HOTBIN Composting - Using the Hot Water Bottle

 There's no time like the pleasant - George Bergman

A while ago, I received one of the regular e-mails from hotbincomposting.com which discussed when/how to use the supplied hot water bottle that is included with every new HotBin. Following Sod's Law, I'm now having trouble locating that e-mail. From memory, the advice was to use the hot water bottle only during the colder months when the outside temperature was below 15 ℃. This seemed unnecessarily restrictive.

Photo 1: Newer & Older HotBin Versions

I regularly use hot water bottles during the winter to help maintain the active (i.e. top of the heap) composting temperature above 40 ℃. It is important to keep the active region above this temperature when the kitchen caddy (possibly containing cooked food) is the main source of green waste.

Photo 2: Hot Water Bottles for the HotBins

Sometimes, I also use the hot water bottles to start the hot composting process when starting up a 'new bin' at any time of the year. I always like to get the HotBin up and running as quick as possible. Often because I have a ton of garden waste to process because: (i) Mary has had a weeding/pruning blitz, (ii) the neighbours have had a pruning/weeding blitz, or (iii) my local garden maintenance guy, Paul, has just mowed/weeded/pruned one of his client's gardens.

To get a 'new bin' going asap, I use one or more (and sometimes all four) activation methods. In all HotBin start-ups, I assume a suitable mix of brown/green materials is available along with some form of bulking material (bark) and an absorbent (e.g. shredded/torn paper/cardboard) to take up excess moisture:

1. CompStix is the woody unsieved material recovered after sieving my finished compost which I use as a free bulking agent and compost activator.                                                                                                                                    

   

   Photo 3: CompStix Collected after Sieving Finished Compost

   
   I use this material as the base for a new bin start-up and also as a blending component. It performs two main tasks: (i) aeration by ensuring there are pockets of air throughout the heap and resisting compaction, and (ii) inoculating the new heap with thermophilic bacteria (from previous HotBin runs) to give the hot composting process an early boost. Other bulking agents (e.g. bark or twigs) fulfill the first task but not the second.
2. Fresh Hot Compost from a concurrently running and active HotBin. This material adds both heat and active bacterial cultures to the start-up bin. Not always available!
3. Hot Water Bottle (HWB) or bottles are added to provide a heat boost. I often add two 2.5L litre bottles, neither of which is an original HotBin water bottle. Cheap replacements are available (e.g. used detergent/fertilizer bottles) but only use sturdy polypropylene (PP) bottles that can withstand hot/boiled water. My preferred method is to take a kettle of boiled water down to the HotBins (about 50 metres) and use a funnel to fill the empty HWBs that I have already buried in the upper compost layer (Photo 3). I use HWBs to kickstart a new HotBin, recover a partially-full cold bin, to convert a HotBin chugging along at 30-40 ℃ (mesophilic bacteria) into a HotBin racing along at 50-60 ℃ (by activating the thermophilic bacteria), or as a prophylactic measure to prevent the bin temperature dropping too low on a frosty night in winter.
4. Heat Reflective Cover (i.e. aluminium foil) to retain heat within the compost heap. This is a slightly controversial issue. While the foil undoubtedly reflects electromagnetic radiation (in the form of heat energy) back into the compost heap, it also partially restricts the upwards air flow that is a fundamental to the efficient operation of the HotBin.

Photo 4: Aluminium Foil Top Cover

The key is to ensure the foil cover is loose fitting with plenty of gaps around the edge to allow some hot air (and water vapour) to bypass it and exit via the vent. If the foil has small holes in it then that is also fine. 

A word of caution, however. Water retention within the heap is greater with the reflective foil in place. This is a benefit when the temperature of the heap is 55 ℃ or higher. Firstly, it helps maintain constant high temperatures due to the high specific heat capacity of water (effectively, its own internal hot water bottle). Second, it increases the rate of enzymatic (amylases) hydrolysis (saccharification) of biopolymers (e.g. starch) to produce simple sugars for the bacteria to feed upon. A pH of around 5-6 and a temperature of 55-60 ℃ is optimum for fungal and bacterial amylases found in the natural environment.

One thing I'd like to see is the addition of a heat reflective coating to the underside of the HotBin (visible in Photo 4).  This would eaid heat retention without restricting air flow.

According to HotBin, you should expect your freshly half-filled bin to warm up to 40 ℃ or higher in 48 - 72 hours. The actual time will depend on your mix of green and browns. Typically, I would expect a 'new' HotBin to be operating at 55-60 ℃ within 36 hours when using one or more of my 'accelerators'. As an example, I started to prepare a new HotBin run on the 10th June 2026:

•  Spread 4L Compstix on the base, followed by 20L hot partially-composted material from my other HotBin and 80L of mixed shredded garden waste (note: as a general rule I always shred my garden waste, apart from grass cuttings, before adding to the HotBin; this could, I suppose, also be added to the list of accelerator processes). A reflective foil cover was added but no HWB.
• After about 12 hours, the lid thermometer read 17 ℃ with the compost heap itself at around 30-35 ℃. A further 20L hot partially-composted material was added and the foil cover replaced.
• After 36 hours, the lid thermometer read 25 ℃ and the compost had heated up to 35-40 ℃.
• Time for drastic action. Two HWBs (each filled with 2.5L boiling water) were added along with a further 10L hot partially-composted material and the foil cover replaced.
• After 40 hours (i.e. just 4 hours later), the lid thermometer read 55 ℃ and the bulk compost temperature was 55-60 ℃. The HWBs were removed and 20L shredded garden waste added and the foil cover replaced.
• After 60 hours, the lid thermometer read 63 ℃ and the bulk compost was 60-70 ℃**.

This example shows how the addition of HWBs can, in as little as 4 hours, take a HotBin operating at mesophilic temperatures (35-40 ℃) up to the thermophilic (55-60 ℃) stage. It is important to remember we are dealing with a natural process and such interventions will not always provide such rapid dividends.

** After 26 days, the bin is still operating at 60 ℃ (bulk compost temperature) having consumed over 500L of garden waste.