Expansion tank - wikipedia Expansion tank This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. ( June 2012 ) Expansion tank in central heating system 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 and absorbs excess water pressure caused by thermal expansion. Contents 1 Description 2 Domestic applications 3 Automotive applications 4 Larger systems 5 See also 6 References Description 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 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 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 oxygen within the pipes, reducing corrosion in the system. Domestic applications Expansion tank for boilers 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, and the trapped air acted as a crude expansion tank. In new and upgraded systems, expansion tanks are used 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 ; these had the disadvantage of being exposed to the 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 they use for heat transfer. A minimum pressure of 4 - 5 psig at the top of a closed hydronic system is suggested. In Europe the design and the construction of expansion tanks are ruled by EN 13831 according to Pressure Equipment Directive ( PED ) 97 / 23 / EC. Automotive applications Expansion tank in a 1987 Saab 90 ( brown plastic tank with white lid at top of picture ) An expansion tank 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. Larger systems Similar devices are used in large - scale pumping stations, where they may be called expansion chambers or hydrophores, to maintain an even pressure and to reduce the effects of water hammer. See also Hydronics References Jump up ^ ACHR News Pressurization Of Closed Hydronic Systems Jump up ^ Pumping Station Design, By Garr M. Jones, Robert L. Sanks, Bayard E. Bosserman, George Tchobanoglous, Ch. 22 p. 44 ( 1 ) retrieved 15 May 2012 Heating, ventilation and air conditioning Fundamental concepts Air changes per hour Bake - out Building envelope Convection Dilution Domestic energy consumption Enthalpy Fluid dynamics Gas compressor Heat pump and refrigeration cycle Heat transfer Humidity Infiltration Latent heat Noise control Outgassing Particulates Psychrometrics Sensible heat Stack effect Thermal comfort Thermal destratification Thermal mass Thermodynamics Vapour pressure of water Technology Absorption refrigerator Air barrier Air conditioning Antifreeze Automobile air conditioning Autonomous building Building insulation materials Central heating Central solar heating Chilled beam Chilled water Constant air volume ( CAV ) Coolant Dedicated outdoor air system ( DOAS ) Deep water source cooling Demand - controlled ventilation ( DCV ) Displacement ventilation District cooling District heating Electric heating Energy recovery ventilation ( ERV ) Firestop Forced - air Forced - air gas Free cooling Heat recovery ventilation ( HRV ) Hybrid heat Hydronics HVAC Ice storage air conditioning Kitchen ventilation Mixed - mode ventilation Microgeneration Natural ventilation Passive cooling Passive house Radiant heating and cooling system Radiant cooling Radiant heating Radon mitigation Refrigeration Renewable heat Room air distribution Solar air heat Solar combisystem Solar cooling Solar heating Thermal insulation Underfloor air distribution Underfloor heating Vapor barrier Vapor - compression refrigeration ( VCRS ) Variable air volume ( VAV ) Variable refrigerant flow ( VRF ) Ventilation Components Air conditioner inverter Air door Air filter Air handler Air ionizer Air - mixing plenum Air purifier Air source heat pumps Automatic balancing valve Back boiler Barrier pipe Blast damper Boiler Centrifugal fan Chiller Condensate pump Condenser Condensing boiler Convection heater Cooling tower Damper Dehumidifier Duct Economizer Electrostatic precipitator Evaporative cooler Evaporator Exhaust hood Expansion tank Fan coil unit Fan heater Fire damper Fireplace Fireplace insert Freeze stat Flue Freon Fume hood Furnace Furnace room Gas compressor Gas heater Gasoline heater Geothermal heat pump Grease duct Grille Ground - coupled heat exchanger Heat exchanger Heat pipe Heat pump Heating film Heating system High efficiency glandless circulating pump High - efficiency particulate air ( HEPA ) High pressure cut off switch Humidifier Infrared heater Inverter compressor Kerosene heater Louver Mechanical fan Mechanical room Oil heater Packaged terminal air conditioner Plenum space Pressurisation ductwork Process duct work Radiator Radiator reflector Recuperator Refrigerant Register Reversing valve Run - around coil Scroll compressor Solar chimney Solar - assisted heat pump Space heater Smoke exhaust ductwork Thermal expansion valve Thermal wheel Thermosiphon Thermostatic radiator valve Trickle vent Trombe wall Turning vanes Ultra-low particulate air ( ULPA ) Whole - house fan Windcatcher Wood - burning stove Measurement and control Air flow meter Aquastat BACnet Blower door Building automation Carbon dioxide sensor Clean Air Delivery Rate ( CADR ) Gas sensor Home energy monitor Humidistat HVAC control system Intelligent buildings LonWorks Minimum efficiency reporting value ( MERV ) OpenTherm Programmable communicating thermostat Programmable thermostat Psychrometrics Room temperature Smart thermostat Thermostat Thermostatic radiator valve Professions, trades, and services Architectural acoustics Architectural engineering Architectural technologist Building services engineering Building information modeling ( BIM ) Deep energy retrofit Duct leakage testing Environmental engineering Hydronic balancing Kitchen exhaust cleaning Mechanical engineering Mechanical, electrical, and plumbing Mold growth, assessment, and remediation Refrigerant reclamation Testing, adjusting, balancing Industry organizations ACCA AMCA ASHRAE ASTM International BRE BSRIA CIBSE LEED SMACNA Health and safety Indoor air quality ( IAQ ) Passive smoking Sick building syndrome ( SBS ) Volatile organic compound ( VOC ) See also ASHRAE Handbook Building science Fireproofing Glossary of HVAC terms Template : Home automation Template : Solar energy Retrieved from `` https://en.wikipedia.org/w/index.php?title=Expansion_tank&oldid=781340208 '' Categories : Plumbing Engine cooling systems Heating, ventilating, and air conditioning Hidden categories : Articles lacking in - text citations from June 2012 All articles lacking in - text citations Talk Contents About Wikipedia Català Čeština Deutsch Español Italiano Nederlands Norsk Русский Svenska Edit links This page was last edited on 20 May 2017, at 16 : 48. 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Question: purpose of expansion tank on a hot water heater
Answer: protect closed (not open to atmospheric pressure) water heating systems and domestic hot water systems from excessive pressure