I was looking at my geyser consumption stats and noticed my one geyser is running at about 216V, while the rest of the house is at or around 230V. Why would this be, and is it something I should be concerned about?
Because high current causes voltage drops over conductors. I would not worry about it too much.
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Thanks. I am aware of voltage drop, I guess my question is when does it become problematic.
Probably not the best thread to handle this, but more than 5% drop is a problem as far as I know. You’re on the edge of that as far as I can see. But then, it also depends on the length of the conductor (a long conductor spreads the heat generated in the conductor over a larger distance), and then for things that are not voltage-sensitive it might not matter if it is a little more. Best to ask a sparky I think, I don’t know the full nitty-gritty of this. But I don’t think 216V is a serious problem.
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SANS regulations require volt drop not exceeding 5%. More than 5‰ you start risking overheating of cables that might cause fires. I would definitely look into that drop.
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How are you measuring this voltage at the geyser? If using some sort of smart switch, then I would double check using a proper tester at the geyser and then compare it to the voltage at the db. If you indeed have such a big drop between the 2 ends, then something isn’t right.
But aren’t all offerings relays when it comes to switching the element supply??
Relays are fairly common, but power measuring capabilities are not as common in the cheaper, off-the-shelf smart relays / switches.
I was thinking the same as @Vassen - if @Speedster measured the voltage drop with the same multimeter and measured at multiple spots, then good, but if he’s relying on a Sonoff POW or Shelly PM or a CBI astute or something similar, then it may just be an uncalibrated meter.
Yes but some have voltage / power meters built into the switch. For example the sonoff pow can tell you the power consumption and voltage as well.
My point being that different devices like these could report voltages differently.
I agree with @Vassen… In my case I gained about 13V after the smart relay 
Was quite excited until the multimeter, uhm, brought me back down to earth.
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The long and the short of it, use the same meter (your multimeter) to measure both ends. If the calibration is off, then hopefully it will cancel out. Assuming the temperature is more or less stable 
Pro tip measure with increasingly cheaper multimeters until you achieve conformation bias
Use one for each end instead of the same meter. See if you can exceed 25%.