Which is the input and output dial on my Creda storage furnace?

I have a Creda storage radiator and I cant tell which is the input and which is the output dial. It looks approaching one of the dials was changed at some point as they are different. Also ill-fatedly I cant seem to find a model number anywhere on the space heater. I have tried experimenting beside the dials but Im still not sure.


Answers:    The output will open a flap, look through the grill and or listen and you should hear the flap orifice / closing.

Update

Looking at the drawing linked below the output will be LH knob and the RH will be the input.

Having have these heaters for a few years the effects of turning either knob is not early.
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