For a fire door to be certified the door and frame must both be replaced at the same time. The reason being that the doors and frames are manufactured together and have been tested as a package. If we were to simply replace the door and not the frame then we cannot guarantee that they will work together as they should, this being as we do not know the age of the frame and its structural integrity.
A fire certified door is a custom made to measure door and frame that have been approved and certified by the manufacturer meaning you will get a physical certificate with the door.
Just to recap, for the door to be certified, the frame must be replaced as well. This is what is taught when engineers undertake the fire door training courses.
Fire rated or in other words uncertified
This is a slightly cheaper option and is an off the shelf door blank ordered to the nearest measurement available. It then must be cut down to fit the opening and re‐lipped with hardwood. We then have to decorate it (primer, sand it, undercoat, sand it, topcoat). A fire rated door is still rated to 30 minutes etc but because we cut it down and re‐lip, we are essentially messing with the integrity of the door and no‐one will be able to certify it anymore.
How do I know which one I need?
It would depend on the fire risk assessment for the building as to whether it needs to be a certified door or simply fire rated. Usually a fire route would require a certified door which is why it's always best to check with the fire risk assessment for the building to see what type it needs to be, sadly we cannot advise either way.