Using Compost to Heat Water

Reader Contribution by Sean Mitzel
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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.

Volume and Performance

We used 125 feet of 3/4-inch garden hose in the pile. The volume of hot water in the pile is approximately 2 gallons at any given time. I say approximately because it’s hard to be precise when it comes to the pile because the temperature differences at different levels, both vertically and horizontally, vary. The bottom line is we get about 2 gallons of hot water at any given time. The ground water, during this time of year, comes up at 50 degrees, goes through the pile, and comes out right around 110 degrees. We actually add a little cool water to have the perfect shower. This works out to be right at 2.5 gallons per shower. As a side note, the average shower taken uses 10 gallons of water.

Using an 18 degree compost thermometer we have documented the temperature of the pile in the same location. Unfortunately, this is only one location and not representative of the whole pile.

24 hours

90 deg

72 hours

155 degrees

2 weeks

145 degrees

3 weeks

126 degrees

Lessons Learned

Compost Hot Water Experiment 2.0 coming soon!

Our main lessons learned:

  1. Increase the volume of tubing in order to increase the amount of hot water
  2. Use PEX or CPVC as piping instead of garden hose
  3. Increase the size of the pile for increased “burn” times
  4. Employ a small jet pump to recirculate water to increase the amount of hot water production

Overall, this has been a wonderful and fun experiment. Not to mention, we have saved on heating costs, increased water conservation and stacked functions to make our property a little more self-sufficient. Happy compost hot water making!

Sean and Monica Mitzel are the proprieters at Huckleberry Mountain Homestead & Breakfast and homestead with their family on 40 acres and are using permaculture principles and strategies for the property. The property is a demonstration and education site where they teach workshops and raise dairy goats, sheep, pigs, rabbits, chickens, and ducks. To learn more about the Mitzels, visit The Prepared Homestead. Read all of their MOTHER EARTH NEWS posts here. 

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