in Canada we general do not use specialized cylinders so much (but i guess it getting more common now) - most places just use normal hotwater tank and attach as side arm . some are passive and other are not there a version that you take out the bottom heating element and you attach a flatplate exchanger - but that one uses a small pump(20w) to drive the circulation ( i’ve used those but they are a little expensive to do but you can uses on any tank you want) . I myself I build hotwater tank inserts they work much better they generally not that expensive to build ~$50 and a couple hours of time and are passive exchange
