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Hot Composting - Converting a cold winter compost heap into a warm winter heap

Tony Callaghan 19/01/2017

Hot Composting - Converting a cold winter compost heap into a warm winter heap

Here are my top tips on how to convert a cold winter compost heap into a warm winter compost heap.

Moving air (wind) cools compost 10 to 30 times faster than still, cold air; wind chill affects compost heaps! Block all holes and gaps in walls and ensure you have a robust lid pressed down on your compost. You can use plastic sheeting and old carpet. 

Go one step further and use sheets of insulating material, but take care not to use insulation materials that absorb or hold water; no Rockwool or PU roof insulation sheets. 50mm polystyrene sheet is a good choice but does break easily and you’ll be using fork and spade so this option needs to be thought through.

How can the compost breathe if sealed up with sheets? It sounds like this will create an anaerobic, slimy smelly mess. Here’s a simple principle you need to add: warm air rises and it will draw cold air in at the base. Place a couple of those water irrigation pipes under the heap (for instance, pipes with holes every few centimeters. Then place a tiny hole in the lid, and I do mean tiny, no bigger than a pencil prick. You have just created and hot airflow system.

My heap is already stone cold. No bacteria are working so it won’t heat up even if insulated. Assuming you have done steps one and two, here’s a very simple way to get the bacteria working again. Give them a hot water bottle. It may sound bonkers but it works (IF / WHEN you have (i) a fully insulated heap or a HOTBIN composter, (ii) some fresh compostable waste. Fill a one or two-litre plastic container with boiling water and seal it with the lid. Place the bottle in the centre of the compost heap/bin. (H&S warning: do not use a glass bottle - risk it shatters, do not use PET (1) “coke/soda” type bottles or LDPE (4) as they will melt. The heat given off will heat up the middle compost for a couple of hours – long enough to get bacteria going, once going they generate heat.

Will these tips work? You bet. They are the basic science concepts behind the award-winning HOTBIN®. You won’t get the same heat (HOTBINs work at 40 to 60C) but you will get a significant improvement; things will tick over in winter at 10-30C. And that means if you start now, you will have more ready for March! 

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