8 Step Compost Bin Build with Hotbin Mk2 for Kitchen Waste

Why this bin runs hotter than an open pile

The HOTBIN Mk2 body is made from expanded polypropylene, the same closed-cell foam used in cycle helmets, with walls around 50mm thick. The insulation keeps bacterial heat inside the unit instead of losing it quickly to the surrounding air. A standard plastic dalek bin sheds that heat within hours, so it rarely rises above ambient temperature and usually decomposes through cold, slow fungal action over many months.

The Mk2 has a thermometer built into the lid and a removable valve for airflow control. When the contents are working well, the dial sits in the green band, between 40C and 60C. In that range, thermophilic bacteria dominate the mix. They break food down faster and kill most weed seeds and pathogens that survive in a cold pile.

The design also includes a bottom hatch for removing finished compost and a separate raised plate inside the base. That plate leaves an air gap beneath the waste. Oxygen matters as much as food and moisture once the bacteria begin generating heat.

Steps 1 and 2: site the bin and set up the base

Put the bin on a hard, level surface such as paving slabs, concrete, or decking. Bare soil is a poor base because the HOTBIN drains leachate through its base hatch into a tray, and soft ground can let the unit lean as it fills with 50kg or more of wet waste. A south-facing wall gives a small thermal advantage in winter, although the bin can run once it is hot.

Assembly is brief. The Mk2 arrives as the base unit and lid, with the raised floor plate, the rake supplied for mixing, and a bottle of HotBin bulking agent. Drop the perforated plate into the bottom so it sits clear of the floor, slide the leachate tray under the front hatch, and clip on the lid. Set the valve in the lid roughly half open as the starting position. The whole job takes under ten minutes with no tools.

Step 3: give the first load enough mass

A cold start is the most common failure with this bin, and it usually comes from underfeeding. Bacteria cannot raise the temperature of a thin layer of scraps. The HOTBIN needs enough fresh material to ignite, which the manufacturer puts at around 5kg added in one go, sitting on a warm or pre-loaded base.

In cold months, many users prime the first load with a kettle of hot water or add a HotBin Hot Water Bottle, a sealed reusable pouch made to sit inside the bin and seed warmth. Once the core reaches 40C, the process becomes self-sustaining and relies on the regular additions that follow.

The first charge should be mixed. Pure food waste quickly becomes a wet, airless mat, then drops into anaerobic sludge within days. The rotten-egg smell of hydrogen sulphide is the clearest sign that the ratio has gone wrong.

Step 4: use bulking agent and paper by volume

Every food addition needs two companion materials. The supplied bulking agent is a woodchip product that keeps air channels open so oxygen reaches the centre. Shredded paper or torn corrugated card absorbs surplus moisture and adds carbon to balance nitrogen-rich food.

HOTBIN guidance gives a practical ratio of roughly equal parts by volume. For every caddy of kitchen waste, add a similar volume of bulking agent and a similar volume of shredded paper. Office shredder waste works, and cereal-box cardboard torn into strips works too. Glossy magazine paper performs poorly because its coating resists wetting.

Keep the mixture loose. Pressing it down with the rake crushes the air spaces made by the bulking agent, and the temperature can fall within a day.

This is the point where many bins drift off balance. A week of heavy cooking produces far more food scraps than paper, and the contents can turn wet and cold. Keeping a stocked container of shredded paper beside the bin fixes the missing input before the food caddy overflows.

Steps 5 through 8: feeding, moisture, liquid, and harvest

Once the bin is running, feed it little and often. A daily or twice-weekly caddy keeps the bacterial population supplied at a steady rate. Add the food, bulking agent, and paper together, then lightly fork only the top 100mm to 150mm with the supplied rake. Do not dig to the bottom. The lower layers are maturing, and disturbing them cools the whole column.

Judge moisture by squeezing a handful. It should feel like a wrung-out sponge, damp with no water running out. A dry mix stalls bacterial activity. Waterlogged material loses its air spaces and turns anaerobic. Add more shredded paper to dry the mix, or use a splash of water in summer when the surface crusts.

Leachate is the dark liquid that drains into the front tray. It is concentrated, so empty the tray before liquid backs up. Diluted at around one part leachate to ten parts water, it can be poured onto borders. Used neat, it is strong enough to scorch leaves.

Harvest from the bottom hatch. Mature compost forms at the base while fresh material is added at the top, so the unit runs continuously with no full emptying. Pull a few litres of dark, crumbly, earthy-smelling material from the hatch, and the column above settles down to fill the space.

A month of household waste

For a two-person kitchen producing around 1.5kg of food waste a week, the month’s total is roughly 6kg. Vegetable peelings, coffee grounds, eggshells, bread crusts, and tea leaves can all go in. The HOTBIN also accepts cooked food and small amounts of meat because the heat handles them. Cold bins commonly attract rats when those materials are added.

Across that same month, add a comparable volume of bulking agent and a comparable volume of shredded paper. At that feeding rate, the bulking agent from a single 5 litre bottle stretches across several weeks.

Through a January with night frosts, a bin that was already hot held its core around 45C to 50C while the air dipped below freezing. Once the column had matured, the output was a few litres of compost a month from the base hatch, while the surface still steamed on cold mornings.

Keeping it running through frost

When a hot bin goes cold in deep winter, restarts usually come after a single large 5kg feed plus a hot water bottle inside. Small daily additions can disappear into cold contents with little movement on the dial.

Winter leaves an awkward mismatch: the core can still look alive while the feed rate has already slipped behind the kitchen.