A complete cracking Heater (radiant and convection sections)is supplied with C-E marking accoerding PED. Is the replacement of the radiant coil considdered as a repair (not subject to PED) or is it a new equipment and shall it fulfill PED.
You mean that the Heater "was" supplied with CE mark and now it's replaced.
Is it a CE marked machine? Is it a gas burning equipment?
You may consider it new equipment if you are manufacturer.
If you don't, It will depent on local regulations, and requirements may vary in each country though similar tecnical requirements will apply.
Best to ask to authorities