You have two T&P valves, one on the boiler circuits, and one on the WH. They each protect their individual components. As stated, the water between the two do NOT (or at least should not) mix. A typical boiler is a closed system, so it needs an expansion tank. A WH MAY need one, but only if the potable water system is closed (a check valve in the incoming water line, or a PRV). If you do not have a check valve or PRV on the incoming potable water supply, you do NOT need an expansion tank connected to it to account for the WH creating the hot, expanded water.
So, which T&P valve is releasing water?
Your boiler typically has a pressure gauge...what does it read? If you have an auto-fill valve, it could be shot, raising the boiler's water pressure. Or, as Redwood indicated, the WH tank could have a leak between the potable water and the boiler water, which would raise the pressure on the boiler, and cause its T&P valve to release (the SuperStor is a good tank, and unlikely but anything is possible - plus, the pressure relief setting on the boiler is a lot lower than the one on the WH). You probably have a valve before the autofill valve...you could shut that off to see if the water stops flowing.
Air in the WH isn't a problem...as you use hot water, it will be purged. Air in the boiler's circuit to the WH might create problems, and it could prevent any heat from getting to it. If you have hot water, that's unlikely since to get hot, it needs flowing hot water from the boiler, and an air lock could stop that. Any boiler should have an air extraction device. There are various kinds, and some work better than others. Verify what you have, if you have one, is working and not gunked up. I like Spirovents, but there are others. If water isn't flowing through the boiler, it would likely quickly reach its high temperature limit and shut off. It might trip the T&P if there is a lot of mass to the boiler as steam is a lot bigger than liquid water.
Maybe a picture or a diagram of the layout would help. Way back at the beginning, when working properly, it was indicated that a T&P valve can release under any one of two conditions: excessive pressure, or excessive temperature. There is a manual release valve, and if the valve has corrosion or mineral buildups, if it does get opened, it might not close. So, the conditions that caused it to start leaking may have been resolved, but the valve may be bad, and the valve should be replaced.
We're getting some conflicting info, so it's hard to evaluate exactly what is going wrong.