At fyre can mean different things: to use a certain type of fuel is at fyre med, so at fyre med olie ("to burn with oil") means to use oil as a fuel for a fire or heating system. At fyre op ("to fire up") is the expression you'd use in relation to lighting or igniting a fire or turning on the heating: jeg fyrer op i brændeovnen means "I'll light the fire(place)". During the fyringssæson, landlords are required to ensure heating and hot water are adequately supplied to buildings, houses and apartments (this is obviously also the case year-round, but rooms are not usually heating during the rest of the year).