Using Compost to Heat Water

| 7/27/2017 10:17:00 AM

Function Stacking

Jean Pain’s work, and many that have followed, inspired hot water compost experiments to commence on our homestead! We were not happy with the performance of our solar shower so we set out to find a better way to get “free” hot water.

Solar Shower

We are located in North Idaho on our 40 acre homestead. We continually look for ways to live simply and self-sufficiently. Making hot water with electricity and propane are quite easy and we will do that when needed but we would like to figure out ways to reduce our dependency on this kind of power. During the warmer months, we especially look to reduce our energy requirement with natural processes. A solar shower along with using the gray water from the shower seemed like a natural fit. However, due to the cool nights here in North Idaho, even in the summer, we would frequently only get water temperatures up to 85-90 degrees. This is good and I’m sure with some tweaks we could improve that but it’s not good enough. Quite frankly, I like to take hot showers! The ideal temperature for a shower, for me, is 105 degrees. We had to find another way.

Function Stacking

As a permaculture practitioner, I don’t know why I didn’t see the function stacking potential sooner in this instance. We have six large composting bins that we use extensively. These bins are near our solar shower/dry composting toilet structure. We were viewing these elements independently and never considered how each element could benefit or work synergistically with one another. Element placement and integration is probably the number one area for improvement we see when working with clients. In our case, I missed the function stacking opportunity for compost hot water integration with the seasonal outdoor shower. Let’s stack the functions of the compost to not only make excellent soil, let’s use the compost to also make hot water for showers but then let’s also use that gray water to provide moisture for plants!

Compost Hot Water Experiment

The experimental hot water compost pile we built was simple and the goal was to test and document the results. We are monitoring the temperature of the pile, the temperature of water entering and exiting the pile, the approximate amount of hot water we can make and finally how long the pile will produce hot water. Our prediction is that the pile will produce enough heat for hot water for 30-45 days.

Building the Pile

We built a pretty standard compost pile following the generalized recommendations regarding carbon to nitrogen ratios. Our pile is composed of manure straw, plant material, and animal offal (unused parts from animal processing). We layered the materials appropriately and built the pile using a cattle panel bent into a circle. The pile started at about 4’ high and 5’ in diameter. We knew this was a relatively small pile but we were anxious to get the experiment underway. In order to run the hot water through the pile we used standard garden hose as tubing. This is not ideal and when we build 2.0 we will change this part of the build. We used it because we had it and it was simple. The best materials for this application are probably PEX, CPVC or even coiled copper tubing. Essentially, you want to use something that can withstand high temperatures even though we are not talking about really hot water.

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