MACHINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only ENGINES positive displacement rotary or oscillating piston liquid and elastic fluid or elastic fluid liquid only reciprocating piston or other liquid and elastic fluid or elastic fluid liquid only non-positive displacement liquid and elastic fluid or elastic fluid liquid only PUMPS positive displacement rotary or oscillating piston reciprocating piston or other non-positive displacement In this class, the following terms are used with the meanings indicated: "stove" includes apparatus which may have an open fire, e.g. fireplace; "range" means an apparatus for cooking having elements that perform different cooking operations or cooking and heating operations. STOVES OR RANGES, NOT RESTRICTED TO SOLID FUEL General characteristics With single kind of fuel or energy supply With more than one, or unspecified kind of fuel or energy supply With additional means for heating water With self-cleaning provisions Combinations of stoves or ranges DETAILS OF STOVES OR RANGES IN GENERAL In this subclass, the following expression is used with the meaning indicated: "central heating system" means a system in which heat is generated or stored at central sources and is distributed by means of a transfer fluid to the spaces or areas to be heated. CENTRAL HEATING SYSTEMS With heat-transfer fluid: steam; hot water; hot air or exhaust gas; other fluid ; ; ; Combinations District heating systems By heat storage Other systems OTHER DOMESTIC- OR SPACE-HEATING SYSTEMS Electric; Other ; DOMESTIC HOT-WATER SUPPLY DETAILS In this subclass: air-humidification as auxiliary treatment in air-conditioning, i.e. in units wherein the air is also either cooled or heated, is covered by groups or ; air-humidification per se, e.g. "room humidifiers", is covered by group . In this subclass, the following terms or expressions are used with the meanings indicated: "air-conditioning" means the supply of air to rooms or spaces by means which provide for the treatment of the air in at least two of the following ways: heating cooling any other kind of treatment, e.g. humidification; "ventilation" means the supply of air to, or its extraction from, rooms or spaces, and systems for circulating air within rooms or spaces, but does not cover the mere treatment of air being supplied to, extracted from, or circulated within, rooms or spaces. Processes using enzymes or micro-organisms in order to: liberate, separate or purify a pre-existing compound or composition, or to treat textiles or clean solid surfaces of materials are further classified in subclass . AIR-CONDITIONING Room units; central systems; other systems or apparatus ; ; AIR-HUMIDIFICATION VENTILATION SCREENING BY AIR CURRENTS COMMON DETAILS Control, safety Use of energy recovery systems Other details The distinguishing feature of the air heaters covered by this subclass is that the heat is predominantly released to the air by convection, mostly by forced circulation of the air. The domestic stoves or ranges covered by subclass , may also be fired or electric air heaters but they release their heat to a considerable extent by radiation and only to some extent by natural convention. In this subclass, the following terms or expressions are used with the meanings indicated: "water" includes other liquids and means always the liquid to be heated; "air" includes other gases or gas mixtures and means always the gas to be heated; "furnace tubes" means tubes inside the heater wherein combustion is performed; "fire tubes" means tubes inside the heater through which flue-gases flow from a combustion chamber located outside the tubes; "heater" means apparatus including both heat generating means and means for transferring the generated heat to water or air. All storage heaters are classified in group . WATER HEATERS AIR HEATERS; STORAGE HEATERS ; FLUID HEATERS USING HEAT PUMPS COMBINATIONS OF WATER AND AIR HEATERS FLUID HEATERS FOR EXTRACTING LATENT HEAT FROM FLUE GASES DETAILS Processes using enzymes or micro-organisms in order to: liberate, separate or purify a pre-existing compound or composition, or to treat textiles or clean solid surfaces of materials are further classified in subclass .