Does fermentation actually produce heat?
Yes, and it is not close. There is a long-running forum thread titled "Fermentation produces heat??? Or not!", which tells you how settled the question is among homebrewers — but it has been a number in the professional literature for decades. Yeast turning sugar into alcohol and CO₂ is exothermic, and the brewing textbooks put it at roughly 570 kJ for every kilogram of extract fermented.
Run that on an ordinary batch — 21 L going from 1.050 to 1.010, which is 2.2 kg of sugar — and you get 1,247 kJ. If none of that escaped, it would raise the beer 13.5 °C (24 °F). It does escape, of course. The point of the number is the size of the thing your fermenter has to shed.
How many watts is that?
This is the version that makes it click. Narziß also publishes the rate at which yeast works at its peak — 4.5 kg of extract per hectolitre per day for ales, 2.5 for lagers — so the heat output during the vigorous phase comes out at 6.2 W for a 21 L ale and 3.5 W for a lager — sustained for about 2.3 days, which is how long this batch's extract lasts at that rate.
Your fermenter contains a six-watt heater that runs for three days. That is a night light. It is also exactly the size of thing that lifts a bucket a few degrees above the room it is standing in — which is what brewers report and never quantify. The commonly quoted "4 to 6 °F above ambient" is what six watts does against the heat loss of a plastic bucket, and it explains the two things that make it worse: a bigger batch makes proportionally more heat but has less surface per liter to lose it through, and a fermenter wrapped in a blanket or sitting in a closed cupboard loses it more slowly still.
Turning watts into "your beer will run X degrees over the room" needs the heat-transfer coefficient of your particular fermenter in your particular spot, and nobody publishes a U-value for a plastic bucket in a garage. Anyone who gives you a single number for that has guessed it. What this page gives instead are two things that are not guesses: the heat that has to go somewhere, and the coldest evaporation could possibly get you. Put a thermometer on the fermenter, not on the wall — that is the only way to know the difference for your setup, and it costs a few dollars.
Which heat figure is right?
The two standard references disagree by more than a factor of two, which is worth knowing before you trust any single number:
| Source | Heat per kg extract | This batch | Adiabatic rise |
|---|---|---|---|
| Narziß (2005), Kunze | 570–587 kJ | 1,247 kJ | 13.5 °C · 24 °F |
| Briggs et al. (2004) | 1217 kJ | 2,663 kJ | 28.9 °C · 52 °F |
Computed by this page's engine for 21 L at 1.050 → 1.010. Both textbooks are paywalled; the figures are as quoted by Ken and Dot's "Cooling requirements in the Brewery", which cites them directly — we have not read the originals and do not claim to.
We default to Narziß because it is the one that checks out independently. The enthalpy of glucose fermenting to ethanol is about 100 kJ per mole, which works out at 555 kJ/kg — within a few percent of his figure. And his own published peak cooling demand (2.5 kg of extract per hectolitre per day needing 1,465 kJ) implies 586 kJ/kg, which is Kunze's number almost exactly. Briggs' figure is closer to the total free energy of the reaction than to its enthalpy, which may be the difference. Either way the conclusion holds — the heat is large, and if you are sizing a chiller rather than satisfying curiosity, use the cautious one.
Will a swamp cooler work where I live?
A fermenter in a tub of water with a wet t-shirt over it and a fan on it is a swamp cooler, and it is the standard first answer to temperature control. It works by evaporation — which means it cannot get below the wet-bulb temperature, and the wet bulb depends on humidity, not just how warm the room is. Nobody recommending the method mentions this, and it is the difference between a technique that transforms your beer and one that does nothing at all.
| Room | Relative humidity | ||||||
|---|---|---|---|---|---|---|---|
| 20% | 30% | 40% | 50% | 60% | 70% | 80% | |
| 20 °C · 68 °F | 9.1 | 10.8 | 12.3 | 13.7 | 15.0 | 16.3 | 17.5 |
| 24 °C · 75 °F | 11.8 | 13.8 | 15.6 | 17.1 | 18.6 | 20.0 | 21.4 |
| 26 °C · 79 °F | 13.2 | 15.3 | 17.2 | 18.9 | 20.4 | 21.9 | 23.3 |
| 28 °C · 82 °F | 14.5 | 16.9 | 18.8 | 20.6 | 22.2 | 23.7 | 25.2 |
| 30 °C · 86 °F | 15.9 | 18.4 | 20.4 | 22.3 | 24.0 | 25.6 | 27.1 |
| 32 °C · 90 °F | 17.3 | 19.9 | 22.1 | 24.0 | 25.8 | 27.5 | 29.0 |
| 35 °C · 95 °F | 19.3 | 22.1 | 24.5 | 26.6 | 28.5 | 30.3 | 31.9 |
The coldest a swamp cooler can reach, °C, from Stull (2011). Shaded where it reaches 20 °C (68 °F) or below — a workable ale fermentation temperature. This is a floor, not a promise: it assumes a soaked wick and moving air, and a real setup lands somewhere above it.
At 30 °C (86 °F), a swamp cooler reaches 18.4 °C in 30 % humidity and only 25.6 °C at 70 %. The same summer afternoon in Denver and in Orlando: one lands on a clean ale temperature, the other misses it by 5.6 °C and leaves you with fusel alcohols. In a tropical 35 °C at 80 % the whole method is worth 3.1 °C. If you live somewhere humid, skip the wet t-shirt and budget for a fridge — the physics is not on your side and no amount of technique fixes it.
How much ice does it take?
If you are dropping frozen bottles into the tub instead of relying on evaporation, the arithmetic is simpler: ice absorbs 334 kJ per kilogram as it melts. Absorbing this batch's entire 1,247 kJ takes 3.7 kg (8 lb) of ice across the whole fermentation — about a kilogram a day for the first few days, when most of the heat comes out.
That is the fermentation heat alone. Whatever leaks in from a room warmer than your target is on top, and in a hot garage that is usually the larger number — which is why swapping two frozen two-liter bottles a day is the figure people converge on rather than the one this calculation gives. Treat 3.7 kg as the floor: if you are not putting at least that much ice in, you are not removing the heat the yeast is making, let alone the room's.
What should I be aiming for?
Most ale strains are published for 18–22 °C (64–72 °F) and most lagers for 9–13 °C (48–55 °F), but the number that matters is the beer's temperature in the first 72 hours, not the room's and not the average over two weeks. That is when the yeast is working hardest, when it is making the most heat, and when fusel alcohols and esters are decided — a beer that spends its first day at 26 °C and its next fortnight at 19 °C tastes like the first day.
Which is the practical conclusion of everything above: the peak heat and the sensitive window are the same 72 hours. Cool deliberately at the start and you can be relaxed afterwards. The pitch-rate calculator covers the other half of a clean fermentation, and the attenuation calculator tells you whether it finished where it should have.