Would it be OK to have only 1.5" space above a P15 in an open rack?
Son House
That is what I have above my P20.
I think space around the sides of the P15 might be a bit more important. I think you’ll be fine.
Really? Are you really Son House? Please sing us a song, Mr House. 
My P20 has been so Housed for years and it seems happy.
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As long as there’s open ventilation front and back—somewhere for the hot air to go, you’re fine. Plus, the P15 doesn’t really get that hot. So, I think you’re all good.
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I have 2 meters just to be sure. 
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