You wouldn’t install any equipment or appliances on property that is not your own so the same goes for installing it on common property as that is owned by all owners not just one. I think I would be requesting the owner , through the rental agency, to sign a document accepting full responsibility for any damage the HWS does to the common property as well as for their own HWS ( after discussing this at the SC and then the OC of course). It seems that you also need to speak to your Strata Manager perhaps requiring that any unusual costs be discussed with the treasurer before payment is made.