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I recently had some plumbers install an exterior (natural gas) tankless water heater here in central Texas, in case the jurisdiction and / or climate matters.

For the purposes of getting it installed quickly, they used a pre-existing outdoor outlet with an in-use cover, but I want to run a dedicated circuit (15 amp, 120v, and GFCI) and I'm planning on hard-wiring the heater instead of using a plug and cord.

I've mounted an electrical disconnect box near the water heater, fed via 3/4" EMT to the back of the box (with rain tight fitting), and have run 12 AWG THHN / THWN wire (including a dedicated ground wire) to it. The final step is to remove the cord and pipe from the disconnect to the water heater.

I've noticed that most outdoor residential equipment is hooked up with one of the several varieties of flexible conduit, such as LFNC. Part of the value of Liquid Tight is ease of installation; there's no need to reach for PVC glue, EMT benders, etc. so the labor cost is much lower.

But the other value is that for equipment that vibrates or moves to function, such as AC condensers, the little bit of give is pretty important, so LFNC and friends would typically be preferred over hard pipe anyways.

My question is: since I already have 3/4" EMT, rain tight fittings, paint and primer to make it last longer outdoors, etc., would it be a mistake to run EMT from the disconnect to the tankless hot water heater instead of some sort of flexible conduit?

I want to make sure my installation is up to code so I can get my work (and the plumbers' work) inspected.

Thanks in advance!

EDIT - here's what the manual says:

  • Do not rely on the gas or water piping to ground the water heater. Ground locations are provided inside the water heater.
  • The water heater requires 120 VAC, 60 Hz power from a properly grounded circuit.
  • If using the 5 ft (1.5 m) power cord (supplied with internal/indoor water heaters), plug it into a standard 3 prong 120 VAC, 60 Hz properly grounded wall outlet.
  • On external (outdoor) models, a disconnect switch must be provided and installed for the incoming 120 VAC power. The switch should be a type that is suitable for outdoor use. Check the National Electrical Code, ANSI/NFPA 70 and your local codes for a proper switch type to use in your area. Power connections must be protected from the weather and flexible cords must use an appropriate strain relief.
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  • Step Zero: Read The Fine Manual and make sure that a "cord and plug" connection is not specified/required by the instructions before eliminating that. Following the instructions is a required part of code, somewhere around Article 110 or so.
    – Ecnerwal
    Jan 8 at 16:53
  • That's good advice. In this case, the manual does not specify that cord-and-plug is required, nor does it specify any specific type of conduit, cable, etc. It does say that with a cord I'll need a gasket to keep it water proof, but that's it (other than the 15 amp + GFCI requirement)
    – ZachS
    Jan 8 at 16:54
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    Can you get a piece of solid pipe into position on two boxes that are already mounted on the wall without using a lot of fittings or bends that will look bad? Seems like a 1/2 inch flex conduit with a fitting at each end is what you need. Why do you think it will not last as long as the EMT, or at least as long as the water heater and probably its first replacement?
    – jay613
    Jan 8 at 17:34
  • My question is more if it's better to run flex instead of EMT due to vibration or other issues. I have leftover EMT that I have already bent to run from box to box (single piece, no fittings needed except at each box), but I'd have to run to the store to grab some flex (which I'm more than willing to do if it is a superior installation). There is no concern about the longevity of the conduit, exposure to damage, etc.
    – ZachS
    Jan 8 at 17:45
  • Regarding vibration, I've seen discussion about Flex when talking about powerful pumps and fans that seriously vibrate, but a water heater? And only providing DHW so doesn't even have a circulator right? What's vibrating? If the thing is wearing out your EMT you have bigger problems than what choice of conduit! If I were you, and already had the EMT and watertight fittings and I know it fits and happy with the appearance, I'd go with that.
    – jay613
    Jan 9 at 17:02

1 Answer 1

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As you describe the situation, I would not hesitate to use EMT.

You likely already have or could have a rigid gas line connected to the heater. The manual does not specify a conduit type. I'm not aware of any code requirement for flexible connection to a tankless water heater. Provided that you've got the required disconnect, you should be good to go.

The one exception would be if the manual requires a flexible gas connection. If one is required by the installation instructions, it implies that a flexible electric line would also be a good idea. The Rennai and Bosch units I've worked with did not require a flexible gas connection.

That said: I removed my 14 year old tankless last year which was plug and cord connected. The plug and cord was never an issue.

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  • Thanks for the answer. Seems like EMT is good to go. I'm sure plug and cord is fine, but something about the possibility of a curious little neighborhood kid unplugging my hot water heater makes me lean a different direction. Thanks again!
    – ZachS
    Jan 11 at 15:46
  • I wouldn't worry about it - disconnect switches are on the outside of buildings all the time, and you never hear about buildings losing power because someone got curious and flipped the switch. But if you have the supplies and want the security, go for it. Personally I'd leave it plug and cord simply so I can easily hook it up to a generator in a backup situation, because I've done that before.
    – KMJ
    Jan 11 at 17:20

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