Water heated by compost

Instead of the usual compost pile to heat water with wire mesh frame and wood chips we are testing this with a wall of straw bales filled with woodchips, giesta (scotch broom) and sheep manure with straw bedding. 

The pile took half a day to construct plus a further day making woodchips. We made it with this method due to lack of woodchips so instead we used the materials that we had available on the day.

Compost water heater
Drawing of the compost pile that we built that is heating water

How we assembled the hot water compost heater:

  1. Straw and hay bale frame was used because:
    1. It is quick to construct and we had spares on the farm
    2. They should act as an insulator for the pile
    3. They are not just a frame but part of the compost pile
    4. By using this thick frame space inside where the compost pile gets hotter is smaller which means we need less woodchips (which we don’t have right now due to no sun for our solar-powered woodchipper)
  2. Inside we layered 50% fresh acacia (mimosa) woodchips with 25% scotch broom (giesta) and 25% sheep manure with bedding
  3. We put 100 meters of coiled 1-inch irrigation pipe inside
    1. When we had this with the cold going in at the bottom and hot out of the top we could not clear the airlock inside the pipe so we have reversed this and have cold water going in at the top and hot out of the bottom
    2. I added a tap at the highest point to clear the air inside the water heater, I will connect this to a drip irrigation to go over the top of the pile

The results of the hot water compost pile

  1. Day 2: the water is coming out piping hot
  2. The question is will the pile keep on heating water for one week, one month or one year
  3. Check back for an update

Connecting to the house

Currently, the pile is not connected to the house because we wanted to see if it worked first. So far it is very very hot and we will connect next week, but we are currently seeking advice.

We have three options for connecting it to the house which we are currently deciding as it requires digging up some pipes and buying fittings. Maybe you have some advice here?

  1. Replace the hot water feed from our wood powered insulated water tank (Ballerina). Instead, have the hot water go straight into the house from the compost pile
    • Pro: The is easy to do and requires no purchase of fittings
    • Con: We have no way of knowing how hot the water is before we get in the shower
    • Con: When the pile stops working we have to change the plumbing again
  2. Replace the cold water feed into our wood powered insulated water tank. Instead, have hot water from the compost pile go into the tank
    • Pro: this is also easy to do
    • Pro: we can see the temperature of the water before we get into the shower as it is displayed on the hot water heater
    • Pro: If the compost pile doesn’t heat the water enough we can top it up by burning wood. However, we may not need to use as much wood as normal as we should have hot water coming in
    • Pro: If the compost pile stops working we don’t have to change any plumbing as water will still come in the wood-heated water heater.
    • Con: We would need to keep using the hot water taps to ensure a constant supply of hot water to the tank
    • Con: if we do not use the hot water for a while, the tank will lose it’s heat and we would need to light a fire to heat the water
  3. Have both the compost pile’s input and output feed into and out of the hot water tank. Then maybe the hot water will transfer into the water tank by convection or a pump, then the tank will be heated as much as the pile can heat it.
    • Unknown issue: The bottom of the water tank is about halfway up the compost pile. the top of the water tank is higher than the top of the compost pile. Will convection (heat rising) fill the tank with hot water? If not we need to buy a pump
    • Unknown issue: the compost pile has around 60 litres of hot water, the water tank is 200 litres, is it strong enough to heat all 200 litres through the day
    • pro: If it works then we are storing 260 litres of hot water
    • Pro: we can see the temperature of the water on the thermometer on the water tank before we get into the shower
    • Pro: If the compost pile doesn’t heat the water enough we can top it up with wood but we may not need to use as much wood as the base heat is higher than our usual cold water
    • Pro: If the compost pile stops working we don’t have to change any plumbing, we can just turn off the plane and connect to a new pile or solar heater

How the compost heater works

Amazing microscopic life (bacteria and fungi decomposers) are breaking down the compost pile into compost, when they do this they generate heat. This heat then warms up the water in the water pipes.

Next experiment

To make a water heater with 100% scotch broom (giesta)

Watch this space for updates

Thank you to our volunteers who helped cutting broom, woodchipping and wheelbarrowing all the material to me so I could make this.


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