Hotbox Composters Do An Excellent Job But Are Expensive, Often Around £300, But Can Be Built From Scrap At Nil Cost. Here’s How to Make a Hotbox Composter From Construction Waste And Parcel Tape!
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I hate waste. I hate unnecessary spending. But I want a really good, fast and efficient composting system that will swallow as much organic garden and household waste as we produce every week of the year.
So, having researched hotbox composters, and been shocked by the price of them, I decided to build one from waste materials.
With so many people rightly complaining about the quality of purchased compost now might be a good time to learn How to Make a Hotbox Composter from rurally low cost to scrap materials.
What Are Hotbox / Hot Bin Composters?
Basically a hotbox composter is an insulated plastic composting bin. Being insulated the heat generated by the composting is retained and this speeds the composting process. Temperatures in the bin can rise to as high as 170F/76C., though slightly lower temperatures are to be preferred.
Many boxes also have a tap built into the design, to allow leachate to be removed from a storage chamber at the bottom of the bin. This type of design clearly adds to the complexity of the design and home manufacture, which makes such bins so expensive.
My bin is much simpler in design and build complexity, which makes it easy to build from scrap. I’m planning on raising it off the ground so I can catch the leachate in a try below.
The advantage of a hot bin is that it makes compost much quicker, due to the heat. And that the process allows for cooked food scraps to be included. Hence much less food waste needs to be sent to recycling sites, saving the cost of transport.
Hotboxes copy nature, but speed the process up to make sterile compost free from weed seeds and the potential of roots surviving; and all without turning. Various manufacturers and users make claims that the compost can be ready in 4-6 weeks whilst other claim 10-12 weeks.
One last thing. Some hotbox devotees claim there is no need to mix the materials whilst others say it decomposing materials need a good stir now and again to move the outside to the inside and ensure everything heats up well. Time will tell if this is necessary in my case and I’ll be writing another article on how to fill and manage the hotbox very soon. Until then enjoy reading the rest of this How to Make a Hotbox Composter article.
Scrap Materials For HotBox Construction
The insulating material used in my case was recovered, with permission, from a construction site skip. It is a sheet of Xtratherm insulation board that is used in walls, floors and roofs and comes in various thicknesses from 20-170mm. The full sheets measure 1200 x 2400mm but clearly finding a complete sheet is unlikely so you may have to skip dive to find suitable offcuts.
In my case I managed to find two large sheets and cut them down to the size needed.
Being an insulation board the sheets are relatively easy to damage so if you can find some scrap plywood or similar to wrap the insulated box in that would be great. So far I’ve only made the insulated box and it’s not covered.
The Xtratherm, or similar material, is used to make an upright box with an insulated top and bottom. If can be glued with a non solvent glue. In my case I had some wrapping tape and I’ve used it to join the sides of the box together and found it works very well. It also means I’ve used something that I didn’t have to buy!
Constructing The Hotbox
This is very simple and needs no special skills. I’ve made a box of four equal size pieces of board and taped them tother to form an upright rectangular box. To plug the top and bottom Ive cut inserts that fir inside the box and glued them to a lid that fits the end section of the box. They are the same shape and size for the top and bottom of the box.
The bottom “plug” I’ve drilled with a series of air holes to allow air in and in the top I’ve put a single small hole to allow very limited air circulation (and to hold a thermometer). The process is aerobic but I don’t want so many holes that too much heat is lost.
My theory with the bottom plug is that it shouldn’t be secured in place as then I can remove the box by lifting without damage, should I need to.
The size of the box is up to you and what you can find to recycle. Mine is 1200mm high which is the width of a full board and I’ve gone for 450mm wide as my sheet would split into that size without wastage.
The size I’ve chosen is smaller than some commercial bins but there are only two of us, so kitchen waste is limited. The other factor is that the compost bin chomps through waste very quickly and I didn’t want it to be too big. If I find I need more space I can build a bigger one sometime.
Cutting the board is very easy. It will cut with a sharp knife or a saw.
The downside of using a knife is that it is hard to cut a straight line, my knife often strayed a bit off the line I’d marked and this means the fit of the boards will not be snug and heat will be lost. The alternative is to use a saw. It saws very easily and if you use a strip of wood as a guide it is easy to keep straight. The downside of a saw is the sawdust which is fine insulation material. That isn’t great but bear in mind the whole sheet was in skip and going to who knows where!
Because I’ve only used insulation board to make my bin, and not yet given it a more solid and durable skin it’s a good idea to ensure the corners are protected. To do this I’ve found some plastic corners that I’ve recycled from some waste packaging.
Some people might decide packing tape isn’t secure enough and decide to put buckle straps around the bin. These would cut into the insulation boards so I suggest using another recycled product, plastic padding corners. These are the blue items in the photo.
What If I Can’t Scavenge Recycled Construction Materials?
Products such as Xtratherm can be bought from builders merchants. Depending on the thickness used they are around £30-35 a sheet. I’ve used just over a full sheet to make mine. The tape I’ve used costs about £4-5 a roll. So even if not built from scrap this can be a very cost effective way to start hotbin composting.
Conclusion: How to Make a Hotbox Composter
OK, so that’s it.
Except of one thing. As you’ve probably spotted removing the finished compost is going to be impossible as I’ve not added a door.
So, to make harvesting simple I have cut the whole front sheet in to two sections. The top section is held permanently in place whilst the bottom one is temporarily held in place by tape and can be removed to access the compost when ready.
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