Venturing gently into the Boffins' Corner, I am after some help to try and come up with a heating / water / ventilation system for rentals which will:   1 - Not involve gas (trying to get rid of Annual Checks, and the plain worry of gas appliances going wrong). 2 - Will not slug the EPC figure. I am regulated to be D or better from 2025, and C from 2030, and not to comply will be a criminal offence - though there are exceptions. 3 - Will not be overtly expensive.  4 - WI