Abstract: In an assembly of fusion furnace and fusion head for applying heat to plastic pieces to be fused, the furnace being fired with gaseous fuel fed through a conduit to a burner head in the furnace from a source of fuel, a temperature sensing probe projects into the furnace. The temperature sensing probe is operatively connected to a gas flow control device in the conduit from the source to the burner. The flow control device acts to throttle the gas flow, to maintain a predetermined variably selective temperature of the probe, from a high pressure, high rate of flow of gas to a low rate of flow of gas to the burner head. A fusion head block has a well in it to receive the temperature sensing probe when the fusion head block is in a predetermined heat-receiving position with respect to the burner head, the well being spaced from the burner head. The burner head is provided with a wire screen to ensure the maintenance of a flame at a reduced flow rate of the gaseous fuel.