Cool Box Mash Tun

DIY brewing equipment

What is a Mash Tun?

A mash tun is a crucial piece of brewing equipment used to create the malt extract from the grain. It does this by mixing the grain with water and keeping the mixture at a steady temperature, usually around 70°C, which allows enzymes in the grain to break down starch into sugar for the yeast to consume during fermentation. A good mash tun also provides an easy way to draw out the extract leaving the grain behind.

Up until now I have been using a stock pot wrapped in a duvet to keep my mash (the mixture of grain and water) at the right temperature. This has worked really well but is quite small and has no way to easily draw off the extract. Additionally, being able to draw the liquid extract out from beneath the grain has one interesting benefit in that I can pump the extract back into top of the mash tun, circulating it through the grain bed which acts as a filter to remove sediment from the extract before diverting the extract to the pot for boiling.

My plan is to use a cool box to build my own mash tun that will let me brew bigger batches and filter the extract.

A photo of the grey, black, and neon pink tartan material

Cutting the Hole (and breaking it)

I started by drilling the hole through the side of the cool box for the tap. This got off to a bad start when the brittle inner layer cracked out from the hole in three directions.

Not ready to write off the cool box just yet, I cut a rectangle of silicone that acted as both a seal around the inside of the tap and a patch over the cracks.

Cutting the Pipe

I needed something in the bottom to evenly draw out the liquid extract while leaving the grain behind so I made a loop of copper pipe and sawed slits into the pipe. This would be sufficient, as smaller pieces of grain husk and sediment would be filtered out by circulating.


To check the mash tun was watertight, I filled the bottom with water and measured the depth. After leaving it for a few hours there were no drips around the tap and the water level had not decreased.

First Run

Ready for my first 25 litre brew, I got to work sterilising my equipment and activated my packet of yeast ready for later. This is when disaster struck and sterilising solution started dripping out of the seal around the tap.

I was devastated, but I still had 25 litres of porter to brew and my yeast was already bubbling away. Taking the loop of copper pipe out of the cool box, I placed it in the bottom of a spare cool box. Cutting a short length of silicone tube I threaded it into the pipe, added my grain and water, and put the lid on.

While the enzymes in the grain broke down the starch, I dug out a small 12V pump and wired it up. When the mash was ready, I plugged the silicone tube into the pump to draw the extract up from the bottom and I was on my way again!

Making the skirt pattern AKA template

To the Future

After inspecting the mash tun, it seems that the weight of the water caused the walls of the cool box to deform when I lifted it up, ripping the tap out of the connector inside. My temporary solution had worked, and I was over the moon at how well filtered my porter was, but the tap would provide more functionality.

I don’t have a clear plan to fix it yet and I can’t seem to find taps with longer threads which means a slight redesign but even with my pump hack this is a huge improvement for my brewing and just one step in the ongoing process of increasing my capabilities.