Your central heating system is probably just filled with liquid and is unlikely to operate at much more than atmospheric pressure. An air conditioning system is mostly filled with pressurized gas and a relatively small amount of liquid. The operating pressure is probably of the order of 10 times atmospheric pressure.
Gas is much better at finding leaks than liquid.
By necessity, the compressor on a car air conditioning system has a seal through which a rotating shaft passes to operate the pump internals. This seal will leak but this can be minimised by running the aircon regularly which distributes refrigerant oil around the system whichstops the seals from drying out and becoming leaky. This applies to the other seals where various components are joined together.
By contrast a domestic central heating pump can be designed so that the stator is on the outside in the dry with the rotor and pump entirely submerged in the liquid and no seal needed since the moving part of the motor is on the wet side.
Car air conditioning systems also include flexible hoses and these too can be slightly porous allowing the gas to slowly leak away. Again running the system regularly keeps them coated with oil which makes them less leaky.
Having said that, a system in good order should hold its charge for a few years but it will, inevitably, leak.
Fridges and freezers don't usually leak because the compressors are hermetically sealed and there are no flexible pipes.
Martin
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