Why do I get a whiff when I open the HOTBIN lid?

Bacteria do not decompose complex sugars and cellulose straight to carbon dioxide, they break it into smaller chemical compounds first. Many of these intermediate "volatile organic chemicals" (VOCs) have an odour. In cold composting, these chemicals are produced slowly and they do not vaporise to the same degree, hence you rarely notice them.

At 60°C, they are produced faster and more importantly, they vaporise. The hot steam/carbon dioxide leaving the HotBin contains small amounts of odorous ‘volatile organic chemicals’ (VOCs). They are a sign of a highly efficient heap, but can also be a nuisance if not addressed.

The filter removes these below nuisance levels, but when you open the lid you may catch a whiff as the gases exit the bin.