I'm considering using an electric hot water heater (EHWH) with upper and lower heating elements as an electric diversion load. This EHWH would be for pre-heating the water bound for the standard HWH.

Often the total watt rating of the EHWH is larger of the two elements - instead of the sum. Is it allowed (*) to energize both heating elements at the same time?

  • 'allowed' means - general technical limitation of the EHWH - not necessary code related. Also I'm also interested relevant code limitations and their reasons.

** Might also be relevant to post in a different forum?

  • More information? Make and model, for instance.
    – SDsolar
    Apr 12, 2018 at 6:20

2 Answers 2


Although different heater manufacturers have and could still do different things, they rarely would configure the heaters to operate both elements at the same time because amperage draw doubles, wire gauge size increases, fuse size increases and little is gained in heat recovery.

Pic below illustrates operation of elements on standard dual-element heaters with non-simultaneous operation.

enter image description here Pic courtesy of A.O. Smith Co.

Is it allowed (*) to energize both heating elements at the same time?

NO, unless the heater was originally designed for that. You can never modify equipment (especially a boiler) without manufacturer's permission.

  • A bit new to this DIY community, but in others, modification of equipment without permission is the norm! However, boilers are a special case. The purpose of the question is to understand the technical limitations so that and others can make an informormed decision. The limition then appears to be internal wiring in block "power enters high-limit" the solution to this would be to disconnect the existing connection to lower thermostat and replace with a new connection to provide a 2nd power connection there. Of course depending on the elements, 30 circuit might not feed both.
    – pathfinder
    Apr 12, 2018 at 15:39
  • Also, not part of the question, but background, the element(s) would likely be powered through a triac circuit acting like a lamp dimmer under computer control such that it would adjust power delivered to the elements to 'exactly' match the excess production.
    – pathfinder
    Apr 12, 2018 at 15:43
  • @pathfinder, yes you are right, a creative "hack" would be appropriate (hopefully with a disclaimer). Thinking through the scenario here your proposed solution would work except that the high-limit cutout would then not work for the lower thermostat, leaving no safety current cutout. Most models have integrated stat/cutout; maybe a model with a seperate cutout then wire through it to both stats (but is the cutout rated for that much current?). Should we assume excess electrical production above max element wattage (I think yes)? Apr 13, 2018 at 1:16
  • better than kludge! Not sure the exact meaning of some of the words. My mental models of the EHWH is possibly a fuse and an upper and lower thermostat each connected to an element. Are the "cutouts" additional safety circuits? "stats" are thermostats? Yes - max excess when off grid rather large - but there are multiple safety limits in the solar system that will step in before things get too ugly.
    – pathfinder
    Apr 13, 2018 at 13:51
  • yes, they are equipped with auto-reset high temp cut-off switch ("cutout"); can be separate but commonly integrated with the upper thermostat, so if you wired the stats/elements separately you would be bypassing the cutout for the lower stat/element. I think it could be safely done, the tank should have a temp/pressure relief valve too... so some safety redundancy. Apr 13, 2018 at 18:12

To run in parallel, you would need the high temp cutoff on both elements so use two upper thermostats along with bigger wiring and bigger circuit braker. Easily make a faster heating water heater.

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