For instant hot water you can't beat a gas fired instant water heater.

The Morco will give you up to 6 litres a minute of lovely hot water. It is self lighting from a pilot light and must be flued through the roof.

It includes a draught diverter within the casing.

58e LPG heater available WITH or WITHOUT draught diverter.

Instantaneous gas water heater.  For boats, caravans and in holiday park homes where LPG is available.
Dimensions: 322mm high x 270mm wide x 197mm deep.
L.P.G. only, not suitable for mains water.