Separate clearance from preservation
The radiator manufacturer's clearances address the appliance and surrounding objects. They do not automatically establish a suitable environment for a particular artwork. Follow those requirements, then consider the medium, frame, and exposure separately.
The Library of Congress advises keeping framed paper objects away from radiators. Glazing and a backing board do not make a paper print immune to heat or environmental changes. An irreplaceable drawing, photograph, or family document deserves a location with more stable conditions.
Look beside rather than directly above
An adjacent wall may offer a better position while still relating to the radiator-side furniture. Use a template to test the picture where it can be seen comfortably without sitting in the rising warm air. Keep valves, vents, and controls accessible.
Do not rest a frame on the radiator or on an improvised cover. A cover has its own installation and ventilation requirements, and a flat surface does not make it a safe art shelf. Avoid blocking heat circulation with frames or decorative objects.
Judge the room during heating season
A wall that seems harmless in summer may behave differently when the radiator is operating. Observe the actual conditions and check for nearby leaks or condensation. If you cannot establish a suitable location, choose another wall rather than assuming occasional use removes the issue.
You can still make the area feel considered through furniture placement or a clear stretch of wall. Artwork is not required above every appliance. If a replaceable reproduction is used nearby, it still needs secure mounting and appropriate care; replaceability is a choice about consequences, not evidence that the conditions are good for the print.
