18 Kilograms of Finished Compost from a HotBin Mk2 in Eight Weeks

I bought the HotBin Mk2 to see whether a small insulated bin could turn ordinary household waste into usable compost in time for spring lawn repair. The bin is sold as capable of holding 40 to 60 degrees Celsius from microbial decomposition alone, helped by its 50mm expanded polypropylene wall and a bung-and-valve lid system that lets moisture out without throwing all the heat away.

I started with woodchip in the base for drainage, then added a handful of the supplied bulking agent to keep air channels open. The bin sat against a south-facing wall. By day four, the built-in thermometer had climbed to 48 degrees; by day nine, it had settled at 55.

That heat range is the useful zone. It kills most weed seeds and pathogenic organisms while thermophilic bacteria stay active. Below 40 degrees, the contents drift into a cold, slow rot. Above 60 degrees, the working microbes start dying and the mass cools itself. The eight-week finish and the 18-kilogram yield came from regular feeding, moisture checks and a decision to stop adding fresh material before the final curing stretch.

Feeding ratio and moisture

The carbon-to-nitrogen balance decided whether the HotBin heated properly or sulked. The manufacturer recommends roughly equal volumes of nitrogen-rich material, such as kitchen scraps, grass clippings and coffee grounds, and carbon-rich material, such as shredded corrugated cardboard, torn paper and dry leaves. Volume mattered more than weight, because a bag of grass clippings weighs far more than the same bag filled with cardboard.

Grass clippings caused the easiest mistake. Added on their own, they compacted into a wet, airless mat, dragged the core temperature down within a day and produced the ammonia smell of stalled decomposition. I mixed every batch of clippings with an equal volume of shredded cardboard and a scattering of bulking agent. That kept enough structure in the heap for air pockets to survive after the fresh load settled.

Moisture needed almost as much attention as the feed mix. When the material looked glossy and clumped, I left the lid valve open a quarter turn so steam could escape. When it looked dry and the thermometer sagged, I added a litre of water with the next feed. The feel I wanted was a wrung-out sponge: damp to the touch, with no free water squeezed out by hand.

Overwatering caused more temperature drops during the run than any shortage of nitrogen. Feeds went in every three to four days, usually in batches of two to three kilograms. Larger single loads buried the hot core under cold material, knocked ten degrees or more off the thermometer reading and needed two days to recover.

The thermometer record

Day 1 to 4: the bin climbed from ambient temperature to 48 degrees.

Day 5 to 21: it stayed between 54 and 58 degrees, the active thermophilic phase when most of the breakdown happened.

Day 22 to 40: it eased down to 42 degrees as fresh feeds tapered off and the remaining material moved into mesophilic curing.

Day 41 to 56: it sat between 30 and 38 degrees. Earthworms that had migrated up from the base were visible through the lower material by then.

That fall in temperature matched the feed pattern. Once I stopped adding fresh scraps, the bin cooled as the easily available carbon and nitrogen were consumed. A fresh load in week six would have restarted the heat and pushed the finished-compost date back by another fortnight, so the last feed went in on day 38.

The south-facing wall probably helped the result. It absorbed daytime heat and gave some of it back overnight; the same bin in an exposed, shaded corner might never have cleared 45 degrees in early spring, and the cycle would have stretched accordingly. I kept the bin in that one spot for the whole run, so the thermometer record belongs to that placement as much as to the bin itself.

Screening and weight

At week eight, I opened the lower hatch and pulled out material that was dark, crumbly and smelled like forest floor. I could not find recognisable food fragments apart from a few avocado stones and pieces of eggshell. A 10mm garden riddle separated the finished compost from the coarse pieces, and the unfinished bits went straight back into the top of the bin as bulking material for the next cycle.

The screened compost weighed 18.2 kilograms. I had weighed the inputs on a kitchen scale through the eight weeks, and the total came to roughly 55 kilograms of mixed material. That works out as a mass loss of about two thirds, which fits the water and carbon dioxide driven off during active decomposition. A one-to-one return from waste to finished compost uses the wrong arithmetic; the finished mass is always only a fraction of the feedstock.

Eggshells survived the process almost intact and needed crushing before they would break down in soil. Citrus peel and onion skins decomposed fully at these temperatures. That went against the common instruction to keep them out of a compost bin, an instruction that makes more sense for cold heaps where those scraps can linger for months.

Wormery liquid on the same lawn

I ran a separate stacking wormery at the same time, stocked with Dendrobaena veneta and fed with the softer kitchen scraps. It produced a small amount of vermicast and a dark leachate from the sump tap. I diluted that liquid at roughly one part to ten with water and put it on one half of a brown-patch repair area on the back lawn. The HotBin compost went onto the other half, worked into the top 25mm of soil. Both halves were overseeded with a dwarf rye and fescue mix.

The liquid feed gave visible green-up within eight to ten days, faster than the compost side, because its nutrients were already dissolved and available to the grass roots. The compost side took longer to colour up, then held the improvement longer and produced denser tillering by week five. Its effect came through better soil structure and water retention instead of a quick soluble hit.

My lawn soil is sandy, drains fast and loses nutrients to leaching. In that soil, the compost’s structural contribution mattered more than the speed of the liquid after the seed had established. On heavy clay, the balance might shift, since clay already holds nutrients and needs drainage improvement that a quick liquid feed cannot provide. Used on the same repair patch, the liquid helped the new seed show early colour while the compost improved the root zone for the dry weeks that followed.

Wormery leachate is not the same as commercial worm-tea, and its nutrient concentration changes with the worms’ feed. The one-in-ten dilution was only a starting strength. I watched the grass for scorch and did not see it at that dilution.

Timing and the bed edge

An eight-week compost cycle started in early spring finished in time for the main overseeding window, when soil temperatures were above 8 degrees Celsius and new seed germinated within two weeks. A cycle started in high summer would have finished in the drought-stress period, when top-dressing a lawn with compost achieves little because the grass is not actively growing. The bin’s output had most value when its arrival matched the turf’s growth curve.

Bark mulch had a different job on the beds bordering the lawn. I put down a 50mm layer of composted bark, which reduced surface evaporation and kept the soil beneath measurably damper between waterings. That buffered the young grass along the bed edges from the drying that had killed the first overseeding attempt. Fresh, uncomposted bark would have locked up nitrogen as it decomposed, so I used only the aged product. The bed edge no longer browned first, which was the most visible change from the first attempt.