Heat pumps work most efficiently when they can circulate water at lower temperatures than gas boilers normally do. For many homes this means that you'll need to swap out some of your radiators for larger versions to provide the same level of comfort. These radiators don't necessarily need to cover more wall area than your old ones - they can just be thicker. Any new radiators will normally be installed at the same time as the new heat pump. Depending on the amount of new pipework or extra radiators required you may need to redecorate.