Have a look at this thread.
And this Zigbee item.
https://www.develcoproducts.com/products/smart-relays/smart-relay-30a/
Also there are projects around to handle this type of board 16A load take a look at projects and boards that use SLA-05VDC-SL-A eg: Aliexpress example
So I don’t know what electrical supply / outlets are normally available in your location but here in Australia most domestic 240VAC power outlets are rated up to 10A, you can have installed additional power circuits that can do larger outlets but in most cases for lights etc… 10Amp is all that is needed, but yes a Hotwater boiler is most probably in domain of 16Amp to 30Amp depending on the requirements.