I did a brew at the weekend with my usual kit (electric boiler). Everything was identical, apart from the location (a shop) and the water (I usually use filtered water from a Brita tap, but this time I used plain tap). I usually get about 19 litres, but this time, I got closer to 15, so it seems like the boil off rate was much higher. What factors could influence such a dramatic change in boil off rate?

2 Answers 2


Boil off is effected by three main things.

  1. Boiling temperatures: Effected by elevation For example: I sometimes brew at sea level (212°) and sometimes in the high desert (206°) 3000ft the two areas have a few degree difference in boil temp. Effected by BTU efficiency, voltage. As mentioned, voltage especially 220v has a wide range here in the USA. 218-240v. Elevation and humidity will effect the BTU of gas burners.
  2. Humidity In the same way humidity can make evaporative coolers useless, it also effects boil off rate.
  3. Ventilation If an open kettle has a cross breeze you can expect more boil off.

Best practices Set your boil based on its action, not a temp setting or kettle burner setting. Its best to have a low rolling boil just a touch above a simmer.

If under unknown conditions watch it and record the boil off for the first 1/4 of time (15min in 60min boil). You can then adjust temperature to reach you desired post boil volume.

If you have more boil off than expected, add water! This can be done at any point before pitching as long as the water is sanitary (bottled or boiled).


Two main reasons I can think of (only considering boil off, not speculate "you did something else wrong"):

  1. Higher voltage from the grid than at home.

Most EU countries at least you should expect 230V +/- 6% from your mains. This means the actual voltage can be as high as 244, or as low as 216. US and other first world places likely have similar tolarances.

Power goes (in the simplest case) as P = U^2 / R for a resistive load. Notice theres a square in there with the voltage, so a small change in voltage makes large impact in power.

Example: A simple heating element rated at 2kW @ 230V gives an R of 26.45Ohms. This means that the actual power is anywhere between 1760W and 2250W without anything being outside of their specifications.

  1. Lower air preassure (elevation and weather).

Probably a small effect unless there's a big difference in elevation between locations.

  • Ah, I did wonder if it was a voltage thing, but my understanding of electricity is weak, to say the least!
    – Pezholio
    Commented Oct 13, 2016 at 10:22

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