You would be surprised. I know someone that buys them simply because they are cheap enough that can be repaired or used for parts, if you sell them cheap enough (around $200) it is in fact the last chance to get rid of them...
As heaters you can always play with BraiinsOS and force them to different speeds, if they get too hot just slow them (UC/UV) down... And perhaps removing the middle board helps too.
How much can you sell your miners for locally ? I know S9s are becoming kind of worthless to many people out there, but they still hold some value, why not get rid of them for 100-150$ while you still can ? it's better than "torturing" them with low fan speed.
as for the winter heating , it does make sense, but if say nothing major happens in BTC price till winter ends, then you won't be able to get a 100$ not even a 50$ for those S9s.
The problem is I was getting offers of at best $100 / $125 shipped for old S9.
Hell I could not sell someones CLEAN AND 100% PERFECT T9+ for $150 shipped:
https://bitcointalk.org/index.php?topic=5189815S9 units with wonky hash boards and dirty fans were not even getting any looks.
But I did get offers for the controllers for $50 to $75 shipped. So a small board that I can ship easily anywhere in the US for under $10 that I will never hear about again is a lot easier then selling a heavy miner that has issues. But you do wind up with a pile of hash boards in the end.
I would like to thank everyone on the fan advice I have a few fans coming in to play with.
-Dave