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Hydronics () is the use of
liquid water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a s ...
or gaseous water ( steam) or a water solution (usually
glycol A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified. The most common industrial diol is e ...
with water) as heat-transfer medium in
heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
and
cooling Cooling is removal of heat, usually resulting in a lower temperature and/or phase change. Temperature lowering achieved by any other means may also be called cooling.ASHRAE Terminology, https://www.ashrae.org/technical-resources/free-resources/as ...
systems. The name differentiates such systems from
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and refrigerant systems. 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 Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
.
Chiller A chiller is a machine that removes heat from a liquid coolant via a vapor-compression, adsorption refrigeration, or absorption refrigeration cycles. This liquid can then be circulated through a heat exchanger to cool equipment, or another p ...
s and
cooling tower A cooling tower is a device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat an ...
s are used either separately or together as means to provide water cooling, while
boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...
s heat water. A recent innovation is the
chiller boiler system A chiller boiler system includes a chiller, a boiler, pumps and HVAC controls in a single pre-engineered package. This reduces complexity for the field installer and increases the simplicity of installation overall. Most chiller boiler packaged s ...
, 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 high temperature hot water 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 can include the following kinds of distributions: * Chilled water systems * Hot water systems * Steam systems * Steam condensate systems * Ground source heat pump systems


Classification

Hydronic systems are further 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

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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
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 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 (2.5 to 5cm) 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 preferred. These systems depend on the proper operation of thermostatic air-venting valves located on radiators throughout the heated area. When the system is not in use, these valves are open to the atmosphere, and radiators and pipes contain air. When a heating cycle begins, the boiler produces steam, which expands and displaces the air in the system. The air exits the system through the air-venting valves on the radiators and on the steam pipes themselves. The thermostatic valves close when they become hot; in the most common kind, the vapor pressure of a small amount of alcohol in the valve exerts the force to actuate the valve and prevent steam from leaving the radiator. When the valve cools, air enters the system to replace the condensing steam. Some more modern valves can be adjusted 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.


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
valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
s.


Two-pipe direct return system

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


Advantages

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


Disadvantages

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 , include balancing and flow-measuring devices at each terminal or branch circuit; and use control valves with a at the terminals.


Two-pipe reverse return system

The two-pipe reverse return configuration which is sometimes called 'the three-pipe system' is different from the two-pipe system in the way that water returns to the boiler. In a two-pipe system, once the water has left the first radiator, it returns to the boiler to be reheated, and so with the second and third etc. With the two-pipe reverse return, the return pipe travels to the last radiator in the system before returning to the boiler to be reheated.


Advantages

The advantage with the two-pipe reverse return system is that the pipe run to each radiator is about the same, this ensures that the frictional resistance to the flow of water in each radiator is the same. This allows easy balancing of the system.


Disadvantages

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


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 Air conditioning, often abbreviated as A/C or AC, is the process of removing heat from an enclosed space to achieve a more comfortable interior environment (sometimes referred to as 'comfort cooling') and in some cases also strictly controlling ...
. 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 cannot 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 Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span * Frigid zone, ...
connected to a thermostat. In most water systems, the water is circulated by means of one or more
circulator pump A circulator pump or circulating pump is a specific type of pump used to circulate gases, liquids, or slurries in a closed circuit. They are commonly found circulating water in a hydronic heating or cooling system. Because they only circulate li ...
s. 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 valve Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span * Frigid zone, ...
s.


Improved efficiency and operating costs

There have been considerable improvements in the efficiency and therefore the operating costs of a hydronic heating system with the introduction of insulating products. Radiator Panel system pipes are covered with a fire rated, flexible and lightweight elastomeric rubber material designed for thermal insulation. Slab Heating efficiency is improved with the installation of a thermal barrier made of foam. There are now many product offerings on the market with different energy ratings and installation methods.


Balancing

Most hydronic systems require balancing. This involves measuring and setting the flow to achieve an optimal distribution of energy in the system. In a balanced system every radiator gets just enough hot water to allow it to heat up fully.


Boiler water treatment

Residential 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 Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
* Prevent freezing of the water in the system * Increase the boiling point of the water in the system * Inhibit the growth of
mold A mold () or mould () is one of the structures certain fungi can form. The dust-like, colored appearance of molds is due to the formation of spores containing fungal secondary metabolites. The spores are the dispersal units of the fungi. Not ...
and
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
* Allow improved leak detection (for example, dyes that
fluoresce Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...
under
ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
)


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 noises, and interrupts proper heat transfer to and from the circulating fluids. In addition, unless reduced below an acceptable level, the
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
dissolved in water causes
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
. 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.


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 In physical chemistry, Henry's law is a gas law that states that the amount of dissolved gas in a liquid is directly proportional to its partial pressure above the liquid. The proportionality factor is called Henry's law constant. It was formulate ...
) of the incoming water. On average, tap water contains between 8-10% dissolved air by volume. Removal of dissolved, free and entrained air can only be achieved with a high-efficiency air elimination device that includes a coalescing medium that continually scrubs the air out of the system. Tangential or centrifugal style air separator devices are limited to removal of free and entrained air only.


Accommodating thermal expansion

Water expands as it heats and contracts as it cools. A water-loop hydronic system must have one or more
expansion tank An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure. The tank is partially filled with air, whose compressibil ...
s 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
cistern A cistern (Middle English ', from Latin ', from ', "box", from Greek ', "basket") is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns are distinguished from wells by t ...
s open to atmospheric pressure are also used. Water also expands drastically during as it vaporizes, or flashes, into steam. Sparge pipes can help accommodate flashing that may occur as high pressure condensate enters a lower pressure region.


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 Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its wat ...
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 A relief valve or pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system; pressure might otherwise build up and create a process upset, instrument or equipment failure, or fire. The pressure is re ...
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. Rapid condensation of steam can also lead to water hammer, which during rapid volume change from gas to liquid leads to a powerful vacuum force. This can damage and destroy fittings, valves and equipment. Proper design and the addition of vacuum breakers reduce or eliminate the risk of these problems.


Typical schematic with control devices shown


See also

*
Aquastat An aquastat is a device used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too frequently, aquastats have a high limit temperature and a low limit. If the thermostat A thermostat is a ...
*
Central heating A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning (short: HVAC) systems, which can both cool and warm interior spaces. ...
* Hydronic balancing *
Radiant cooling Radiant heating and cooling is a category of HVAC technologies that exchange heat by both convection and radiation with the environments they are designed to heat or cool. There are many subcategories of radiant heating and cooling, including ...
*
Radiant heating Radiant heating and cooling is a category of HVAC technologies that exchange heat by both convection and radiation with the environments they are designed to heat or cool. There are many subcategories of radiant heating and cooling, including: ...
*
Uniform Mechanical Code The Uniform Mechanical Code (UMC) is a model code developed by the International Association of Plumbing and Mechanical Officials (IAPMO) to govern the installation, inspection and maintenance of HVAC (heating, ventilating and air-conditioning) ...


References


External links


Fluid Handling Representatives Association
- Hydronics association website.
Hydronic Heating System Red Deer
- Drain Doctor
Fluid Handling Representatives Association
- Hydronics association website.

* ttps://www.cambrohydronicheating.com.au/hydronic-heating Melbourne Hydronic Heating- Cambro Hydronic Heating.
Uniform Mechanical Code Website

Uniform Solar, Hydronics & Geothermal Code Website
{{HVAC Heating, ventilation, and air conditioning Plumbing