In your worm bin, there is very little composting taking place, or else it would be a fairly sterile environment, and certainly devoid of worms. The heat produced from a proper compost pile is 140 to 150 degrees. This will kill worms. You are likely only seeing solar gain, or a low temp(non optimal) compost. Now think of the potential with 150 degrees. Problems are constant feeding of the pile, aeration, and the fact that removing heat via a hydronic system, will snowball, and slow the composting, and the heat produced. So, though this is a great technology, it is far from low maintenance.