MWBC for gas furnace and water heater

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jeff48356

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Would this be a good use for a 15A MWBC? Since the water heater and furnace are usually located near each other, and each require a separate 15A circuit, I just had a thought come to mind that I could use an MWBC for this. I could run 14-3 to the junction box for the furnace in the basement ceiling, then jump off the red wire and common neutral to an outlet in the ceiling nearby to plug the high-eff. water heater into. Any thoughts on this?
 

mbrooke

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Would this be a good use for a 15A MWBC? Since the water heater and furnace are usually located near each other, and each require a separate 15A circuit, I just had a thought come to mind that I could use an MWBC for this. I could run 14-3 to the junction box for the furnace in the basement ceiling, then jump off the red wire and common neutral to an outlet in the ceiling nearby to plug the high-eff. water heater into. Any thoughts on this?

I've seen it done on new home builds no problem. Handle tie as 480 said.


BTW, does the AHJ require that both be separate? Technically a single 20 will do, but its up to you.
 

peter d

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New England
I've done many new houses and we always put the furnace and on-demand water heater on the same 15 amp circuit. The on-demand water heaters use virtually no power and we've never had a problem doing it this way.
 

mbrooke

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I've done many new houses and we always put the furnace and on-demand water heater on the same 15 amp circuit. The on-demand water heaters use virtually no power and we've never had a problem doing it this way.

I like the way you wire :cool:
 

mbrooke

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Yep, sometimes we forget the things we already know.:)
And given the issue of needing a multipole disconnect, and while you could use use a MWBC, I would not.

Yup. Especially when you are no longer wiring new homes- the mind tends to over-write memory that is no longer deemed essential- at least for me it does.

While a good electrician can make a splice which does not fail, a bad neutral can do several thousand in damages. It cost me something like $650 for a new electronic control on my water heater when it failed for no reason (part + labor), hate to think what it would cost to have the control boards in my furnace replaced on top of that.
 

jeff48356

Senior Member
I've done many new houses and we always put the furnace and on-demand water heater on the same 15 amp circuit. The on-demand water heaters use virtually no power and we've never had a problem doing it this way.

According to the label on the unit, it draws 12A of current, so I had to put it on a separate 15A circuit.
 

mbrooke

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I wire based on the code of common sense. :thumbsup:

Indeed. For a second I thought the OP was basing it on instructions. If we wired to instructions every home would literally have 30 more circuits. Nearly everything you buy has "separate circuit" in the instructions, alebiet its often "recommended"
 

mbrooke

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This is not a tankless water heater.. it's a regular tank one with an electric vent on the top. All high-efficiency water heaters have these, and it's a motor that calls for a separate circuit.

Calls- the motor is listed at 2 amps max- reality is it will do fine sharing a circuit like the basement lights.
 
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