NOVELTY - Transferring (M1) heat involves: transferring heat from a body to a first heat transfer system by using secondary loop passageway comprising a heat transfer fluid and, optionally transferring heat from a second heat transfer system, to the body by using the secondary loop passageway comprising the heat transfer fluid. The secondary loop passageway comprises at least one polymer sealant selected from rubber seal and plastic pipe. The heat transfer fluid comprises at least one linear perfluoropolyether.    USE - For transferring (M1) heat (claimed) for cooling circuits on board of aircrafts, refrigeration systems for supermarkets or heat transfer circuits in industrial plants; for cooling compartments e.g. food stuff compartments in supermarkets or in transport trolleys on board of aircrafts, vehicles or boats, devices and reactors, especially in the cryogenic surgery or in the nuclear and energy industry, e.g. lithium ion batteries and fuel cells or for storing fuels, particularly liquid hydrogen; for operating devices and reactors e.g. for operating freeze drying devices.    ADVANTAGE - The heat transfer fluid is circulated through the secondary loop passageway in first step for transferring the heat from the body to the first heat transfer system and, optionally, in the second step for transferring the heat from the second heat transfer system to the body. The method uses fluorinated ether heat transfer fluids suitable for use in secondary loop heat transfer systems which provides viscosities as low as compared with viscosities of hydrofluoro(poly)ether fluids in a wide operating temperature of -120 degrees C up to 180 degrees C; boiling points up to 250 degrees C at atmospheric pressure; low permeability and high chemical compatibility and resistance towards polymer sealants, particularly at high temperatures; inherent chemical inertness and stability, and low solubility of water, thus is used in the secondary loop heat transfer systems so as to comply with outstanding safety standards by successfully reducing or even eliminating risks of leaks through polymer sealants and thus corresponding emissions of the fluids to the atmosphere, while increasing efficiency of the systems and minimizing its maintenance. The method leaks through rubber seals and plastic pipes are successfully reduced due to low permeability and high chemical compatibility and resistance of the heat transfer fluid towards polymer sealants, thus advantageously increasing efficiency of the secondary loop heat transfer system while complying with outstanding safety standards and also minimizing its maintenance by advantageously decreasing routine defrosting work and routine replacement of polymer sealants.    DETAILED DESCRIPTION - Transferring (M1) heat involves: transferring heat from a body to a first heat transfer system by using secondary loop passageway comprising a heat transfer fluid and, optionally transferring heat from a second heat transfer system to the body by using the secondary loop passageway comprising the heat transfer fluid, where the second heat transfer system is the same as or different from the first heat transfer system. The secondary loop passageway comprises at least one polymer sealant selected from a rubber seal and plastic pipe. The heat transfer fluid comprises at least one linear perfluoropolyether of formula R'f-(OCF2)n(OCF2CF2)m(OCF2(CF2)zCF2)p-OR'f (I). The heat transfer fluid is circulated through the secondary loop passageway in first step for transferring the heat from the body to the first heat transfer system and, optionally, in the second step for transferring the heat from the second heat transfer system to the body.    R'f=1-3C perfluoroalkyl;    m,n,p=0-100 (preferably 0-50);    m+n=higher than 0;    z=1 or 2.