In the centre of the house between the door and the dresser is the hearth. But what did they burn it it?
While there is plenty of usable peat in Orkney today, this didn't form until several centuries after the settlement was abandoned. Fuel was probably a mixture of animal dung, dried seaweed, heather, bracken, and maritime mammal bone (rich in oil) - such as whalebone.
They also may have burned some wood. But not much. Virtually no charcoal has been found. Wood was probably too precious to burn.