A picnic cool box makes a good mash tun because its food safe and has a degree of insulation which will help maintain a constant mash temperature, usually around 64-66 deg c. Despite this insulation though, it is necessary to throw a few old blankets over it and make sure the underside is insulated from the surface it is sitting on by putting a few news papers or old towels underneath it. Using this method over a 90 minute mash period the temperature should drop no more than a degree c or less.To separate the wort from the grains a slotted copper manifold is used which is attached to a tap fixed to the inner skin of the cool box. The slots allow the wort to pass through but are narrow enough to hold back the grains.
The items required to make the mash tun are listed on the left.
Step 1. Cut a hole for the tap. Using a hole saw cut a 32mm clearance hole in the outer skin of the cool box followed by a 26mm hole through the inner skin for the treaded part of the fermenter tap to pass through. Fasten the tap with the back nut. The sealing washer should go on the outside.
Step 2. Make the manifold. The sizes of the various pieces of the manifold will depend on the size of the cool box. The dimensions shown will produce a manifold suitable for a 28L/32L Thermos Cool Box but may need to be adjusted for different sized cool boxes. The tube can be partially soldered into sections for ease of assembly and cleaning as shown, or simply pushed together as minor errors in cutting and measuring the will tend to hold the parts together perfectly well without it. The fittings can be crimped a little if necessary to make them more secure. Once the tubing has been cut (and soldered as required), slots are cut with a hacksaw blade halfway through the tube at around 7mm intervals to allow the wort to pass through. If sections have been soldered make sure the slots are cut so when assembled they all face the same way. (Best results are achieved when the slots face downwards in the cool box).
Step 3. Connect the manifold to the tap. Braided PVC tube can be used for this, it has a degree of flexibility so will allow the manifold to sit flat on the bottom of the cool box even if there is a degree of misalignment with the tap due to the positioning of the hole. It is also relatively unaffected by mash tun temperatures and unlike lighter weight siphon tubing it will maintain its shape under the weight and heat of the grist. The size of PVC tube depends on the type of tap used. If using a WD drum tap, 5/8” braided pvc hose is a snug fit in the back and fits neatly over the 15mm section of the reducer tee and is secured with a stainless steel jubilee clip. If using a WD quick serve lever tap then 1/2 hose can be used but requires a little modification as its a little loose in the back of the tap. It can be expanded by boiling in water briefly then pushing it at least 55mm of it over some 15mm copper tubing and letting it cool. It will be tight on the copper but with a little patience can be removed and pushed into the back of the tap, the other end will be a snug fit over the 15mm portion of the reducer
tee.