Heat pumps work by extracting heat from the air or ground and by means of a compressor turning this into usable heat to perform heating functions. They require both external components and internal components as the heat is collected from the air outside or pipework buried on the ground.
Because of the way they work heat pumps work best when heating using lower flow temperatures as this means the compressor works less hard to increase the temperature of the heating fluid in the radiator/heating pipework. They are perhaps most suited to new build and renovations where insulation levels are high and often underfloor heating is used as a heat emitter rather than radiators.
If you have applied in the past with no success you may be accepted now as the qualifying criteria was changed dramatically at the beginning of the year (2017) to help more households pass for free funding.