Hi all
Call me silly but has anyone looked at the principles of geyser heater pumps? Basically it's a reverse refrigiration/ac unit that warms the water. Have you felt the back of your refrigerator ? I think this could work for small setups coiling tubing around the condensor/evaprator (not sure what it's called ) & circulating water through it. Thats an apliance we have running in most of our homes 24/7.
Another Idea I was planning to use for my project (large scale) was to have a compost heap between the 2 tunnels with pvc pipes running through it. the compost would be fed by veggie waste etc. those things get damn hot inside
Anyway just some ideas I am/have considdered

G