Well lets see if I can explain.
The hot water loop is in the coal stove, it collects heat from the coal fire any time there is a fire in the stove. It will collect heat from the coal fire even if the water in the loop is near boiling.
There is no thermostatic control of the water temperature in the hot water loop, it just thermo-siphons the hot water to the water tank any time the loop in the stove is hotter than the water in the tank, which is any time there is a fire in the stove.
Said another way, the hot water loop is transfering heat from the coal fire to the water tank anytime there is a coal fire.
So... any time the hot water tank hasn't been asked to give up hot water for several hours, and the coal fire is burning, the heat collected by the loop can exceed 212* and the relief valve will open.
The advantage to this system is that the water in the tank will always be quite hot if the coal fire is going. Unlike KTM's and my system, where if no hot water was used for say 18-20 hours, the water temp in the tank may drop low enough to trigger the electric or propane heater to raise the water temp up to the tank's minimum setting.
What KTM, myself and others do is run the hot water for a few minutes before going to bed at night, this fills the tank with hot water from our heat exchangers and hopefully the water temp stays high enough through the night that no electricity or propane is used while we sleep.
I think that explains it... Greg L
What I would recommend, is if the PRV opens often, then add a length of radiant baseboard finned tube to the return line from the tank. Then the return will give up a fair amount of heat to the room, less heat with warm water, a lot of heat with very hot water. This will help 'regulate' the water temperature.