Examples relate to a 3D printing system comprising a connection to a 3D printing chamber; an energy source to apply energy to a 3D build material and an energy source chamber separate from the 3D printing chamber. The energy source chamber contains the energy source. The 3D printing system comprises an air valve system, the air valve system comprising a first valve element selectively connecting the energy source chamber and the connection to the 3D printing chamber and a second valve element selectively connecting an external air input and the connection to the 3D printing chamber. The 3D printing system also comprises a first sensor measuring a temperature representative of a temperature in the 3D printing chamber and an air valve controller connected to the air valve system to regulate the temperature in the 3D printing chamber in function of data provided by the sensor.