Electrical – How bad is it to attach a gas water heater to a gas furnace electrical circuit


I'm looking to replace my existing water heater with a more efficient model. One of the things that makes better models more efficient is the replacement of the thermocouple / pilot control system with electronic control and ignition plus electrically controlled dampers. Thus the heater requires some form of electrical hookup for this function.

My understanding is that in this scenario code requires the gas furnace and gas water heater to be on dedicated circuits. (Is my understanding wrong?)

I'd rather not (pay someone to run) a whole new circuit when I'd expect the water heater to need less than 4W. As a result I'd like to reuse the furnace's circuit and install a SPST switch and plug combination in the junction box where the furnace's existing disconnect switch is located, and connect the water heater to it.

How worried should I be about this? Am I going to cause serious safety hazards or equipment damage?

Best Answer

According to the National Electrical Code, a gas furnace (or any other central heating equipment) must be on its own circuit. So connecting a water heater (or anything else), would be a code violation.

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.