Cold vs. Hot Composting in Winter

Hot (104-140°)

Cold (32-104°)

The right hot composting system will work all year round – which is relevant when composting food waste.

Cold heaps 'stop' in winter (23 to 41°F). All waste added piles up until spring when the sunshine warms up the heap. Piling up food waste is not an option – it is just a free rat take away.

Hot Composting can take a wider variety of food waste types without causing issues.

In addition, the higher temperature inside a hot composter results in water (excess) removal, which is the prime cause of mushy/anaerobic food waste.

70% all household food waste is not added to 'cold' compost bins as it is likely to cause issues with odor, rats, and flies.

Cold compost bins tend to be constructed with open vents and hatches —so any odor not only attracts but also allows access to the food— creating the infamous 'swarm' of flies when the lid is taken off, or even worse finding a nest of rats when the heap is broken open.

Hot composting kills weed including invasive seeds faster. The higher the temperature the more seeds are destroyed.

Many seeds will survive in 'cold' composting heaps.

 The problem is made worse as the seeds are planted in nature's best growing medium - humus/compost.

 It's not just weed seeds but seeds from melons and tomatoes plus weeds like couch grass and dandelion will also survive and grow.

Hot composting kills pathogens and unwelcome bacteria. Which and how many is a function of both time and temperature. Defra (Dpt. Environment & Rural Affairs) offers a robust analysis, and summarizes recycling at 104°F for 1 hour, to equal good sanitation,

To achieve the same level of sanitation and bacteria to die off, you need to leave the cold produced compost for as long as 12-18 months.

It is recommended to wear gloves and wash your hands to reduce chances of infection.

There is a lot of confusion and hype about various herbicides wrecking vegetable plots when treated plants have been added to compost heaps.

 Herbicides and pesticides are broken down in composting. They are broken down 32 times faster at 104°F than at 50°F.

The risks of pesticide infection via use of domestic compost are low because the concentration and volumes used are low. However, the extra security of fast destruction in hot composting is an extra level of security.

Speed is not important to everyone – for the most, gardeners are known to be a patient group.

However, if you are short of space, speed means more compost produced with less composters. We often hear about cold bins that are overflowing, requiring a second, third or 4th unit. If you want more vegetable plots and garden and less compost plots – then hot composting quickly is a benefit.

A UK ‘outdoor’ heap averages the same temperature as UK ambient air temp - i.e., 10°c. At 10°c the heap it is 32 times slower than at 60°C. If a soft waste like grass takes 6 months in a cold hep it will take about 2 days to reach the same state in hot heap. In general, cold = 12-24 months and hot = 1-3 months (because it does not stay at 60°c for the whole period).

A hot heap rarely produces putrid odors. Hot heaps transfer water away from the heap as steam, (if the waste has free air space which is normally provided by bulking agent/mulch).

All composting produces other odors - known as volatile organic chemicals, which often have an odor.

This is more noticeable in hot composting as they are made faster, and the heat vaporizes the chemicals.

Hot composting needs a carbon-filter to reduce odors while you are adding more food. HOTBIN comes with a charcoal filter that reduces odors that ordinarily attract unwanted animals.

In a cold heap, water is not evaporated; it must drain to the ground. This happens slowly and poorly in compost, so it gets waterlogged and turns anaerobic and putrid. The main solution is not to add 'wet' food waste to cold heaps - most of our diet is foods with 80% water. Cold composting releases VOC more slowly and they do not vaporize as much.

Hot composting will kill fly eggs & larvae - so swarms of flies or maggots do not survive inside a hot composter. In addition, it's too hot at the top for ants, rats, and most things you don’t like to see in a heap.

Flies will lay eggs and larvae (e.g., maggots) will be present in a cold heap, and often swarms of flies can come out when opening the lid.