I have a similar system with my boilers and indirect fired water heater.
I have a coil in my indirect fired water heater that uses the 180 to 190 degree boiler water to heat the water surrounding that coil in the water heater up to 135 degrees C.
Basically, when the indirect fired water heater calls for heat, it sends a signal to the Tekmar 265 controller that controls both of my Weil McLaine Ultra 310 boilers. One of the boilers will fire up and heat water up to about 180 degrees in a jiff. ONLY the circulating pump that pumps water through the indirect fired water heater will turn on, NOT the main circulating pump that pumps water through the radiators of my apartment block apartments. This indirect fired heating process will continue until the water temperature in the hot water heater is at 135 deg. C, when the Amtrol 120 gallon tank then tells the Tekmar 265 controller that it no longer needs to be heated, and the Tekmar controller then shuts the boiler off, but keeps that pump mentioned earlier on for 5 minutes more so that the remaining 180 degree heat in the boiler can be transfered to the 135 degree heat in the hot water tank. Thereafter, both the boiler and the Amtrol indirect fired water heater just sit there losing heat to their surroundings until the temperature of the water in the Amtrol 120 gallon tank falls below 120 degrees C. Then the dance starts anew.
That's how my system works, but my system uses two high efficiency boilers that only hold about 10 gallons of water each in a 15 pound aluminum heat exchanger to heat up 120 gallons of water in my hot water tank. I am not heating up a 400 pound cast iron boiler with 100 pounds of boiler water in it every time I want to heat up 40 gallons of potable water. There would be a tremendous difference in the thermal efficiency of our respective systems, I'd venture to guess.
Still, your system has to be reasonably efficient (either thermally or economically) for it to have become as popular as it is.