i assume the piano will be outside of the greenhouse?
the simple answer to your question is with a fan and vents.
it is a more complicated answer about which fan and how to set it up.
since it is indoors and you have electricity there i would simply make it so there are two vents in the greenhouse that can be opened or closed, one into the house and one into the window which can still be opened.
then the fan can be used to create a positive or negative environment depending on the time of year and the effect you are going for
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Mercurianmad 
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