OK, it's not exactly in the same class as a space heater, but...
discovered why the heater was blowing feebly. The heater box has 4 sets of holes, pipes for the entry of: I assume this is to allow it to have the pipes top or bottom and left or right.
Originally, I dare say these holes had stuff fitted inside to stop the hot (OK, warm) air blowing out from them. Mine are now taped up on the outside of the box with insulting tape, and now the warm air, such as it is, actually gets inside the vehicle.