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?

  • I haven't seen anything in the NEC that mandates that a gas furnace be on its own circuit -- in fact, most gas furnaces are on the same circuit as the Class II transformer for the thermostat, and nobody bats an eye at it. Commented Jul 15, 2015 at 3:48
  • 1
    Is this in a garage or unfinished basement type area? If so, you will need GFCI protection on any 120V receptacles you use, even if they are intended to be "dedicated" to something like your water heater.
    – mjohns
    Commented Jul 15, 2015 at 12:05
  • 1
    @ThreePhaseEel The thermostat transformer is part of the gas furnace, or at least part of the control system for the gas furnace. That's why nobody bats an eye.
    – Tester101
    Commented Jul 15, 2015 at 13:01
  • NEC is as stupid as the EPA is.. I'm sure your Keurig probably uses 10x the amount of power or better than the primary control on that gas water heater.. Just like how a HVAC system that contains less than 50 lbs of R22 refrigerant is allowed to leak all it wants, but cutting open a refrigerator containing 12 oz of 134a will get you a big fine and loss of your license if someone catches you lol.. it is what it is.. Commented Dec 7, 2019 at 22:27

2 Answers 2


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.


The national electric code requires a dedicated circuit for fixed ELECTRIC space-heating equipment, but does not require a dedicated circuit for fixed natural gas space-heating equipment.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. Commented Dec 7, 2019 at 22:35
  • This is not supported by the text of NEC 422.12, which states that all central heating equipment, save for fixed ELECTRIC space-heating equipment, requires a dedicated circuit, as well as common practice with fixed electric space-heating equipment such as baseboard heaters. (Oftentimes, two or three smaller baseboard heaters are put on the same 240VAC circuit.) Commented Dec 8, 2019 at 2:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.