Oh...
Okay, so the boiler heats water in a closed system to 190 degrees. Pumps then pump this hot water through the house to baseboard radiators. Basically a copper tube with little fins crimped onto it. The radiators passively heat the air through convection...no fan involved.
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Since we already have a unit making hot water, it's pretty trivial to make another loop passing that hot water through a tank to heat up "domestic hot water" for bathing/cooking. There's 5 heat zones and one more for DHW.
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Now to make things even more needlessly complicated, the baseboard radiant heat is more of a backup. Most of the house is heated & cooled through forced air. The actual furnace though is a little different. It's a geothermal system. It's a heat pump, so an air conditioner that can pump heat into or out of the house. Instead of using an air conditioner coil outside (an "Air-Source Heat Pump"), it uses water passed through loops that are drilled 60' into the ground, taking advantage of the fact that the earth stays about 60 degrees year round, and making it more efficient than an ASHP in both heating and cooling. This is the pump that circulates the water through the loop, and the box on the right is the actual heat pump and air handler unit.
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The geothermal system was put in about 20 years after the house was built, so it doesn't reach all areas, necessitating the boiler to still be in service. Neither system reaches the room above the garage that wasn't intended to be living space originally, so that room has it's own traditional forced air furnace, like yours. Then this year we put a mini-split ASHP in that space as well for air conditioning and more efficient heating when it's not super cold. So I have 4 HVAC systems to look after
