Abstract: This invention discloses a furnace in which hot gases are discharged from heaters to provide uniform heating to printed circuit boards commonly used in the electronic industry. Outside air is prevented from flowing into the furnace, so as to reduce and to stabilize the concentration of oxygen. The furnace 1 is designed with an air tunnel 2 to separate it into upper and lower sections which are fitted with hot air blow type heaters 5a, 5b. The hot air discharged from the heaters in the upper and lower sections flow in opposite directions to create a counter-flow of gases. Each individual heater can produce its own hot air circuit. They are designed with the blowing outlet and suction inlet in the same structure with the blowing outlet fitted with a redirectional flow plate 18 which directs the hot air to the suction inlet.