Hot Compost Water Heater Results

Question 1 – Can we make hot water 24/7 using renewable resources?

We are experimenting with a system to heat our shower and tap water using hot compost. We have made our first compost and it has reached the desired temperatures so as we run each test we will keep the results here on the blog for people to see.

Question 2 – Can we supply our hot water for less money than if we worked to pay for gas heated water?

Another important factor for myself is that this way of heating water works out cheaper than working for the money to pay for the gas to heat the water. I have valued my time at 5 euros per hour (in Portugal actually a basic wage I would expect would be 3.50 p/h) and I have valued gas heated hot water at 50 euros per month (I am open to suggestions here as I have not seen someone’s water heating bill, I am basing this on a UK average from the which.com website). With this in mind I cannot work on the pile for more than 10 hours each month.

The cost of the pile structure was negligible and as it should last many years the infrastructure cost will not be accounted for. In a modern gas system the installation and maintenance would be far higher, with gas boilers costing thousands in the UK at least. Our structure cost less that 100 euros.

And finally the cost of the compost produced as a byproduct of the whole process will also not be accounted for. This has the ability to pay for the structure itself (with 50L of bagged compost costing 3.50 to 4.50 euros here). The compost is incredibly important to us as it will fertilise our garden each year but for the sake of keeping the monetary account straightforward I will only factor in time spent working on the pile.

Research

I started by researching the composting process. A few books, videos and discussions later it seemed as though the carbon/nitrogen ratio was a major determining factor in the composting process. However it seemed the Nitrogen rather than the Carbon played a stronger role in deciding whether the organic material would compost successfully. There were reports and videos online of people making hot compost water heaters using only wood chips as their organic material. They also said the compost lasted longer this way. Wood chips contain little to no nitrogen so a lack of Nitrogen doesn’t seem to affect the composting process too heavily. However too much nitrogenous materials, such as an abundance of food scraps and no dry carbon material can make a soggy smelly anaerobic compost pile. We know this from research as well as experience.

So for the organic material mixes we would focus on collecting the carbon element of the organic material to begin with, and add nitrogenous material when available.

We currently have a local free source of sawdust for our toilet, and a source of pine bark for the mixes too. We have yet to find a free source of animal manure to add fertility and nitrogen to our final compost but we are still looking. We bought 4 cubic metres of horse manure a few months ago and will use the rest of that for our first mix.

Compost Heat Duration

We heard a lot of conflicting accounts of the length of time that a compost pile can hold a good temperature. A friend told me one year with horse manure and hay, a volunteer said a couple of months with wood chip and a month perhaps with manure/carbon mixtures. So we really had to start experimenting with mixtures ourselves as it was difficult to get definitive answers. What I did notice online though was that the people claiming to have a hot compost which held its heat for a long time had large piles, with a wood chip/ large particle size carbon element.

Experimentation

A few things will stay constant during the various tests and they are as follows:

  1. The hose coil will most likely stay at its current length and structure. We made a wire frame using a piece of fencing. This is 1.2m high with a diameter of 1.5 metres. The pipe starts at the top of the coil and winds down, that is to say the water enters the coil at the top and is pumped downwards and out. There is approximately 90 metres of pipe in contact with the compost pile.
  2. After leaving the compost pile the pipe travels underground and is insulated for the first 5 metres. After entering the house it is uninsulated and reaches the kitchen tap after about 5 more metres. When the ambient temperature was 20 degrees outside, the water was coming out of the hot tap at 20 degrees so we know it can carry the heat all the way to the furthest tap in the house. We are building our shower block immediately next to the compost pile so that part will have only a few metres to travel from composter to shower head.

So the piping infrastructure will stay more or less the same between tests.

Here is the composter structure. Four posts with a sturdy metal fence wrapped around. The pipe was then wrapped around the fence

The variables will be:

  1. The mixtures of organic materials added to the compost pile
  2. The insulation on the outside and above
  3. The amount of  water the pile receives during its use
  4. The ambient temperature

TEST 1 – Compost made  – 24/02/2019

 

Here is the composter when full and active. As you can see the pipe is exposed and as such the water did not heat although the pile itself was at the desired temperature. 

 

Variables

  1. Pile composed of the following:
    1. Pine tree bark (added per bag, each bag contains approx 15kg).
    2. Humanure 1 wheelbarrow, approx 60 litres (humanure composed of pine sawdust, poo and wee).
    3. Horse manure/straw mix 1 wheelbarrow.
      1. The layers were: Bag of pine/Wheelbarrow of Humanure/Pine/Wheelbarrow of Horse Manure/Pine/Humanure/Pine/Horse Manure etc…
  2. No side insulation was used but on top was a 10cm layer of straw, mainly to reduce drying through evaporation more than insulating heat.
  3. We had the water sprinkler spraying on the pile from the start and through building the pile for three hours. It was soaked through from the beginning. In week 2 we had 30mm of rain
  4. Ambient temperature ranged from 2 degrees at dawn to 20 in the afternoon.

Results

The results below are on a weekly basis, and shown in degrees centigrade.

Week Compost temperature Water temperature
1 – 01/03/2019 40 14
2 – 08/03/2019 49 22
3 – 15/03/2019 40 17
4 – 21/03/2019 20 16
5 – 29/03/2019 Compost ended
6 – 04/04/2019

 

Hours

Task Time (hours) Value
Construction 3

€15.00

Maintenance 1

€5.00

Compost removal, site clean 1

€5.00

Total

€25.00

Conclusion

Having built and observed this pile, I do not expect any hot water beyond the ambient day time temperature. As such this pile will be allowed to compost, and we will keep taking weekly internal temperature readings in order to observe the compost heat, until it starts to cool down. The next pile will be heavily insulated, slightly larger and with the pipes inside the structure we expect successful water heating. 

TEST 2 – Compost made  –

Further research carried out:

From looking in to one of the pioneers work in this field (Jean Pain), his relatives recommend that the best materials to use would be fresh trees chipped evenly. We do not have access to that volume of trees, or a chipper but it is good to know the best state of the material to be used. We could look in to fresh olives leaves though as a major part of the mixture.

They also recommend to soak the mixture with 700 litres of water for each cubic metre of wood chip. Using the same water continuously as changing the water would wash away important nutrients and tannins etc which are important to the process. 

Variables

Results

The results below are on a weekly basis, and shown in degrees centigrade.

Week Compost temperature Water temperature
1 –
2 –
3
4
5
6

Hours

Task Time (hours) Value (hours x 5€)
Construction

Maintenance

Compost removal

Total

Conclusion

TEST 3 – Compost made  –

Variables

Results

The results below are on a weekly basis, and shown in degrees centigrade.

Week Compost temperature Water temperature
1 –
2 –
3
4
5
6

Hours

Task Time (hours) Value (hours x 5€)
Construction

Maintenance

Compost removal

Total

Conclusion

Further Reading 

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