Abstract: A system for creating an alarm whenever the pressure inside of a combustion furnace is higher than atmospheric pressure. The system uses a pressure reference comprising a chamber which is designed to average out momentary wind-induced fluctuations of pressure in the atmosphere and to provide a substantial volume of air inside the chamber which is maintained at an average value of atmospheric pressure. A conduit is passed through the wall of the furnace to the point under the furnace arch where the pressure is to be measured. This conduit is connected by a tubing to the chamber. A temperature sensing means is inserted in the conduit so as to measure the temperature of gas flowing through the tubing from, or to, the furnace. Normally, because of the draft in the furnace, air will flow from the chamber to the furnace.