Here come some pics , both bottoms are of high density poly , They both rely on a cold draft over the floor to ventilate damp air
This hive bottom (nacka botten ) will ventilate even when the hive is covered in snow , Its also good in the fight against varroa mite because they can fall through the mesh.
One of the draw backs is if a lot of dead bees accumulate on the floor ventilation can bee restricted , and in a worst case suffocation of the hive.
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ImageShack.usThis is the one I use (universal botten ) I think it was designed by a group of professional Swedish beekeepers. costs about 22 dollars
It is what we call a high or deep bottom .
In the front there are 2 entrances , with 3 different settings , the top setting is wide enough for one bee , the bottom is 2 bees wide , but there is a wooden bar that can reduce the opening to 1 bee width , This also acts as a mouse guard under the winter season.
The hive should slope slightly down towards the back of the bottom board , this will allow any moisture to drain out of the hive.
The metal grate causes the cold air to condense and drain out. Also i acts as a blind lure entrance to fool robbing bees.
The metal grate is easy to remove to inspect the hive , treat varroa , clean the floor.
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ImageShack.usmvh edward :P