Hydronics
Encyclopedia
Hydronics is the use of water
as the heat-transfer medium in heating
and cooling system
s.
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
. Chiller
s and cooling tower
s are used separately or together as means to provide water cooling, while boiler
s heat water. A recent innovation is the chiller boiler system
, which provides an efficient form of HVAC
for homes and smaller commercial spaces.
In the oldest modern hydronic heating technology, a single-pipe steam system delivers steam to the radiator
s 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.
s as well as gravity-induced flow. The flow of steam to individual radiators can be modulated using manual or automatic valve
s.
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.
.
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
connected to a thermostat
.
In most water systems, the water is circulated by means of one or more circulator pump
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
s.
, this involves measuring and setting the flow to achieve an optimal distribution of energy in the system.
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.
, 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.
) 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.
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
s open to atmospheric pressure are also used.
of system water (and any water treatment chemicals) into the water supply.
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.
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
as the heat-transfer medium in heating
Heating system
A heating system is a mechanism for maintaining temperatures at an acceptable level; by using thermal energy within a home, office, or other dwelling. Often part of an HVAC system. A heating system may be centralized or distributed.-See also:...
and cooling system
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
s.
Some of the oldest and most common examples are steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
and hot-water radiators. Historically, in large-scale commercial buildings such as high-rise and campus
Campus
A campus is traditionally the land on which a college or university and related institutional buildings are situated. Usually a campus includes libraries, lecture halls, residence halls and park-like settings...
facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
. Chiller
Chiller
A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required.-Use in air conditioning:...
s and cooling tower
Cooling tower
Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or in the case of closed circuit dry cooling towers rely...
s are used separately or together as means to provide water cooling, while boiler
Boiler
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
s heat water. A recent innovation is the chiller boiler system
Chiller boiler system
A chiller boiler system is a chiller, 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...
, which provides an efficient form of HVAC
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
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.Classification
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
Radiator
Radiators are heat exchangers used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in automobiles, buildings, and electronics...
s where the steam gives up its heat and is condensed
Condensation
Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....
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 pumpCondensate pump
A condensate pump is a specific type of pump used to pump the condensate produced in an HVAC , refrigeration, condensing boiler furnace or steam system...
s as well as gravity-induced flow. The flow of steam to individual radiators can be modulated using manual or automatic valve
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
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 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.Advantages
This system is often considered "self balancing", however, valves should always be included.Disadvantages
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
HVAC
HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
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 conditioningAir conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...
.
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 valve
A zone valve is a specific type of valve used to control the flow of water or steam in a hydronic heating or cooling system.In the interest of improving efficiency and occupant comfort, such systems are commonly divided up into multiple zones. For example, in a house, the main floor may be served...
connected to a thermostat
Thermostat
A thermostat is the component of a control system which regulates the temperature of a system so that the system's temperature is maintained near a desired setpoint temperature. The thermostat does this by switching heating or cooling devices on or off, or regulating the flow of a heat transfer...
.
In most water systems, the water is circulated by means of one or more circulator pump
Circulator pump
A circulator 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...
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 valve
A zone valve is a specific type of valve used to control the flow of water or steam in a hydronic heating or cooling system.In the interest of improving efficiency and occupant comfort, such systems are commonly divided up into multiple zones. For example, in a house, the main floor may be served...
s.
Balancing
Most hydronic systems require balancingHydronic Balancing
Hydronic Balancing is the process of optimising the distribution of water in a building’s heating or cooling system so it provides the intended indoor climate at optimum energy efficiency and minimal operating cost....
, 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:- Inhibit corrosionCorrosionCorrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
- Prevent freezingAntifreezeAntifreeze is a freeze preventive used in internal combustion engines and other heat transfer applications, such as HVAC chillers and solar water heaters....
of the water in the system - Increase the boiling point of the water in the system
- Inhibit the growth of moldMoldMolds are fungi that grow in the form of multicellular filaments called hyphae. Molds are not considered to be microbes but microscopic fungi that grow as single cells called yeasts...
and bacteriaBacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals... - Allow improved leak detection (for example, dyeDyeA dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....
s that fluoresceFluorescenceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
under ultraviolet light)
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
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
dissolved in water causes corrosion
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
. 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 fittingSchrader valve
The Schrader valve is a brand of pneumatic tire valve used on virtually every motor vehicle in the world today. The Schrader company, for which it was named, was founded in 1844 by August Schrader...
, 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 LawHenry's law
In physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...
) 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 tankExpansion tank
An expansion tank or expansion vessel is a small tank used to protect closed water heating systems and domestic hot water systems from excessive pressure...
s in the system to accommodate this varying volume of the working fluid. These tanks often use a rubber diaphragm pressurised with compressed air
Compressed air
Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....
. 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
Cistern
A cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns are distinguished from wells by their waterproof linings...
s 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 backflowBackflow prevention device
A backflow prevention assembly is used to protect water supplies from contamination or pollution.In water supply systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower etc...
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 valveRelief valve
The relief valve is a type of valve used to control or limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire....
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.
See also
- Radiant heatingRadiant heatingRadiant heating is a technology for heating indoor and outdoor areas. Heating by radiant energy is observed everyday, the warmth of the sunshine being probably the most commonly observed example. Radiant heating as a technology is typically more narrowly defined...
- Radiant coolingRadiant coolingA radiant cooling system refers to a temperature-controlled surface that cools indoor temperatures by removing sensible heat and where more than half of heat transfer occurs through thermal radiation....
- BoilerBoilerA boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...
- ChillerChillerA chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required.-Use in air conditioning:...
- AquastatAquastatAn aquastat is a device used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too often, aquastats have a high limit temperature and a low limit. If the thermostat is calling for heat, the boiler will fire until the high limit is reached, then shut off...
External links
- Fluid Handling Representatives Association - Hydronics association website.
- Hydronic Heating Systems Pty Ltd - Hydronic Heating Installations Melbourne