Water to Heat is a great, easy and almost 100% usage of the input energy converted to heat.
From solar or grid.
Use the Hydrogen Reactor System.
In a simple sand heater or water tank.
The radiant heat is around 300°C. The exhaust, water, can be reused for further cycles.
Hydrogen and oxygen react with certain elements spontaneously, under room-temperature conditions.
Standard car catalytic converters contain such elements to turn hazardous pollutants into water. Hydrogen and oxygen convert into water and release energy as heat when routed through a catalytic converter.
An advantage of using a converter is that, compared to a flame, there is no risk of flame-outs. The disadvantage is that not all hydrogen is converted and may reach the open atmosphere — this must be considered for risk reduction when setting up such systems.