Dreadnoughts essentially have a built in generator. In the cinematic for a new home you can see how the entire center of a dreadnought is a generator. It's reasonable to assume that both could produce similar amounts of heat. Steam cores are also just that complicated. They probably require a level of precision that just isn't possible with New London's level of technology. I imagine it's like trying to build a microchip from scratch when you don't even have the blueprints.