Turning a 5 Gal Water Cooler into a Mash Tun

5 gallon Igloo water cooler with
KettleValve and barbed hose adapter

I've been an extract brewer from the get-go. I was perfectly content to steep specialty grains, but I wondered how steeping was different from mashing.

Mashing uses the enzymes in malted barley (its "diastatic power") to convert starch in the grains into fermentable sugars. Water and grain are held at a certain temperature (148°-158°F) for 45-90 minutes while the enzymes do their thing. Temperature and duration have significant effects on the resulting wort (e.g. higher temperature leads to less sugar) so these need to be managed carefully. Mashing also extracts color, flavor and body from the grains.

Steeping is like a crude mashing. It also extracts color, flavor and body from specialty grains, but the time and temperature are less important. The grains are steeped for at least 30 minutes at roughly 150°-170°F. These grains don't have any diastatic power, so precise temperature control is not necessary.

Mashing requires a fair amount of water. The single infusion method popular with homebrewers needs 1.25 quarts of water for every pound of grain, and that adds up quickly. Rather than try to maintain mashing temperatures in the brew kettle, some clever homebrewer got the idea of converting a cooler into a mash tun. The cooler functions like a giant thermos, holding the mash at the desired temperature.

Now, you can buy a cooler that's already been converted to a mash tun (or even a specially-made tun if you've got the scratch). But if you already have a cooler, you can make it a mash tun pretty easily by replacing the push-button spigot with a ball valve. (You don't have to replace it, but just know that you'll wear out your thumb pretty quickly trying to drain several gallons.)

The Once and Future Mash Tun

Original spigot assembly and the
indentation around the hole

I have a 5-gallon water cooler I bought at a big-box store over 10 years ago, lightly used and yet no other use for it. There are plenty of how-to guides and conversion parts out there, but interestingly, none of them used my model of cooler (Igloo 5-gallon Maxcold).

I removed the spigot by removing a plastic nut inside the cooler and sliding the assembly out. I needed an adjustable wrench to loosen the nut since it wasn't easy to grab, but otherwise it was straightforward.

The spigot assembly is made up of four parts: The spigot itself (the business end), a black rubber washer, a white plastic washer and the white nut. The spigot shank is 0.85" wide and it neatly fits the hole in the cooler. The shank isn't very long either. The horseshoe-shaped indentation around the spigot hole means the cooler wall is thin there, not much more than the inner liner and outer shell. Whatever valve I put here can't be very heavy.

Enter The KettleValve

I took the spigot to MyLHBS to see if they had anything that would fit. Fortunately, the clerk had made a mash tun cooler several years before. He took me over to one of their hardware shelves and handed me the KettleValve.

The KettleValve is what is known as a weldless valve. Rather than attach it to a welded port on the side of a brew kettle, it simply screws on through a hole that's been punched or drilled. I imagine there are many more DIY homebrewers who have a drill vs a TIG welder.

The valve isn't very big but it feels substantial. The ball valve has a 1/2" MPT (male National Pipe Thread) intake to attach a kettle screen and a 3/8" FPT (female National Pipe Thread) outlet to attach all manner of drain tubes. The valve has a built-in gasket on the intake side, so it looked like I might not need any other parts to install it. I got a 3/8" barbed hose adapter so I could connect a length of PVC hose and I was all set.

Note the gap (orange circle) left when
the valve doesn't close fully.

I unboxed the valve and gave it a once-over. Like any other ball valve, it's just a quarter turn from open to closed. The valve moves smoothly but has a subtle, satisfying pop when it's fully open or shut. I put teflon thread tape on the male end then attached it to the cooler. Easy peasy.

[Sidebar: Teflon tape is your friend. It makes loosening and tightening metal pipe connections sooo much easier. Ball valves, regulator check valves, showerheads, garden hoses ... you can find a use for it. The stuff is cheap too. Go get some.]

Remember that indentation around the hole? It's pretty deep. Deep enough that it kept the valve from closing completely. (See photo on the right) That clearly won't do. I had two options: Add some spacers on the outside to move the valve stem out, or cut a notch in the indentation to allow the handle to reach the closed position. The spacers option was the safer of the two, but where would I get them?

Black and white washers added to
KettleValve

Off of the original spigot!

I took both the black and white washers off and stacked them on the male end, then reassembled the KettleValve.

And it worked. But only just. I could only manage two complete turns on the nut that goes inside the cooler, and I doubt I can find a washer thin enough to go underneath it. Best to leave the nut finger-tight and not jostle the valve too much.

Leak Testing

Weldless valves need a tight connection to prevent leaks, but I didn't know if finger-tight was good enough, nevermind my scavenged washers. I poured in about 2 gallons of water and let it sit for an hour with the valve closed.

Not. One. Drop.

I put teflon tape on the barb adapter then screwed it in. I opened the valve and out came a nice steady stream of water. No more holding open a spring-loaded spigot for 10 minutes. Looks like this thing will work!

Brew Day

Satisfied that it wouldn't leak, I got ready for my first ever partial mash, a tweaked version of Snowzilla, my low-ABV oatmeal stout. The plan is to mash at 155°F for 60 minutes. I warmed up my strike water to 170°F and crushed my grain. I put the water in the mash tun then slowly lowered the grain bag. I used my 22" whisk to stir the grain and break up any dough balls. I dropped the thermometer probe in and it's too warm at 161°. The grain absorbs some heat, but not enough this time. I added some chilled filtered water and got it to 151°F. Too far. Crap. It would be futile to chase the temperature, as it were, so I left it.

I checked back every 15 minutes or so and gave the bag a quick dunk each time. The temperature was dropping, but barely. After 60 minutes, the temperature had dropped just 3° to 149°F. Just on that, the cooler mash tun is a success.

I opened the valve to empty the tun, then rested the grain bag in a colander on top to drain. I gave it a short sparge with warm water to get me up to 3 gallons. The mash gravity came out 1.024, for an efficiency of 55.7%. Not bad for the first go round.

Saving a Tun

Even after just one batch, I'm happy I made a mash tun. It was inexpensive to make (roughly $25 in parts - the cooler was effectively free) and it will save me some money over using all extract. Partial mash isn't much more effort than steeping grains and it gives me more control over my recipes. It also means I have more variables to work with (mash temperature, grain temperature, figuring out available diastatic power to convert other grains, etc.) which is also more things to screw up. As long as the mistakes aren't furry or acrid, I'll be fine.