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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 compressibility cushions shock caused by water hammer [ citation needed ] and absorbs excess water pressure caused by thermal expansion. [1]
The modern vessel is a small container or tank divided in two by a rubber diaphragm. One side is connected to the pipe work of the heating system and therefore contains water. The other, the dry side, contains air under pressure, and normally a Schrader valve (car-tire type valve stem) for checking pressures and adding air when needed. When the heating system is empty or at the low end of the normal range of working pressure, the diaphragm is pushed against the water inlet; as the water pressure increases, the diaphragm moves, compressing the air on its other side.
An older style of expansion tank was larger, oriented horizontally, and had no rubber diaphragm separating the water from the air pocket. This now-obsolete style would gradually transfer air from the tank to the highest point in the system, due to air dissolving in the water, and then coming out of solution elsewhere in the system. This in turn required periodic draining of the expansion tank, as well as periodic bleeding of unwanted air from other sections of the system, to maintain its effectiveness.
The rubber diaphragm in modern expansion tanks prevents this undesired transfer of air, and helps maintain low levels of dissolved oxygen within the pipes, reducing corrosion in the system.
When expansion tanks are used in domestic hot water (DHW) systems, the tank and the diaphragm must conform to drinking water regulations and must be capable of accommodating the required volume of water.
In the past, domestic plumbing systems often contained more air than they do currently[ citation needed ], and the trapped air acted as a crude expansion buffer. In new and upgraded systems, expansion tanks are designed in and installed [2] more frequently than in the past.
In the UK, prior to the use of sealed expansion tanks, "open" tanks were installed in the roof space to accommodate the water's expansion[ citation needed ]; these had the disadvantage of being exposed to cold air in the roof space. This, without effective loft insulation, could fall below freezing, and could cause the pipework supplying the tank to freeze. However, with good pipe and tank insulation, this was in practice quite rare. Although such systems were remarkably trouble free, there are concerns about the potability of water from roof tanks due to the possibility of contamination. The other major disadvantage is that the water pressure from a roof tank is considerably lower than mains water pressure, making the use of mixer taps sometimes unpredictable.
Domestic hydronic heating and cooling systems generally include an expansion tank to buffer pressure changes due to expansion and contraction of the water used for heat transfer.
A minimum pressure of 28–34 kPa; 0.28–0.34 bar (4–5 psi) at the top of a closed hydronic system is suggested. [3]
In Europe the design and the construction of expansion tanks are ruled by EN 13831 according to Pressure Equipment Directive (PED) 97/23/EC.
An expansion tank, also known as "overflow bottle", is also used in the cooling system of most internal combustion engines, to allow the coolant, the antifreeze, and the air in the system to expand with rising temperature and pressure. The tank is also called a "coolant recovery tank", since it prevents venting and permanent loss of coolant, by allowing it to be sucked back into the cooling system as the engine cools.
Similar devices are used in large-scale pumping stations, where they may be called an expansion chamber [4] or a hydrophore, to maintain an even pressure and to reduce the effects of water hammer.
A pump is a device that moves fluids, or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy.
A thermostat is a regulating device component which senses the temperature of a physical system and performs actions so that the system's temperature is maintained near a desired setpoint.
A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid to flow through it in only one direction.
Water heating is a heat transfer process that uses an energy source to heat water above its initial temperature. Typical domestic uses of hot water include cooking, cleaning, bathing, and space heating. In industry, hot water and water heated to steam have many uses.
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 systems, which can both cool and warm interior spaces.
A humidifier is a household appliance or device designed to increase the moisture level in the air within a room or an enclosed space. It achieves this by emitting water vapor or steam into the surrounding air, thereby raising the humidity levels.
Hydronics is the use of liquid water or gaseous water (steam) or a water solution as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.
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 liquid within a closed circuit, they only need to overcome the friction of a piping system.
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.
Underfloor heating and cooling is a form of central heating and cooling that achieves indoor climate control for thermal comfort using hydronic or electrical heating elements embedded in a floor. Heating is achieved by conduction, radiation and convection. Use of underfloor heating dates back to the Neoglacial and Neolithic periods.
The wax thermostatic element was invented in 1934 by Sergius Vernet (1899–1968). Its principal application is in automotive thermostats used in the engine cooling system. The first applications in the plumbing and heating industries were in Sweden (1970) and in Switzerland (1971).
A fan coil unit (FCU), also known as a Vertical Fan Coil-Unit (VFC), is a device consisting of a heat exchanger (coil) and a fan. FCUs are commonly used in HVAC systems of residential, commercial, and industrial buildings that use ducted split air conditioning or with central plant cooling. FCUs are typically connected to ductwork and a thermostat to regulate the temperature of one or more spaces and to assist the main air handling unit for each space if used with chillers. The thermostat controls the fan speed and/or the flow of water or refrigerant to the heat exchanger using a control valve.
A freeze stat is a temperature sensing device for HVAC that monitors a heat exchanger to prevent its coils from freezing. Freeze stats can be used on both refrigerant-to-air, and refrigerant-to-liquid type heat exchangers and serve different purposes with similar goals for each.
Radiators and convectors are heat exchangers designed to transfer thermal energy from one medium to another for the purpose of space heating.
Surge control is the use of different techniques and equipment in a hydraulic system to prevent any excessive gain in pressure that would cause the hydraulic process pressure to exceed the maximum working pressure of the mechanical equipment used in the system.
Automatic balancing valves are utilised in central heating and cooling systems that rely on flow of water through the system. They use the latest flow technology to ensure that the design flow rate is achieved at all times irrespective of any pressure changes within the system.
A hot water storage tank is a water tank used for storing hot water for space heating or domestic use.
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 and refrigeration systems. It is designated as an American National Standard.
Tankless water heaters — also called instantaneous, continuous flow, inline, flash, on-demand, or instant-on water heaters — are water heaters that instantly heat water as it flows through the device, and do not retain any water internally except for what is in the heat exchanger coil unless the unit is equipped with an internal buffer tank. Copper heat exchangers are preferred in these units because of their high thermal conductivity and ease of fabrication. However, copper heat exchangers are more susceptible to scale buildup than stainless steel heat exchangers.
The Glossary of Geothermal Heating and Cooling provides definitions of many terms used within the Geothermal heat pump industry. The terms in this glossary may be used by industry professionals, for education materials, and by the general public.