How to Use The Hot Water Bottle?

How to Use The Hot Water Bottle?

The hot water bottle is used in all HOTBIN models except for the HOTBIN Mega. It should only be used when the internal temperature of the HOTBIN is below 15°c.

Why is The Hot Water Bottle Needed?

If your thermometer reads below 15°c despite adding waste on a regular basis then the composting bacteria in your HOTBIN may have become dormant. This is usually most common in one of two situations; when starting your HOTBIN during the winter or continuing use after having a holiday/break where the HOTBIN has not been fed for some time. On these occasions you may need to use the hot water bottle to create a heat to wake up the bacteria and encourage them to start digesting waste and producing heat again.

What If the Temperature is Over 15°c?

If the temperature is over 15°c then the bacteria will already be active so using the hot water bottle will have very little effect. In this instance, we recommend adding fresh waste into the HOTBIN on a regular basis to reach and maintain hot composting temperatures.

Can Hot Water Just Be Added Directly Into the Bin?

No. Do not pour boiling water directly into the heap - the water will simply cool leaving no energy to evaporate the extra water and the contents in an overly wet state.

The Science Behind Why the Hot Water Bottle Works

An understanding of the science behind this process will help understand when it will and won’t work.

  • Heat in composting is produced from bacterial activity (ie. growth). When bacteria are more active, more heat is produced causing bacteria to grow/multiply producing more heat as a result.
  • Bacteria do not grow very quickly below 5°c and not at all at 0°c
  • When waste is cold only a tiny amount of heat is created by the bacteria and this is quickly lost to the cold air - this is why cold composting heaps lie almost dormant through winter.
  • When the hot water bottle with hot water is added into the waste, this heat (1 litre of boiling water contains 4200 J energy as heat) moves to the colder waste.
  • The HOTBIN's insulated walls help retain this heat - just like a thermos flask. The waste around the bottle heats up to 30-40°c for an hour or so and during this short period bacteria warm up and become active. As bacteria actively digest the waste around them they generate heat which increases bacterial activity and consequently the temperature inside the HOTBIN.

Easy to Digest Waste Must Be Added

This hot water bottle technique only works if there is enough easy to compost food waste. We liken it to a human diet - if you eat a high fibre breakfast you get steady energy all day, but drink coke and eat sweets and you'll be on sugar high for an hour!

Since the heat from the bottle only lasts an hour or so, the bacteria need easily digestible food source (vegetable & fruit peelings, grass, nettles, comfrey) during this hour. If your HOTBIN is full of woody material this is harder for the bacteria to digest and therefore no new heat will be generated.

Be aware that in most cases large heat increases will only be seen once the waste level in the HOTBIN is over 40cm deep (above the hatch height). If you are not yet at this stage, stay patient and keep adding waste.

Instructions for Using the Hot Water Bottle

  • Carefully fill the bottle with boiling water and replace the lid tightly.
  • Make a well in the top layer of waste and situate the bottle inside the HOTBIN covering with fresh and easy to digest waste.
  • Close the lid and leave for 24 hours without opening.
  • Check the temperature is above 15°c after 24 hours, if not repeat the process once more.

As a result, you should see an increase in temperature – this should be sustained as long as waste is added on a regular basis with shredded paper/card and woodchip.

Looking for printable instructions?
Downloadable instructions can be found here.

Warning: Once full, bottle is extremely hot so wear gloves and handle with care. The HOTBIN hot water bottle is manufactured from high-density polyethylene (HDPE), a material chosen to withstand the boiling temperatures from water poured inside it. This is not the same as fizzy drink bottles (PET) or milk bottles (LDPE).

DO NOT substitute with a bottle that is not HDPE (plastic/glass) - this will likely warp or break causing injury to the user.

Damaged or Lost Hot Water Bottle?

If you have lost your hot water bottle or it has become damaged, a replacement can be ordered from the HOTBIN shop.

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