Heat pump installation functioning from a cold source, comprising a circuit having at least one compressor, a condenser, pressure release means (3, 3'), and at least one evaporator (4, 4'), a cycle fluid circulating in the circuit, the condenser being cooled by a fluid to be heated, the installation comprising: - at least two exchanger systems (12, 12') each composed of two fluidly disconnected and thermally connected heat exchange zones (Z1 and Z1', Z2 and Z2'), the first exchange zone (Z1, Z1') fulfilling the function of subcooling the cycle fluid and defroster of the second exchange zone (Z2, Z2'), the latter fulfilling the function of evaporator (4, 4'), the two exchanger systems (12, 12') being mounted in parallel relative to the compressor - condenser assembly, on either side of at least two pressure release means (3, 3'); - means for circulating the cycle fluid between the compressor - condenser assembly and the exchanger systems (12, 12'), as well as control means (6, 6') and diversion means (7, 7') for the cycle fluid stream in the circuit, allowing the installation to function according to a mode of alternate circulation of the cycle fluid in the defroster of a first exchanger system (12, 12'), then in the evaporator (4, 4') of the second exchanger system (12, 12'), the heat pump installation being further characterised in that the control means (6, 6') and the diversion means (7, 7') for the cycle fluid stream in the circuit allow the installation to function according to a mode of simultaneous circulation in parallel, of the cycle fluid passing through the evaporator (4, 4') of the two exchanger systems (12, 12'), the installation further comprising a chamber enclosing the components (1, 2, 12, 12', 3, 3') of the heat pump, the chamber thus forming semi-enclosed spaces (53, 53') between each fan (5, 5') and an internal wall of the chamber opposite each fan (5, 5'), each fan (5, 5') being placed against each exchanger system (12, 12') along their height h, the air stream generated by each fan (5, 5') entering via a bottom opening (51, 51') of the chamber and ascending towards an exchanger system (12, 12'), crossing from one side to the other of the second exchange zone (Z2, Z2') of said exchanger system (12, 12'), then descending again and exiting from the chamber via an opening (52, 52') situated under each evaporator (4, 4') in the bottom of the chamber, these semi-enclosed spaces (53, 53') making it possible to promote defrosting of the evaporators (4, 4') by the presence of hot air at the top of the chamber. - at least two exchanger systems (12, 12') each composed of two fluidly disconnected and thermally connected heat exchange zones (Z1 and Z1', Z2 and Z2'), the first exchange zone (Z1, Z1') fulfilling the function of subcooling the cycle fluid and defroster of the second exchange zone (Z2, Z2'), the latter fulfilling the function of evaporator (4, 4'), the two exchanger systems (12, 12') being mounted in parallel relative to the compressor - condenser assembly, on either side of at least two pressure release means (3, 3'); - means for circulating the cycle fluid between the compressor - condenser assembly and the exchanger systems (12, 12'), as well as control means (6, 6') and diversion means (7, 7') for the cycle fluid stream in the circuit, allowing the installation to function according to a mode of alternate circulation of the cycle fluid in the defroster of a first exchanger system (12, 12'), then in the evaporator (4, 4') of the second exchanger system (12, 12'),