Hydronics is the name for the use of water as the heat-transfer medium in heating and cooling systems.
The oldest modern hydronic heating technology, a single-pipe steam system delivers steam to the radiators where the steam gives up its heat and is condensed back to water. The radiators and steam supply pipes are pitched so that gravity eventually takes this condensate back down through the steam supply piping to the boiler where it can once again be turned into steam and returned to the radiators.
In two-pipe steam systems, there is a separate return path for the condensate and it may involve pumps as well as gravity-induced flow. The flow of steam to individual radiators can be modulated using manual or automatic valves.
Modern systems often use heated water rather than steam. This opens the system to the possibility of also using chilled water to provide air conditioning (air cooling).
Domestic (home) systems may use ordinary tap water, but sophisticated commercial systems often add various chemicals to the system water. For example, these added chemicals may:
- Inhibit corrosion
- Prevent freezing of the water in the system
- Increase the boiling point of the water in the system
- Inhibit the growth of mold and bacteria
- Allow improved leak detection (for example, dyes that fluoresce under ultraviolet light)
All hydronic systems must have a means to bleed air from the system.
Water expands and contracts as it heats and cools. A water-loop hydronic system must have one or more expansion tanks in the system to accommodate this varying volume of the working fluid. These tanks often use a rubber bladder pressurised with compressed air. This bladder bears on the water and maintains a roughly-constant pressure in the system across the expected change in fluid volume.
Hydronic systems are usually connected to a water supply (such as the public water supply). An automatic valve regulates the amount of water in the system and also prevents backflow of system water (and any water treatment chemicals!) into the water supply.
Excessive heat or pressure may cause the system to fail. At least one combination over-temperature and over-pressure relief valve is always fitted to the system to allow the steam or water to vent to the atmosphere in case of the failure of some mechanism (such as the boiler temperature control) rather than allowing the catastrophic bursting of the piping, radiators, or boiler. The relief valve usually has a manual operating handle to allow testing and the flushing of contaminants (such as grit) that may cause the valve to leak under otherwise-normal operating conditions.
