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Building a DIY mash tun was the project that moved me from extract brewing to all-grain, and it cost less than $40 in parts. A well-built cooler mash tun holds mash temperature within 1–2°F over a 90-minute mash without any external heat input, better than many purpose-built mash tuns. The fundamental design is simple: a beverage cooler converted to a mash tun with a false bottom or manifold, a ball valve, and a drain fitting. I’ve built three of these over the years and the construction has gotten simpler each time. Here’s the version that works best for 5–10 gallon batches.
Choosing the cooler
A 10-gallon round cooler (Igloo or Coleman) is the standard choice for a 5-gallon batch mash tun, it holds a full 10–12 lb grain bill plus strike water with room to stir. For 10-gallon batches, use a 15-gallon rectangular cooler (typically a 60-quart Igloo MaxCold). The round cooler is easier to convert because the drain fitting goes in the center bottom; rectangular coolers work well but require a manifold (described below) rather than just a false bottom. Buy a quality cooler with good insulation, thin-wall coolers lose more heat during the mash.
Parts list
- 10-gallon round beverage cooler (Igloo or Coleman), $25–35
- 1/2″ stainless ball valve, $8–12
- Stainless braid (from a braided steel toilet supply line, 12–18″ long) or false bottom (stainless perforated screen, 9″ diameter for a 10-gallon round cooler), $8–15 for braid, $20–30 for false bottom
- Stainless nipple (1/2″ × 2″), two stainless nuts, two neoprene or silicone washers, $5–8 total
- Silicone tubing (1/2″ ID × 6″) for the connection between the valve and false bottom, $3
Build steps: braided hose filter method
- Remove the cooler’s factory drain spigot (twist counterclockwise, most come out easily).
- The existing drain hole is typically close to 1/2″, test fit the 1/2″ nipple. If too small, enlarge with a step drill bit to 7/8″.
- Insert the nipple through the drain hole from inside. Place a silicone washer on each side of the cooler wall, then thread a stainless nut onto each end and tighten with pliers until hand-tight plus 1/4 turn. Do not overtighten, silicone gaskets seal with moderate compression.
- Thread the ball valve onto the external nipple end. Use Teflon tape on threads.
- Inside the cooler: cut the woven steel braid from a toilet supply hose (remove the rubber inner tube entirely, you only want the mesh exterior). Crimp one end of the braid closed with pliers. Thread the open end onto the internal nipple end with a hose clamp.
- Test with water: add several gallons of hot water, stir to confirm no leaks at the fittings. Open the ball valve, water should flow freely through the braid filter.
False bottom alternative
A false bottom (stainless perforated screen) provides better flow distribution than a braid for round coolers and is easier to clean. Set the false bottom on standoffs (small stainless nuts work) above the cooler floor. Connect the bottom drain nipple (same installation as above) with a short silicone hose to a port in the false bottom or simply position the false bottom over the drain opening. False bottoms are worth the extra $20 if you plan to brew regularly, they last indefinitely and provide more consistent lautering than braid.
Common Questions
How long will a cooler mash tun hold temperature?
A quality 10-gallon cooler with a full mash (the thermal mass of the grain and water helps) loses approximately 1–2°F over 60 minutes in a typical indoor environment. At 60–70°F ambient temperature, starting at 154°F strike temperature, you’ll end at 152–153°F after 60 minutes, well within the acceptable mash range. In cold environments (under 50°F ambient), heat loss increases, preheat the cooler with hot water for 10 minutes, drain, and then dough in, which warms the cooler walls and significantly reduces heat loss during the mash. Insulating the cooler with a moving blanket or sleeping bag while mashing is additional cheap insurance in cold conditions.
