KERA – looks like Jimmy hasn’t as yet seen your post, so as it’s moving down the list, I’ll respond.
I assume you mean a heat-pump type hotwater heater, in which case your Owners Corporation would be well advised to formally consider any proposal to have such an item on its Common Property, and so that’s what I’ll advise.
The proposal can be formally considered in two (2) ways, and in my opinion the easiest way for a once-off, assuming you’re in NSW, is under Sect 65B of the Strata Schemes Management Act where your Owners Corporation may, only at a General Meeting, consent to the granting of a formal Licence to the Lot Owner covering their use of a specific part (i.e. size and location) of the Common Property for the purposes of installing a heat-pump hotwater heater.
The granting of a Licence requires a vote in favour by a minimum 75% of Owners present at the General Meeting, including proxies, in accordance with the unit entitlement of their Lots (i.e. a special resolution), and be subject to Conditions which in this instance should at least include maximum noise levels (some heat-pumps are noisier than an aircon.), the securing of the unit, how conduits (electrical and water) should be fixed to the wall, and the insulation of the hotwater supply pipe to avoid burning.
If your Owners Corporation anticipates further requests of this type, whether those are for heat-pumps or any other type of hotwater heater, then it should instead consider the above as a generic Special By-Law (SBL), where only the initial proposal is considered at a General Meeting and subsequent proposals may be decided by the Executive Committee alone, but only under the provisions of the SBL thereby avoiding precedents and inconsistencies.