Yeah water is excellent. It's really neat to see the steam rising from the tanks in the middle of the night. Chicken coop is also a good idea. Critters produce a lot of heat. I did another greenhouse at a farm I was managing last year with worm compost bins for heat. They worked great... that one pushed all the way through the cold Taos winter with just a tiny electric space heater as supplemental heat. The house was 10' x 30' in size and the bins were about 4 x 4 x 3 and there were two of them. Thanks for the comment :)