I'm planning to use a hydronic system in a 900 square foot house. The plumbers tell me that I need, or probably need, a water softener "for the boiler". I had thought I could get away with using a normal domestic water heater, given the size of the house. But either way: is a water softener really needed? I did a search for this forums and couldn't find any relevant posts (one addressed 'water conditioners', which is something more than mere softening). thanks!

Water softeners are used in larger systems to limit scale formation on the heat exchangers (which lowers the efficiency) but on small closed loop systems (where there isn't any volume of fresh water introduced until there's a leak), I can't imagine it's needed except in the most extreme water-hardness cases.

If you're planning running potable water through the heating appliance in an "open" system, don't! Stagnation & legionella growth issues make this approach illegal in some places, and in hard water areas you end up with scaling issues in the boiler too. In a combi system it's better to heat the domestic hot water via an isolating heat-exchanger (which you my have to de-scale every few years with a vinegar-rinse/anti-scaling soak on the potable-water side if you have water hardness problems), but as long as the boiler-water is contained in a boiler loop without constant replenishment scale doesn't continue to build up on the boiler-water side.

Conditioned water is used and maintained in commercial boilers systems. For residential boilers we test water to no more than 5 grains hardness and a pH of 8-9.5 satisfying most ModCon boiler specifications. Conventional low-efficiency boilers should be treated the same, but rarely get the attention they deserve.

Of course there is no proper way to address tankless water heaters or open systems, as neither are accepted by the industry, trade, or code officials.

Thanks to both of you for that information. (FWIW, I had been planning to use a closed system.)