Abstract: A building heating system with a plurality of steam radiators is provided with a control unit for the steam boiler. The control unit receives signals representative of the outside temperature and the pressure of the steam in the feeder conduit. The control unit shuts off the boiler when the steam pressure in the feeder conduit exceeds a predetermined value corresponding to a given measured outside temperature. After the pressure has decreased to a value below the predetermined value for the measured temperature, the control unit restarts the boiler. Another control unit receives signals representative of the condensate temperature and the pressure in the return conduit. This control unit operates a vacuum pump in the return line and assures that the vacuum pressure in the return line is less than that which causes the condensate to vaporize.