The single wall heat exchangers have just a single plate between the two fluids. If a leak develops it will allow the two fluids to mix. A double wall heat exchanger has two plates between the fluids with a small air gap between the plates. If a leak develops in this type of heat exchanger, the fluid will leak out of the air gap before the two fluids are allowed to mix. Double wall heat exchangers are often recommended or required for domestic hot water applications.