Jean Pain, a Frenchman who has since passed away, did extensive research into what he called heat heaps. We are using his research to heat water for our outside showers used by our apprentices and students throughout their stays with us. This is about much more than taking the heat and using it productively though.
Our first try at this was done undercover in our concrete lambing pen. Constructed as a dual purpose walled pen, it is used for lambing in the spring and then as a compost bin during the summer. The area is 2x2x1m (6.6x6.6x3.3ft) high, making the heap 4m3 (141ft3), filled with organic material.
We use two ingredients in this pile. The first is oak bark chips from the local saw mill, removed from the logs before they are sawn and is a waste product. We buy this in by the 10m3 (353ft3) load at €8 per cubic meter delivered. We use this material for mulching throughout our vegetable and forest gardens, not just for the compost heaps.
The second material is animal litter from over-wintering sheep and cows in the sheds. This produces a mix of organic material that is carbon rich but with a good amount of nitrogen.
Nitrogen is the fire in a compost heap and the carbon is the fuel, both are important but we have found the higher the carbon level the longer the heap will stay hot. Once the heap starts to cool it’s easy to add more nitrogen from urine separated at source in our dry toilets.
Building the heap is done methodically and on average takes us half a day. The first layer in the heap is 20cm (8in) of bark chips, followed by 10cm (4in) of litter. This insulates the heat exchanger from the concrete floor. The heat exchanger is 100m (330ft) of 20mm (0.8in) rigid polypipe which contains about 18 litres (1.8 gallons) of water. Once the foundation layer of compost is down the first coils of pipe are laid. Then a 10cm (4in) layer of litter is placed on top of the pipe followed by a 10cm layer of bark chips, then another layer of pipe coils. Once each layer of organic material is laid, the heap is wetted down so that you can squeeze water out of a handful of the mix.
The heap is topped with as much bark and litter as we can get in place without it falling over the sides. Then all the pipes are connected back up to shower fittings and the incoming mains water. The blue pipe remains delivering cold water, the black pipe delivers hot water to the shower head.
Once the heap has been built the microbial activity starts to create the heat we are trying to exploit. Four to five days later the heat will be higher than most people can use directly and the water will need to be mixed with cold water to make a comfortable shower.
We also build another heap to heat the water for our campsite shower. In the centre of the heaps where the heat will be highest, we add the dry matter from our dry toilets. Yes that’s right, the mixture of human faeces and toilet paper (humanure). We endeavour to monitor the heat to ensure it gets above 60ºC which renders it inert. Once the heap has finished its decomposing process, we can use the compost.
Four years of using this system in two places on our site has taught us a great deal, both about compost making and the value of good quality compost. Like most aspects of permaculture, no one thing makes a system work and under-standing the balance of what makes compost work well is the key. While many people will tell you exact amounts of carbon and nitrogen to put in your heap to make it get hot, we have learned it’s not an exact science.
Bark chips alone will compost and create heat once the heap gets wet but if it gets too hot for the microbes it will kill them off. While the compost will come back to life again it will go through the same processes over and over again, taking a year or more to finish its process and become useful. This is because its tendency to overheat deters worms and other small creatures taking up residence and finishing the final compost stages.
50% bark and 50% litter will work but we have found it cools quickly and needs remaking and more bark adding to get the heap to finish quickly. 40% bark and 60% litter can get hot but not for long enough to be practical for our purposes. It seems to make a great worm farm however, and piles made to this recipe have proved to produce larger amounts of worms. 60% bark and 40% litter has proven to be the best recipe for our needs. The pile stays hot for longer with the regular addition of urine to keep the nitrogen level up. This recipe will work for 7-14 weeks, depending on the demands made on the shower. Each shower temporarily cools the heap and if people queue for several showers one after another, day after day, it will cool quickly. But it copes well with five or so showers a day for 14 weeks.
Not forgetting the icing on the cake: at the end of the composting process, we have around 3 cubic meters of good quality, 100% natural compost from each heap. This smells fresh and clean and can be used for seed sowing, cuttings, potting up and as a general soil improver: all this on top of 500 showers per heap
Steve Hanson is a permaculture teacher, practitioner and design consultant. A natural builder and professional craftsman for more than 20 years, he is co-owner of Permaculture Eden, a human scale regenerative permaculture farm near Lourdoueix-Saint-Michel in central France.