathtub, the heater must be correctly sized and powerful enough to do the job. Small electric tankless heaters are usually not suitable for high-volume bath filling, especially where fast recovery and strong hot water flow are required.
