What are the working parts involved in a hydronic heating system; we know we’ll have our cast iron radiators, but do I need a boiler?

You’ll need a boiler for sure. Boilers are very efficient appliances, they have the ability to burn up to 98% of the fuel’s energy and convert it to heat for the home, generally forced air can’t do that. Air by its own nature is an insulator, it doesn’t like to absorb heat, it doesn’t like to release heat; water on the other hand, loves to absorb energy. It will take more energy out of the gas than forced air, making it a more efficient heating system.

The footprint of a boiler as opposed to a furnace is significantly smaller. Nowadays, most are wall mounted. The in-house distribution system would be run in pipe, again significantly smaller. We don’t have to worry about bulkheads throughout the house and floor grills full of dirt, blowing around bacteria and dust. The boiler puts the energy into the water and the distribution system pumps it throughout the house. Every room will have a heat emitter of some sorts, whether it’s a radiant floor, a radiator, or a fan coil unit that will disperse the heat from the water into the room itself.