A domestic solar water heater may have a single wall heat exchanger if it uses what type of liquid?

A domestic solar water heater may utilize a single wall heat exchanger effectively when it employs propylene glycol as the heat transfer liquid. This is because propylene glycol is less likely to freeze, which is crucial in regions that experience colder temperatures. A single wall heat exchanger can safely manage the thermal exchange when dealing with propylene glycol since it has a lower viscosity and better heat transfer properties compared to some other fluids.

Water, while commonly used in many systems, is not ideal for single wall heat exchangers in situations subject to freezing conditions. Ethylene glycol, while offering freeze protection similar to propylene glycol, is toxic and more suited for closed loop systems with double wall heat exchangers to prevent contamination of potable water. Mineral oil is typically used in specialized applications but lacks the efficient heat transfer characteristics and properties required for a typical solar water heater system, making it less practical compared to propylene glycol.