The Wallas 30D is a 3kw heater, whereas the Wallas 40D is a 4kw heater.

The 30D has a balanced flue and so draws its combustion air from outside the boat, the 40D draws its combustion air from the environment its mounted in.

The Wallas 30D sucks air in through the grill in the side of its body and blows it down the outlet tubes and into the cabin, the Wallas 40D has air inlet tubes connected so that it will draw air from wherever you want it to.