There was a lot more ice in the heat-pump box than I had thought, a 10 cm cake covering its floor, but getting rid of it proved easy. All I needed was a screwdriver and a small axe. The hot air gun wasn’t much use.
I turned off the power feed, took the hood off the thing, removed the rotor and hacked away the ice, taking care not to bash the fine heat-exchange lamelles lining the walls. The ice was laminated from the many defrosting cycles that had built it up, and it fractured into large easily manageable chunks. After reassembling the box I hacked away most of the remaining ice on the ground beneath it as well and put a piece of a cardboard box there to make ice removal easier in the future. Less than 20 hours after I put the cardboard there, about 0.2 litres of ice had collected on it after a lot of water had soaked into the cardboard. I shall have to get a plastic tray.