Absolutely use the heat to help supplement the home heating if you already have rigs. Why on earth would anybody vent the heat to the outside during the winter? If your rigs are in a room by themselves, just open the door and put a box fan to blow the warm air out. But remember warm air rises and cold air sinks, so its best to elevate the fan off of the ground to allow for better cold air flow into the mining room. If you can get the fan near the top of the doorway, that would be the perfect scenario.

Mine are in the old theater room turned mining room that is in the basement, I close off all ducts in the basement and let the rigs heat the basement and most of the time its still too warm for me and have a window cracked. This year I'm thinking about installing ducting from the mining room into the furnace ducting to help circulate the warm air. The furnace is right on the other side of the wall from the mining room so it shouldn't be too hard. 1 duct near the top of the wall for the hot air and another at the bottom for the cold air return. I'm thinking out loud now: split the cold air return from the house and run it to the bottom vent in the mining room, install damper to shut house cold air off after the split. Vent at top of room goes back into the house cold air that is the inlet (suction) for the furnace blower.
Something like:

   | |
   | | Cold air return from house
   | |

   [ ]___
   [  ___ "T" cold air into mining room vent at floor level
   [ ]

   {\} Damper closed

   [ ]___
   [  ___ "T" Hot air from mining room vent at ceiling level
   [ ]

   | |
   | |
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