A germicidal UV reactor for use in fluid purifiers is provided, in which the UV reactor comprises an UV lamp with an UV transparent primary envelope and also an UV transparent secondary envelope around the primary envelope. The outer surface of the secondary envelope is in contact with a flowing fluid to be purified. There is an isolating space between the outer surface of the primary envelope and the inner surface of the secondary envelope. At least one of the outer surface of the primary envelope and the inner surface of the secondary envelope is provided with electrical means for heating and thus enhancing UV emitting efficiency of the UV lamp.