With any centralized heating system, the goal is to take a source of heat in one location and distribute it throughout a structure. As the name suggests, hydronic heat accomplishes this with water.
You can identify a hyrdonic heating system by the larger radiators located throughout the building, often near sources of cold intrusion (doors and windows). These radiators are made of modular metal segments that are cast with hollow openings throughout, allowing water to pass through them.
The water is fed to these radiators through a system of pipes leading back to the central boiler. Here, water is heated, often to 150–200 degrees Fahrenheit, before heading back out to the radiators. The heat, as the aptly named fixtures imply, is then radiated out into the room.
Three positive elements come out of this distributed heat system using water. One is that it is very easy to control heat on a per-room or per-zone basis. Valves on the sides of the radiators give rudimentary control, and now modern day electrical valves can group several rooms together into a zone with a separate thermostat.
Tandem to this is the scalability brought with individualized control. Buildings with dozens of rooms can still run off of one appropriately sized boiler with no issue.
Because the heat is distributed through a closed water system, there is very little air movement. This greatly reduces allergen and dust movement. Additionally, many people prefer the constant radiant heat from a radiator without a burst of wind that one may have with a forced air system.
There are some downsides to water heat however. One is the obvious size of the radiators. While slimmer radiators have become available recently, most homes in the united states that have radiators still utilized decades old cast iron beasts. These large, and incredibly heavy, objects can be difficult to plan around and occupy a lot of space.
There are also a lack of viable ways of providing cooling via water to a house. In most cases, a separate system must be installed to supply cold air throughout a building supplied by hot water heat.
The future of hyrdonic heating is in under floor heat. This system removes the intrusive radiators while still providing a constant, pleasing heat. Best of all, it is evenly distributed throughout the entire living space. The same zoned advantages apply, and efficiencies using this method can go through the roof, despite starting in the floor.