Hard water is one to be careful of. It's fairly common, really. High levels of calcium or hardness will "plate over" heat transfer surfaces. In the boiler or heat source this will greatly reduce the transfer of chemical energy (heat) to the fluid. It has the same effect on the tubing, radiators, coils, PEX, etc., that are asked to emit the energy from the fluid stream into the space.

If you have ever taken apart a boiler that has run hard water for extended periods, this buildup becomes very evident. Left unchecked it can start to occlude the small passageways in today's high efficiency equipment rather quickly. The hotter the surfaces the faster the sediment falls from the fluid and "glues onto" the transfer surfaces. An element removed from an electric water heater tank is a good demonstration of this.