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Hydronics is the use of water as the heat-transfer medium in heating and cooling systems. Some of the oldest and most common examples are steam and hot-water radiators. Historically, in large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning. Chillers and cooling towers are used separately or together as means to provide water cooling, while boilers heat water. A recent innovation is the chiller boiler system, which provides an efficient form of HVAC for homes and smaller commercial spaces.


District heating

Many larger cities have a district heating system that provides, through underground piping, publicly available steam and chilled water. A building in the service district may be connected to these on payment of a service fee.

Types of hydronic system


Basic types

Hydronic systems are of two basic types:

  • Steam or hot water
  • Chilled water


Hydronic systems are classified in five ways:

  • Flow generation (forced flow or gravity flow)
  • Temperature (low, medium, and high)
  • Pressurization (low, medium, and high)
  • Piping arrangement
  • Pumping arrangement

Piping arrangements

Hydronic systems may be divided into several general piping arrangement categories:

  • Single or one-pipe
  • Two pipe steam (direct return or reverse return)
  • Three pipe
  • Four pipe
  • Series loop

Single-pipe steam

Single-pipe steam radiator

In 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.

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 generally 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.

In order to work correctly, these systems depend on the proper operation of thermally closed air-venting valves located on radiators throughout the heated area. When not in use, these valves are open to the atmosphere, and radiators and pipes contain regular air. When a heating cycle begins, the boiler produces steam, which expands, rises, and displaces the regular air in the system. The regular air exits the system via the air-venting valves on the radiators, as well as air-venting valves placed on the steam pipes themselves. The valves close when steam reaches them, due to a small amount of alcohol in them turning into vapor and exerting mechanical force to close the valve. When the heating cycle ends, the steam in the radiators cools, the air-venting valve reopens, and regular air again enters the system.

To increase heat delivered to an area served by a radiator, a larger air-venting valve can be installed. Some more modern valves can also be adjusted so as to allow for more rapid or slower venting. In general, valves nearest to the boiler should vent the slowest, and valves furthest from the boiler should vent the fastest. Ideally, steam should reach each valve and close each and every valve at the same time, so that the system can work at maximal efficiency; this condition is known as a "balanced" system.

The most common problems with air-venting valves occur when they are painted over, crushed, or clogged with rust, often leading to homeowner frustration. Improperly trained service technicians often respond to complaints by increasing boiler steam pressure rather than replacing air-venting valves. This actually makes matters worse, by causing high pressure steam to leak or otherwise, as well as wasting heating oil and energy. Investing in new air-venting valves for an old or troublesome single-pipe steam system, as well as taking the time to correctly size and adjust them, will reduce or eliminate many headaches once completed and lower heating fuel use and bills.

Two-pipe steam systems

In two-pipe steam systems, there is a 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.

Two pipe direct return system

The return piping, as the name suggests, takes the most direct path back to the boiler.


Low cost of return piping in most (but not all) applications, and the supply and return piping are separated.


This system can be difficult to balance due to the supply line being a different length than the return, The further the heat transfer device is from the boiler the more pronounced the pressure difference. Because of this it is always recommended to: minimize the distribution piping pressure drops; use a pump with a flat head characteristic, include balancing and flow measuring devices at each terminal or branch circuit; and use control valves with a high head loss at the terminals.

Two pipe reverse return system

The return piping takes the same basic path as the supply back to the boiler.


This system is often considered "self balancing", however, valves should always be included.


The installer or repair person cannot trust that every system is self balancing without properly testing it.

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 almost always use heated water rather than steam. This opens the system to the possibility of also using chilled water to provide air conditioning.

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 most water systems, 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.


Most hydronic systems require balancing, this involves measuring and setting the flow to achieve an optimal distribution of energy in the system.

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:

Air elimination

All hydronic systems must have a means to eliminate air from the system. A properly designed, air-free system should continue to function normally for many years.

Air causes irritating system noise, as well as interrupting proper heat transfer to and from the circulating fluids. In addition, unless reduced below an acceptable level, the oxygen dissolved in water causes corrosion. This corrosion can cause rust and scale to build up on the piping. Over time these particles can become loose and travel around the pipes, reducing or even blocking the flow as well as damaging pump seals and other components.

Steam system

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 system

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

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 Schrader-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.

Entrained air

Entrained air is 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.

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, free and 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 as it heats and contracts as it 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. Simple cisterns open to atmospheric pressure are also used.

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.

Typical schematic with control devices shown


See also

External links


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