A heater assembly for an ALD or CVD reactor provides protection for an electrical conductor associated with a heating element by using a purge gas to isolate the conductor from the corrosive environment of the reactor chamber. The purge gas is introduced into a sleeve surrounding the conductor and from there is allowed to leak into the reactor chamber to be pumped out with the process gasses. This arrangement avoids the need for airtight seals at the junction of the sleeve and the heating element easing manufacturing requirements and potentially reducing component costs.