Foremost among them is the development of the hypocaust, a type of central heating where hot air developed by a fire was channelled under the floor and inside the walls of public baths and villas. He gave explicit instructions on how to design such buildings so fuel efficiency is maximized; for example, the caldarium is next to the tepidarium followed by the frigidarium. He also advised using a type of regulator to control the heat in the hot rooms, a bronze disc set into the roof under a circular aperture, which could be raised or lowered by a pulley to adjust the ventilation.