questions about electric water heater used as a diversion load

ken morgan

Solar Addict
I had mentioned in another post of my desire to make a 4k gallon hot water tank to heat hot water for my hot tub, daily use and other uses. Currently I am going through plans to build a mini version so that I can test the control circuits and see what else I might need before I try it on the 4k gallon sized tank. this will involve 3 or 4 inverters and as many elements as I can place on the inverter(s). I originally thoguht about going dc direct to elements with a dc thermostat but after watching some videos the questions about matching the panels to the elements became to much of a PITA to consider so i decided that some el cheapo inverters that are switched off and on are easier.

while doing this the thought occurred to me...what happens when I connect two water heating elements in series? Does wattage stay same and amperage change? or do the elements only put out half power?

I over-paneled so that in the winter months, or when raining I would have plenty of PV to keep my bank charged. in the high production months this means that I am fully charged and the SCC's are shutting off by 1130-1200 depending upon the previous evenings usage. even during the last months typhoon season I found myself charged back to 95% (where I cut off my charging) by 1400 which indicates to me that even in the winter I should be able to heat water as well. I want to capture the extra hours in a hot water tank for hot tub use as well as normal hot water and other possible uses.

So for the sake of the discussion the elements are lets say 2000 watt 120 VAC units the inverter I will be using for this a modified sine inverter that puts out 4000 watts. it will be turned off and on as a diversion load when my battery bank reaches 54 volts and then shut off when the battery drops below 53.8

I cannot wrap my head around how a heating element acts though when i wire them in series and or parallel. if series i would think that they would get half of the power each and their output would drop considerably. If i wired them in parallel I would think that they both would get full power provided the inverter could produce that much.

the other question was how will that affect the thermostat? or should I just run two thermostats? and wire them in series ?

attached is an ugly and I mean ugly paint version of what I was thinking. and to answer no I cannot get a US water heater over here... tried it... It would have made life simpler as I could have stripped it for wiring and parts and then just ordered parts to duplicate the circuit as many times as needed.

cheers
 

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curiouscarbon

Science Penguin
awesome idea!

this resource from filter guy keeps me units clear in the head, since i’m always forgetting https://diysolarforum.com/resources/ohms-watts-law-wheel.193/

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if series i would think that they would get half of the power each and their output would drop considerably. If i wired them in parallel I would think that they both would get full power provided the inverter could produce that much.
each heating element is a resistor of R ohms

two in series will sum the resistances, so R+R=2R ohm equivalent resistor.

since power P is (V*V)/R then if you keep V constant then double R should mean Half P. Due to reciprocal property.

So two elements series of same resistance ought to result in half power total, QUARTER power each. If my amateur math is on track.

but for parallel resistor it’s sum reciprocals.
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from https://www.electronics-tutorials.ws/resistor/res_4.html
the other question was how will that affect the thermostat? or should I just run two thermostats? and wire them in series ?
depending on what you are comfortable with i personally would recommend considering an arduino to control a solid state relay to the heating elements and if it is a zero crossing switch type then it might work really well. anyways just more random thoughts.

attached is an ugly and I mean ugly paint version of what I was thinking.
🎨👍it worked though🙂

hope this helps, can expand on the arduino thermostat stuff if it’s of interest.

good luck on your water heating project!
 

ken morgan

Solar Addict
while it is of interest its way above my knowledge base. I have a couple of friends who are better versed in things like rasberry pi but neither had played with an arduino as of yet, so I was planning on roping them in later so that I can get them to go over something like that and then walk me through it baby steps. why don't we take this to PM so I can hide my ignorance from the rest of the world?
 
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