One of the oldest and most often seen instances are steam or hot water radiators. In large-scale commercial buildings such as high-rise and campus facilities, operations may include both chilled and heated water loops to provide for comfort air conditioning needs. Chillers and cooling towers are used separately or together as means to provide water cooling, while boilers provide for heated water needs. In addition, many larger cities in the world have a district heating system that provides, through underground piping, publicly available steam and chilled water. By paying a service fee, a building in the service district may be connected to these. For example, see Seattle Steam Company.

Hydronic systems may be divided into several general categories:

Single-pipe steam

Single-pipe steam radiatorThe 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.

Despite its name, a steam radiator does not primarily heat a room by radiation. If positioned correctly a radiator will create an air convection current in the room, which will provide the main heat transfer mechanism. It is generaly agreed that for the best results a steam radiator should be no more than one to two inches from a wall.

Single-pipe systems are limited in both their ability to deliver high volumes of steam (that is, heat) and the ability to control the flow of steam to individual radiators (because closing off the steam supply traps condensate in the radiators). Because of these limitations, single-pipe systems are no longer installed.

Two-pipe steam systems
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.

Very large scale systems can be built using the two-pipe principle. For example, rather than heating individual radiators, the steam may be used in the reheat coils of large air handlers to heat an entire floor of a building.

Water loops
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).

In homes, the water loop may be as simple as a single pipe that "loops" the flow through every radiator in a zone. In such a system, flow to the individual radiators can not be modulated as all of the water is flowing through every radiator in the zone. Slightly more complicated systems use a "main" pipe that flows uninterrupted around the zone; the individual radiators tap off a small portion of the flow in the main pipe. In these systems, individual radiators can be modulated. Alternatively, a number of loops with several radiators can be installed, the flow in each loop or zone controlled by a zone valve connected to a thermostat.

In any water system, the water is circulated by means of one or more circulator pumps. This is in marked contrast to steam systems where the inherent pressure of the steam is sufficient to distribute the steam to remote points in the system. A system may be broken up into individual heating zones using either multiple circulator pumps or a single pump and electrically-operated zone valves.

Boiler water treatment
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)

Air Elimination
All hydronic systems must have a means to eliminate air from the system. A properly designed system that is air-free should provide many years of excellent performance.

Air causes irritating system noise in addition to interrupting proper heat transfer as the system fluids circulate throughout the system. In addition, unless reduced below an acceptable level, the oxygen found within water will cause corrosion. This corrosion can cause rust and scale to build up on the system piping. Over time these particles can become loose and travel throughout the system. The particles can reduce flow and even clog the system in addition to causing damage to pump seals and other system components.

In steam systems, individual radiators are usually equipped with a thermostatic bleed valve. At room temperature, the valve opens the radiator to the air, but as hot steam flows into the radiator and pushes the contained air out, the valve heats and eventually closes, preventing steam from escaping into the room.

Water-loop systems can also experience air problems. Air found within hydronic water-loop systems may be classified into three forms:

1. Free air. Various devices such as manual and automatic air vents are used to address Free air which floats up to the high points throughout the system. Automatic air vents contain a valve that is operated by a float. When air is present, the float drops, allowing the valve to open and bleed air out. When water reaches (fills) the valve, the float lifts, blocking the water from escaping. Small (domestic) versions of these valves in older systems are sometimes fitted with a Schraeder-type air valve fitting and any trapped, now-compressed air can be bled from the valve by manually depressing the valve stem until water rather than air begins to emerge.

2. Entrained air. Entrained air is the air bubbles that travel around in the piping at the same velocity as the water. Air "scoops" are one example of products which attempt to remove this type of air.

3. Dissolved air. Dissolved air is also present in the system water and the amount is determined principally by the temperature and pressure (see Henry's Law) of the incoming water. On average tap water contains between 8-10% dissolved air by volume. Removal of Dissolved air and Free / Entrained air can be achieved with a high-efficiency air elimination device that includes a coalescing medium that continually scrubs the air out of the system.

Accommodating thermal expansion
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 diaphragm pressurised with compressed air. The expansion tank accommodates the expanded water by further air compression and helps maintain a roughly-constant pressure in the system across the expected change in fluid volume.

Automatic fill mechanisms
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.

Safety mechanisms
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.