The Physics of Boiling and Cooling in Brewing

The short answer: the boil does real work: it turns hop alpha acids into bitterness, drives off DMS, sterilizes and concentrates the wort. Expect to boil off about 1–1.5 gal (4–6 L) per hour on a homebrew system with a rolling boil. Water boils at 212°F (100°C) at sea level and about 2°F lower for every 1,000 ft of altitude, which slightly lowers hop utilization.

Altitude	Boiling point
Sea level	212°F (100°C)
2,500 ft (760 m)	about 207.5°F (97.5°C)
5,000 ft (1,520 m)	about 203°F (95°C)
7,500 ft (2,290 m)	about 198.5°F (92.5°C)

Heating: how long it takes

It takes 1 BTU to warm 1 lb of water by 1°F, and a gallon of water weighs 8.34 lb. Wort behaves almost the same. So heating 10 gal (38 L) of water from 60°F (16°C) to a boil takes about 12,700 BTU, or 3.7 kWh. A 5,500 W element does that in roughly 40–45 minutes once you allow for heat losses.

Element power	5 gal (19 L) from 60°F (16°C) to a boil	10 gal (38 L) from 60°F (16°C) to a boil
1,650 W (one 120V element)	about 68 min	about 135 min
3,300 W (two 120V elements)	about 34 min	about 68 min
5,500 W (240V)	about 20 min	about 41 min

These are the physics minimums, with no heat escaping. Real times run longer, more so in a cold room or with the lid off.

Boiling: where the energy goes

Once wort is boiling, its temperature stops rising. Every bit of extra heat goes into turning water into steam, and that takes a lot of energy: boiling off 1 gal takes about 8,100 BTU (2.4 kWh), more than six times what it took to heat that gallon from 60°F (16°C) to a boil.

That makes boil-off depend mostly on how much heat you put in, not on the kettle. Each 1,000 W that reaches a boiling kettle evaporates about 0.4 gal (1.6 L) an hour. A 5,500 W element flat out could boil off up to about 2.3 gal (8.8 L) an hour, less what the kettle loses to the room. That’s why electric brewers turn the element down once the boil starts: a steady rolling boil needs far less power than getting there, and less power means less boil-off. A steam condenser cuts boil-off by about 20–30% as well, so measure your volumes and adjust once.

During the boil, hop alpha acids isomerize into bitterness, DMS leaves with the steam, proteins clump into the hot break, and the wort is sterilized and concentrated. Keep the lid off so the DMS can escape. At altitude, the cooler boil isomerizes hops a little more slowly, so the same recipe comes out slightly less bitter.

Cooling: why the last degrees are the slowest

Chilling 5 gal (19 L) from boiling to 68°F (20°C) means removing about 6,000 BTU (1.8 kWh). How fast it goes depends on the temperature difference between the wort and the cooling water: heat moves quickly when the gap is big and slowly when it’s small.

• Immersion chiller. The whole kettle cools together, so the gap keeps shrinking. With 55°F (13°C) tap water, it takes about as long to get from 100°F (38°C) down to 68°F (20°C) as it did to get from boiling to 100°F.
• Counterflow chiller. Wort and water flow in opposite directions, so the coldest water meets the coolest wort at the outlet and the gap stays large along the whole coil. The wort can leave within a few degrees of the incoming water, in one pass. Slow the wort down for a colder outlet.
• The floor. No chiller can cool wort below the temperature of the water going through it. In summer, finish with ice water or let a glycol-cooled fermenter take the last few degrees.

Stainless conducts heat about 1/25 as well as copper, but in a chiller the thin tube wall isn’t the bottleneck, which is why a longer stainless counterflow chiller keeps up. More in copper vs stainless chillers.

Rules of thumb

To	Imperial	Metric
Warm water by one degree	8.34 BTU per gal per °F	4.19 kJ per L per °C
Heat with 1,000 W	1 gal rises about 7°F per minute	1 L rises about 14°C per minute
Boil off water	About 8,100 BTU (2.4 kWh) per gal	About 2,260 kJ (0.63 kWh) per L
Boil-off from 1,000 W of heat	About 0.4 gal per hour	About 1.6 L per hour
Boiling point at altitude	About 1°F lower per 500 ft	About 1°C lower per 300 m

Frequently asked questions

Why does my kettle take longer to heat than the math says?

Heat escapes through the walls and the open top, and more of it the hotter the wort gets. An element also puts out less power if your line voltage runs below its rating.

Does a harder boil make better beer?

No. A steady rolling boil does the work. Beyond that, extra power mostly evaporates more water.

Why does wort foam over at the start of the boil?

Proteins form foam until they clump into the hot break. Turn the heat down as it comes to a boil and stay close for the first few minutes; once the break forms, the foam drops.

Does boiling at altitude change my recipe?

Slightly. The cooler boil gives a little less bitterness from the same hops. Adjust bittering hops to taste after a batch or two.

Related guides

How to chill wort fast, IBU explained, electric brewing for beginners and steam condensers for indoor brewing.