Most combis will have trouble with hard water, as it quickly scales up the
secondary heat exchanger. You should either fit a proper ion exchange water
softener, or a phosphate dosing canister (cheaper, but doesn't have all the
benefits). Don't bother with those ridiculous magnetic/electronic water
conditioners. They are for people who believe in fairies.
Vaillant have a good reputation. I don't know too much about Vokera. You
should really be considering a condensing boiler too. They are much more
efficient. Non-condensing boilers are basically due to be banned soon.
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