De acordo com o Código Elétrico Nacional, um forno a gás (ou qualquer outro equipamento de aquecimento central) deve estar em seu próprio circuito. Então conectar um aquecedor de água (ou qualquer outra coisa) seria uma violação de código.
National Electrical Code 2014
Chapter 4 Equipment for General Use
Article 422 Appliances
422.12 Central Heating Equipment. Central heating equipment other than fixed electric space-heating equipment shall be supplied by an individual branch circuit.
Exception No. 1: Auxiliary equipment, such as a pump, valve, humidifier, or electrostatic air cleaner directly associated with the heating equipment, shall be permitted to be connected to the same branch circuit.
Exception No. 2: Permanently connected air-conditioning equipment shall be permitted to be connected to the same branch circuit.
Chapter 1 General
Article 100 Definitions
Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.