Radiator cover spacer

I have a decorative cover over one of the radiators in my living room, and while it is useful as a shelf, I always think it must be messing up the thermal efficiency of the radiator.
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Putting a chair in front of it isn't helping much either.
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These spacers give a 25 mm air gap . Holding the back of my arm over the gap I can feel hot air rising, so they are doing something.