What Is a Thumper Keg? How It Works, What to Put In It, and What Size You Need

A thumper keg is a sealed second chamber that sits between the pot and the condenser of a still, where hot vapor bubbles through liquid and gets distilled a second time before it is cooled. It is also called a thump keg or a doubler. It gets its name from the deep thumping sound it makes while it works.

This guide explains what a thumper keg does, how it works step by step, what to put in it, and how big it should be. It also covers how to run one safely and what to look for if you want a thumper keg for sale. Think of it as your first lesson in controlling copper, heat and vapor like a real alchemist.

What Is a Thumper Keg?

A thumper keg is a closed container, usually copper, that connects to the pot with one tube and to the condenser with another. The tube coming from the pot runs down inside the thumper and ends near the bottom, under a layer of liquid. The second tube leaves from the top and carries vapor on to the worm or condenser.

On a basic pot still, vapor goes straight from the pot to the condenser. That gives you one distillation per run. Adding a thumper puts a second boiling step in the middle, so you get much of the effect of a second run without starting over.

If you are new to the parts of a still, our guide to how a moonshine still works shows where the pot, thumper and worm each fit.

What Does a Thumper Keg Do? (Thumper Keg Purpose)

The main purpose of a thumper keg is to redistill the vapor from the pot. That gives you three practical benefits.

• Higher proof: the vapor that leaves the thumper is richer in alcohol than the vapor that went in, because it has been boiled a second time.
• Saves time and fuel: you get much of the strength of a double distillation in a single run.
• Adds flavor: whatever liquid you put in the thumper shapes the taste of the final product, which is why some distillers add fruit or grain.
• Catches splashes: if mash foams or spits up the tube from the pot, the thumper catches it before it can reach the condenser.

How Does a Thumper Keg Work?

A thumper works because alcohol and water boil at different temperatures. Ethanol boils at about 173F (78C). Water boils at 212F (100C). When a liquid mix is heated, the vapor coming off it holds more alcohol than the liquid it came from. A thumper simply repeats that step.

Here is what happens during a run, in order:

• The pot heats up and vapor rises through the tube into the thumper.
• The tube ends under the liquid in the thumper, so the hot vapor has to bubble through it.
• At first the liquid is cool, so the vapor condenses into it and warms it up. Little or nothing comes out of the condenser during this stage.
• As more hot vapor arrives, the thumper liquid reaches its own boiling point.
• Now the thumper boils on its own. The vapor leaving the top is stronger than the vapor that came in.
• That vapor moves on to the worm or condenser, where cold water turns it back into liquid.
• The thumping sound comes from vapor bubbles bursting and collapsing in the liquid. Steady thumping is normal and means the keg is working.

Thumper Keg Diagram: How the Parts Connect

If you picture a thumper keg diagram, the setup reads left to right in a simple line. Vapor always flows one way: from the pot, through the thumper, to the condenser.

The key detail is the tube inside the thumper. The inlet tube must reach below the liquid line. The outlet must sit at the top, well above the liquid, so only vapor leaves.

• Pot (boiler): holds the mash or wash and sits on the heat source.
• Cap or onion head: collects vapor at the top of the pot.
• Line arm: carries vapor from the pot to the thumper.
• Thumper inlet tube: runs down inside the keg and ends near the bottom, under the liquid.
• Thumper liquid: fills roughly a quarter to a third of the keg.
• Thumper outlet: at the top of the keg, sends the redistilled vapor onward.
• Worm or condenser: a coiled copper tube in cold water that turns vapor back to liquid.
• Collection jar: catches the finished liquid at the end of the worm.

What to Put in a Thumper Keg

What you put in the thumper keg changes the strength and flavor of the output. There is no single right answer, so distillers pick based on their goal. Fill it about a quarter to a third full. That is enough to cover the end of the inlet tube while leaving plenty of headspace, because the liquid level rises as vapor condenses into it.

Common choices include:

One note on the law: several of these options contain alcohol. Making alcohol for drinking at home is not legal under US federal law without a TTB permit. The same thumper principles apply to legal uses such as distilled water, essential oils, or fuel alcohol under a free TTB fuel alcohol permit. See are stills legal to own? for more.

• Water: the simplest choice. It works, but it dilutes the first output until the thumper heats up and catches up with the pot.
• Fresh mash or wash: adds more flavor and a bit more alcohol to the run. Learn more in what is mash?.
• Low wines or backings: leftover weak distillate, such as tails, from earlier runs. This gives the biggest boost to proof because the liquid already holds alcohol.
• Fruit, grain or other flavorings: added to the liquid to give the final product a particular taste or aroma.

How Big Should a Thumper Keg Be?

A thumper keg should be noticeably smaller than the pot. A common rule of thumb is a thumper somewhere around one quarter to one half the size of the pot. So a 10 gallon pot might pair with a thumper in the 2.5 to 5 gallon range.

If the thumper is too big, it takes a long time and a lot of vapor to heat its liquid to boiling. Your run gets slow, and the pot may run low before the thumper does much work. If the thumper is too small, it can fill up too fast as vapor condenses, and liquid may get pushed toward the outlet.

The easy fix is to buy a still where the thumper is part of the design. Our complete moonshine still kits come with a thumper and worm included, so you do not have to guess. If you are still picking a pot size, start with what size still do I need?.

How to Run a Thumper Keg Safely

A thumper is a closed vessel full of hot vapor, so a few simple habits matter. Most problems come from too much heat, too much liquid, or a blocked path.

In my own shop at Fort Wilkins, I tell every new alchemist the same thing: slow heat makes a calm thumper. For more on heat control and cuts, read how to run a copper still.

Even heat also helps. Our patented Flame Flow stills (U.S. Patent 10,561,960) are built for faster heat-up, even temperature, less wasted propane and less scorching.

• Never overfill: keep the liquid at a quarter to a third full so there is room for condensed vapor.
• Keep the path open: never put a valve or plug that could close off the line between the pot, thumper and condenser.
• Heat slowly: bring the pot up gently. Rushing the heat can make the thumper surge.
• Keep the condenser cold: if cooling water runs warm, vapor can escape the end of the worm.
• Watch it the whole time: never leave a running still alone.
• Ventilate: run propane setups outdoors or in a well-ventilated space, away from anything that can burn.
• Clean after each run: rinse and clean the thumper like the rest of the still. See how to clean a copper still.

Thumper Keg for Sale: What to Look For

If you are shopping for a thumper keg for sale, look first at the copper. Copper reacts with sulfur compounds in vapor and helps clean up the output, which is why it is the classic material. Thick copper also holds heat evenly and lasts. Cheap thin imported stills often use light copper that dents and wears fast.

Every American Copper Works still is hand built at Fort Wilkins in Pittsfield, New Hampshire, from thick 20 oz, 22 gauge American copper and assembled with lead-free silver solder. Each one is cut, rolled, formed, soldered, tested, adjusted and inspected before it leaves the building, and our copper stills carry a lifetime warranty.

You have a few ways to get a thumper:

• Complete copper stills: pot, thumper and worm included, ready to set up.
• Individual thumpers: if you already own a pot and condenser, you can add a thumper from our parts and accessories line.
• EZ KIT DIY kits: for hands-on builders who want to solder their own still.
• Questions? Call the owner directly at 1-502-337-3944. Orders check out through our sister store, MoonshineStills.com.