does any one know the difference between the truma combi 2 4 and 6 it seems the 6 is the newest but it is also the most expensive ,im thinking off taking out my cascade carver room heater and the cascade water heater the room heater seems a bit dangerous as the casing gets very warm and im frightend that the dog burns his tail on it ,any help appreciated and are there any other options for a combi than truma