TL;DR: Our 20-year old heat exchanger leaked 2.7 GJ/year, after upgrading to a better insulated one this was reduced to only 0.65 GJ/year, saving 2.0GJ or 50 euros per year! The upgrade was for 'free', meaning it's included in the yearly fee of 90 euro/year. It's unclear why Eneco does not do this pro-actively, but you can also request an upgrade if you say the old one does not suffice anymore.
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