An airer is better because tumble dryers use a lot of energy. Try timing it so you put your washing out on a clothes horse during the hours your heating comes on. Drying your clothes indoors on an airer can cause problems with condensation and damp, especially in old and poorly-insulated homes, so it is best to dry your clothes outdoors whenever the weather allows.
According to the National House-Building Council, if you need to dry clothes indoors, open the window and close the door of the room where the clothes are drying, so moisture can escape rather than circulate around your home.
