Could the Compost Heap Catch Fire?

It is highly unlikely that a compost heap will self-combust under any normal composting conditions due to the moisture left in the decomposing waste. In fact, most organic chemicals will only self-ignite at around 120°C and the maximum temperature most domestic compost heaps will achieve is 75°C.

The Science Behind the Compost Combustion Theory

Scientific experts report that combustion is possible but only in extremely rare circumstances and it is near impossible to ignite a small domestic heap.

It can happen with very large dry heaps or haystacks and as such sizes over 2x2 meters are considered as a risk. The damp inner core decays creating heat and this heat does not escape fast enough (materials are self-insulating) and airflow becomes restricted. If this heat persists for long enough it can lead to ‘self-oxidation heating’ and push the temperature up to 120°C and hence self-ignite. Most reported compost fires have started with a poorly managed pile of wood or dry plant matter and fires in dry waste and/or compost sites are most often linked to the inadvertent addition of hot ashes, cigarettes and other materials that were already alight.

Worried That Your HOTBIN May Be Too Hot?

The internally generated heat in the HOTBIN will dry the contents to a certain extent, however once the temperature reaches 70-75°C these bacteria cannot survive and heat production is slowly reduced. You will often see a sharp spike in temperature when adding easy to digest items such as chicken pellets and grass which may seem alarming however this will slowly come down as bacteria finish breaking it down.

The HOTBIN is designed to hot compost between 40-60°C (rarely over 70°C but not unheard of). Air is drawn in through the bottom of the bin and rises through the waste to be released as water vapour and carbon dioxide (both flame-retardants) through the valve. The combination of damp waste, steam and carbon dioxide reduce the chance of combustion significantly. The sun will also not create overheating inside the HOTBIN – the insulation properties of the material works both ways keeping bacterial heat in but the sun's heat out.

Although we stress that we are not combustion experts the science speaks for itself, so in summary it is extremely unlikely to happen - especially in a HOTBIN.

Does This Science Apply to All HOTBIN Models?

All HOTBIN models work the same way, insulating the heap to expediate compost production but the HOTBIN Mega operates slightly differently. The HOTBIN Mega is a batch hot composter which due to its size, is a drier process that requires the addition of water. This is in comparison to the HOTBIN and HOTBIN Mini which conversely require more paper/card to control moisture levels.

Even though the HOTBIN Mega units have an increased capacity, ignition of the heap is still extremely unlikely for the following reasons reiterated from above.

1. Moisture – as with the other HOTBIN models, generally, moisture from your waste is locked in by the walls of the bin making a damp heap hard to combust. In the summer the heap can dry out, so moisture levels need to be managed by watering which aids bacterial activity and limits the internal heap temperature
   
   
2. Too hot for bacteria - bacteria generating heat cannot work efficiently at temperatures over 70°C and actually start to die off resulted in a semi regulated heap.
   
   
3. Controlled airflow – compared to an open heap the controlled nature of airflow through the unit limits oxygen; one of the elements that fire needs to burn
   
   
4. Enclosed heap – avoiding direct exposure to sunlight also limits an ignition trigger of heat

Will my HOTBIN Temperature Stay Consistently Above 70°C?

No, not for days on end due to the reasons already discussed around the bacteria’s ability to survive the heat.

Remember, you’ll always get a more accurate temperature reading directly from the heap and the the optimum is 60°C, so if you are approaching temperatures of 70°C you can take steps to cool the heap down. We advise opening the valve on the HOTBIN lid so it’s in a vertical position for a couple of hours (whilst monitoring the temperature gauge). Once the internal temperature has fallen back to within hot composting temperatures of 40°-60°C to return the valve to its original position. Opening the lid and a quick stir of the top layer should also help to lower the temperature if needed.

Please note: We do not recommend leaving the valve open for more than a couple of hours.

HOTBIN Mega Tips:

1. Open the valve fully vertical in the summer and or when the bin is full
   
   
2. Add water - all life needs water, including the microorganisms and bacteria that help your compost pile decompose. Due to the higher volume of waste in the HOTBIN Mega, the heap can dry out more quickly, water as required to maintain moisture levels and or if your Mega bin achieves temperatures over 70°C in the summer).

Guidelines for Watering the Compost Heap

If the heap over dries it will becoming less active, this is almost always going to be because the waste has overdried.

Using a hose with a fine spray attachment or a watering can with a rose will allow you to consistently dampen the heap.

We recommend approx. 10ltrs of water for every 200ltrs of dried waste material. As an example, if your 450ltr Mega is full but only the top half has dried out add approx. 10 ltrs of water.

Initially this may have a cooling effect on the heap, however don’t be concerned as restoring moisture levels will kickstart the process invigorate the bacteria and reactivate the heaps temperature.

If you are concerned or have any questions you can contact the HOTBIN team freephone on 0808 168 8499 or email us at help@hotbincomposting.com.

Updated August 2022