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Hydronics is the use of water or another liquid heat transfer medium such as glycol 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.
- 1 District heating
- 2 Types of hydronic system
- 3 Single-pipe steam
- 4 Two-pipe steam systems
- 5 Two pipe direct return system
- 6 Two pipe reverse return system
- 7 Water loops
- 8 Improved efficiency and operating costs
- 9 Balancing
- 10 Boiler water treatment
- 11 Air elimination
- 12 Accommodating thermal expansion
- 13 Automatic fill mechanisms
- 14 Safety mechanisms
- 15 Typical schematic with control devices shown
- 16 See also
- 17 References
- 18 External links
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
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
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
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 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 preferred.
These systems depend on the proper operation of thermal 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 valves close when they become hot; a small amount of alcohol in the valve vaporizes, exerting mechanical force to close the valve and prevent steam from leaving the radiator. When the heating cycle ends, air enters the system to replace the condensing steam. The valves reopen when they cool, so the air can again be expelled when the next heating cycle starts.
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 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 two pipe reverse return configuration which is sometimes called 'the three pipe system' is different to the two pipe system in the way 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.
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.
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 water may be used in the reheat coils of large air handlers to heat an entire floor of a building.
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 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 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.
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.
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
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)
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, 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.
Water-loop systems can also experience air problems. Air found within hydronic water-loop systems may be classified into three forms:
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 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 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 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 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.
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
- Fluid Handling Representatives Association - Hydronics association website.
- Hydronic and Boiler Water Loops for HVAC - Hydronic and Boiler Water Loops for HVAC
- Problems to look out for when installing a Hydronic Heating system