water filter for shower boiler
What you want is typically referred to as a "whole home water filter".  The name isn't the best description of what it is, but basically they are high-volume water filters that only filter out sediment and relatively large particulate.  They do not bring water to a "drinking water" quality but do filter out anything that would be harmful to appliances.

The good news for you is that they can be cheap enough for a simple installation like yours:



Image from:
http://www.homedepot.com/p/GE-Whole-Home-Water-Filtration-System-GXWH04F/100471282

Only $20, which is a low enough price to put it into the "why not" category.  If you have any reason to believe the water might have some sediment it in, that's enough justification to use one.

To replace the filter, the whole bottom part screws off and a new filter cartridge is put in.  This model says to replace the filter every 3 months, but with your low volume it would probably last much longer.