PURPOSE: To reduce the noise generated by the rotation of a fan by disposing interference structures for inducing a flow resistance in the opening cross sections of a gas intake channel and a gas discharge channel in a side channel for supplying combustion air in a heater.

CONSTITUTION: A fan, in particular a side channel fan 11 for supplying combustion air in a heater has, besides the side channel 12, a gas intake channel 13 and a gas discharge channel 14 in a housing portion 10 thereof. An impeller 15 is mounted on a rotary shaft 16 in the side channel 12. A partition wall 17 of the housing portion 10 is arranged between the opening cross sections 18, 19 of the two channels 13, 14. In such a structure, interference structures 20, 21 for inducing a flow resistance are disposed in the vicinity of the opening cross sections 18, 19. Interference elements 22, 23 forming a V-shaped notch respectively are provided in the interference structures 20, 21. Thus, it is possible to reduce the noise generated by the rotation of the fan.