Here's a home designed and built boiler system:
This is a system that grew over time. It started with two electric boilers. Then a gas boiler was added and the electric boilers were kept as back-up. Plate exchangers were installed either to keep systems separate or because of pressure fluctuations. The second electric boiler and a pump are in a shop and another pump is in the pool house. A fan/coil unit with another pump is out of the picture.
There are several levels of priority: 1) DHW, 2) House, 3) Shop, and 4) Pool. Air conditioning is held off if the house is heating. A call for heat will start either the gas or electric boiler, a main circulating pump, a plate exchanger pump and the appropriate zone.
The controller is a Zelio programmable relay. Before the relay, the owner had an extension cord on each pump.
Unfortunately, the boilers are all too small and they still need to use the fireplace to heat the house.