How To Heat Your Greenhouse With Compost

Compost radiating heat

Learning different methods to capture heat through organic compost for your greenhouse during the colder months is another benefit you can enjoy while producing high-quality organic soil amendment.

Most small or large scale farmers are aware that a large heap of fresh manure and other organic farm scraps can generate substantial heat. And this heat can last for about several weeks or months.

Does Compost Really Generate Sufficient Heat For Your Greenhouse?

Two heaps of wood chip compost piles
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Extracting heat from compost piles is nothing new. Lots of records show decomposing straw can provide heat and carbon dioxide that you can use to improve the health of your growing plants within the greenhouse.

And recently, new techniques were developed and tested on how to extract heat from the compost.

Unlike the old way of using fans to draw heat, the method of using water flow through pipes buried under the slab is efficient enough to pass heat from the compost pile to the greenhouse.

The Principles and Application of Compost Heat Extraction

Extracting heat from compost is considered in three highly interdependent stages:

  • Heat Production
  • Heat Capture
  • Heat Utilization

Heat Production

In the process of composting, microorganisms break down the organic matter in the compost. It produces carbon dioxide, water, heat, and humus-rich products through microbial activity. 

Microbial activity with an optimal temperature of about 130 to 140 degrees produces heat at high rates. This heat is sufficient enough to kill weeds and harmful bacteria. And under optimal conditions, compost heat can provide ideal temperatures inside the greenhouse during winter. 

The heat available from the base of the compost is the same as the heat from the combustion of the substrate you can find in calorimetry

Composting oxidizes organic compounds completely and generates heat. The actual amount of heat generated during the process is determined with the energy content of the organic matter, decomposition, duration, and the actual conditions during the composting process.

Heat Capture

Setting the foundation of the compost heat recovery system
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So far, there are three methods of extracting heat from the compost. And these are either air-based and water-based heat capture, or the recent condenser-type heat exchanger.

1. Air-Based Heat Extraction

In an air-based heat extraction system, the air is pulled down from the active compost piles through blowers. It then forces the resulting compost-heated hot vapor to flow through the duct system and over thermosiphon tubes.

In this method, the greenhouse can benefit from both the heat and carbon dioxide release from the composting vapor.

The thermosiphon tubes will rapidly transfer the heat from the hot vapors within the duct system to potable water in an insulated bulk tank with no direct energy input.

2.  Hydronic Capture Method

A low-density pipe in spiral form on top of the compost to extract the heat through hydronic capture
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Next is the hydronic or water-based heat capture method. In this method, a pipe system is planted under, around, or directly within an active compost pile.

And a water antifreeze solution is pumped through the pipes. The solution itself will heat the water.

Hot water is pumped to a suitable heat load device to buffer the warm temperatures and energy extracted. Such a device can be a flooring slab, water-to-air radiator, or heat exchanger.

The pipe system and heat load device are connected in a heat exchange loop to the expansion tank and pump.

3.  Condenser-Type Heat Exchanger

A coiled LDPE pipe as a condenser-type-heat exchanger to generate heat in the greenhouse
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The third approach is the condenser-type heat exchanger. This is the recent approach to capture a large quantity of compost thermal energy. And this approach becomes beneficial and is widely used by most commercial composting sites.

The heat exchanger should utilize and push the heat into the upper parts of the pile in the thermophilic phase.

When the steam condenses on the heat-exchange surface, the heat passes through the pipe via conduction to the heat-carrying medium.

Heat Utilization

The efficiency of heat extraction from the compost to a large extent depends on the flow rate or how you use the energy and the temperature of the discharging water. The output is always associated with how the heat is recovered.

The higher flow rates of the water temperature to enter the extracting system capture more heat, as the temperatures of discharging fluids decrease.

When using the compost heat recovery system to disperse water within the pipe, the flow rate is configured to extract more heat from the pile. “The Energy Generating Potential of Compost.”

Based on the study conducted by experts in this field, a realistic estimation for the heat generation potential of the active compost heat extraction process can capture a rate of about 1,000 BTU per hour per ton. And the heat can last no longer than 6 months.

So compost heat extraction experts are collaborating to develop low-cost methods to produce concrete estimates of the heat-generating potential of a given compost recipe.

  • Considering The Heat Extraction System

However, when considering a farm-based compost heat extraction system, you must understand the composting process clearly.

A keen understanding is essential before considering the technology for capturing compost heat. And you must know how to utilize the fullest extent of how the heat generation system works.

Compost experts know the parameters involved in the right composting process. And the procedure includes the C: N Ratio, moisture content, the proper decomposition, the porosity of the compost recipe, and the physical structure of the active composts.

  • Compost Heat Extraction For Small-Scale Farm

Most gardeners are familiar with composting to produce humus-rich soil amendments.

Although the practice of compost heated greenhouse is not widely spread due to energy cost, some gardeners use it to extract heat from composts to help extend the growing season.

But utilizing the heat of the compost as the secondary output to provide an ideal temperature within the greenhouse requires a keen understanding. You must know how the system works to make it successful.

Some sophisticated greenhouses use water pipes with large scale manure-based composting to move and extract heat successfully. However, home gardeners use cost-free variations or low-cost heaters with this practice.

  • Cost-Free Variation to Compost Heat Extractions
Compost heat extractor system using 275-gallon plastic industrial totes.
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Home gardeners use compost bins to provide heat to specific locations in the greenhouse.

Others place the plants over the top of the compost to generate microclimates that will keep the heat up through winter.

This practice allows a gardener to achieve various temperatures and provide warmth depending on the specific plant requirements inside the growing facility.

By taking advantage of the sustainable ways to extract heat from compost and keep the air warm inside the greenhouse, it will allow home gardeners to plant year-round continuously, free from the costly heating system.

Using two empty barrels to extract heat from the compost inside the greenhouse is another simple concept you can do yourself. It’ll provide the warm temperatures that your plants need to thrive in winter.

  • Place two 55-gallon plastic barrels and place them several feet apart inside the greenhouse.
  • Make sure each barrel’s top is covered. If the barrel does not have a lid, turn it upside down.
  • Provide adequate space for the compost as you lay a metal wire bench top across the two barrels to support both ends
  • Place the wooden box in the space provided between the two barrels and fill it with both green and brown compost.
  • You need to make it fully loaded by refilling more compost and raise the temperature a few degrees more when it reduces as it starts to decompose.
  • Make sure to add water to the newly established compost bin.
  • Make sure to compress the compost down by either standing on the top with rubber boots or press down with rubber-gloves. Compacted composts can generate heat efficiently for your greenhouse.
  • Place the plants on the top of the wire bench to receive heat as the composts gradually break down and release heat.
  • You can keep a thermometer on the benchtop to monitor the heat.

Final Thoughts On Heating Your Greenhouse

High-quality compost is one of the valuable soil amendments that help improve soil structure, fertility, and biological suppression.

However, extracting the heat from the compost to provide the ideal temperature for your greenhouse is another way to extend your growing season.

Extracting heat from the compost varies from the system scale, the composting methods, and the system you use to recover heat. You can draw heat from compost either through an air-based system, hydronic, or the condenser-type- heat exchanger. If you have any questions, don’t hesitate to ask below!


About Benita Abucejo

Hi there! My name is Benita Abucejo. What can I say? I truly love spending my days in the outdoors, specifically in the garden. Gardening has always been a strong passion of mine since I was a little girl. It has brought me so much joy and happiness that it is definitely safe to say that I will be a gardener for life. For a period of time, I was able to work with people who are into home gardening and I found it to be quite beneficial to my physical health, as well as my mental well-being. Here at Seasonal Preferences, I am going to share with you my experience and ideas so that you can fulfill yourself with the same satisfaction and happiness. Of course, if you have ideas, I would love to hear those as well! Being creative in the garden can really be quite fascinating so let's share our experiences and be the best gardeners we can be. With that being said, thank you for dropping by and please leave me a comment on one of my posts if you would like to get in touch!

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