The basic idea
A hot-water boiler heats water rather than moving heated air through ductwork. The heated water circulates through piping to baseboard radiation, radiators or other hydronic terminal units, releases heat to the home and returns to the boiler to be heated again.
The major components
- Boiler and burner or heat source
- Circulator pumps or other means of water circulation
- Zone valves or multiple circulators where the home has heating zones
- Expansion tank to accommodate water expansion
- Aquastat and other operating/safety controls
- Air-removal components, piping and terminal units
Why one room or zone can be cold
A hydronic system can have a properly operating boiler while a circulation, zoning, air or distribution problem prevents heat from reaching part of the house. That is why diagnosis should consider the complete system, not only whether the burner fires.
Hot-water heat is different from steam
Hydronic hot-water systems circulate liquid water. Steam systems intentionally boil water and distribute steam. The controls, operating pressures and distribution behavior are different.