In a hot water (hydronic) heating systems, water is heated in a gas, oil, or electric boiler and circulated through pipes to convectors; older systems use radiators. When the thermostat calls for heat, a "circulator pump" in the boiler return line moves water through the pipes to the convectors, which give up heat to the rooms. When the circulator is not pumping, a "flow-control valve" stops the water flow to protect the convectors from overheating.

Older gravity systems have no circulator pumps and depend on lighter, heated water rising in the pipes to the radiators; and cooler, heavier water gravitating back to the boiler.

Single-pipe systems, with short branch lines to each convector, are used in small to medium-sized houses. Two pipe systems use separate supply and return pipes, and are more appropriate for larger houses, because cool return water from convectors doesn't pass through others. This maintains a higher water temperature at far ends of the system.

As heated water expands, an air-tight "expansion tank" on the hot side of the boiler takes in the excess; and if the tank fills up, a "safety-relief valve" on top of the boiler relieves the pressure. If this valve leaks under normal operating pressure, you can open it (beware of hot spray) and close it. If the dripping continues, the relief valve needs to be replaced.

Twice a year, if the circulator pump and motor are not permanently lubricated, add a couple drops of oil. If you want to work on your boiler, shut off the power, let it cool down and then drain the system if necessary. Older tanks can be drained when they're too full, but newer "diaphragm tanks" should be drained only by qualified service personnel.

When first heating cold water in a hydronic system, it releases air which becomes trapped in the convectors, preventing hot water from entering. To release this air, most convectors are installed with air vents, which can be opened with a screwdriver until hot water squirts out.

Efficiency can be improved by insulating pipes, or by partially closing convector shut-off valves in rooms requiring less heat. Systems can be divided into zones with one or more rooms controlled by a thermostatic "zone valve". Alternatively, a separate circulator is sometimes used for each zone, or you could have thermostatic valves installed on each convector, but these options might be expensive.

Boilers are more expensive to install than other systems, but are generally trouble-free, very efficient, and last much longer than other types of heating systems.

* Some newer systems use non-metallic piping, installed in/under floors, to distribute hot water in the rooms to be heated. In some cases, a domestic oil or gas-fired hot water tank can be fitted with a circulating pump, and the hot water directed to convectors or a heating coil installed in a hot air plenum.