Cool hacking of semi-dead inverters, Operahouse! I should try that some time. Thus far I've been shy about futzing with HV switching circuits, but if they come cheap enough I can sacrifice some magic smoke, uh?
Kitestrings: thanks for the link to that other page, looks like you've done something like I'm trying to do, to use surplus PV power when available. (And I too am in Vermont, in the town of Essex.)
In my case, the main water heater is an on-demand one that burns propane (Baxi Combi). It heats the house too (except when I burn wood). Alas it does not modulate the flame fully, only partway, and between that and my modest DHW usage I havn't bothered with thermal solar hot water. But if I can use surplus PV why not? Currently I have a small (4 gallon) insulated tank on the output side of the on-demand heater, to smooth out the water temperature which does fluctuate some. (My theory is that small fluctuations get amplified by my waste-water-heat-recovery device since it introduces positive feedback into the system.) Perhaps later (when I find a good deal on a Marathon tank!) I'll add a pre-heat tank on the input side of the main heater, like you have. Eventually the Baxi boiler will die, and the newer models modulate the flame further.
Question: if the pre-heat tank sits around at moderate temperatures, are you worried about Legionaire's disease?
Regarding the other controls in my system, I now have two MPPT controllers, with a pair of 200W PV panels on each (800W total). One of the controllers has an "auxillary load" output (12V derived from the battery) that it can switch on only when the battery is above 13V, i.e., being charged. They call it "refrigerator mode". But I think if I put in the SSR system for a water heating tank I'd rather get the control signal from ghurd's dump load controller circuit, which I currently use to turn on a vent fan for the battery box at about 14V. That very same decision point could serve as the dump load signal. I just hope the sudden withdrawing of 5 amps from the PV to the water heater (translating to at least 15 amps on the battery side of the MPPT controller) won't bring the battery voltage back down under the ghurd controller's hysteresis point, turning the dump load back off, rinse and repeat relaxation oscillator!
I also have the inclination to add two controlled AC outlets off the inverter output. Each would be switched on/off by an AC SSR. One controlled by the 13V aux load output of the PV charge controller (on most of the daylight time), and one controlled by the 14V signal from the ghurd controller (usually on in midday-early afternoon period). That would give me more flexible ways to run other loads. E.g., in sunny weather, I could put the freezer on the 14V-controlled outlet, running it only when the batteries are full in the afternoons, using otherwise-lost power, and not running it at night (storing up the demand for the next afternoon). If that works well enough, I can start running the refrigerator on PV power too, in sunny weather.
My system is intended as a backup power system for when the grid goes down, thus I always have the tension between trying to keep the batteries topped off (in case of an outage) vs. trying to make some use of the PV power other than floating the batteries. That's why I'm concentrating first on using the surplus power on sunny afternoons.