Sanwizard: first thing to do is determine if it is a drain back system, or a closed loop system with glycol. I was just going to type that it should be fairly obvious, but then it occurred to me that most people don't know anything about solar hot water, or even boilers, for that matter! Drain back is the simpler of the two, and is commonly used in cold areas. The problem, as mentioned by self sufficient, is what can happen if you lose power, or the system freezes. Of course, there are other things that go wrong too, like stagnation. But basically, a drain back system has a line going up to panels, and drains into a tank that receives the hot water. There will be a pump to circulate the water back up to the panels, where it will trickle back down. The idea being that if the power goes out, sun goes down, etc...the water just drains back by gravity, and doesn't freeze. There is normally a controller that monitors the temperature, and can turn the pump on and off. This is what I have, and it feeds into the same tank as my boiler. (Buffer tank) I have a heat exchanger mounted on that tank hooked to the panels, so that water never mixes with the rest. The heat exchange is done by the two fluids passing each other in the plate exchanger. Your 80 gallon tank is likely to have a coil in it that allows the solar heated water to heat the other water in the tank. Water to water heat exchanger.
A closed system will look very similar, but a lot depends on how it was set up. If they did a closed system, and you are in a cold area, it will have glycol. The glycol cannot mix with other water in your system, unless you have a boiler and are using glycol in the entire system. Very unlikely. If your tank has a couple sets of ins and out pipes, it is probably a heat exchanger tank with a coil in it. There will almost certainly be some sort of control, and temperature gauges to monitor the panels. A system that freezes with glycol is a real mess, and all it takes is one day, like where we had -50 temps, and you are screwed.
There are a slew of other possibilities, like a prior owner disabled the system, didn't finish it, had a leak, etc. Generally speaking, you are wasting your time with most plumbers. They don't know boilers. I do all my own work, because even when you hire a "pro", they often don't know what they are talking about. It just depends. There are some really smart, talented guys working with controls, etc. I'd call around to your local HVAC places, and ask for tips on a local guy who is good at radiant heat. That will be your best bet.
Properly done, boilers are awseome, and solar panels can be very effeciaent and effective. But frankly, it probaly makes more sense overall to add solar capacity and do a divert load.