Maths was always my worst subject....

I have a fixed lauter-grill/filter at the bottom of my mash tun so i can't stir anything that falls through it = the gas burner gives me a burnt bottom :(

Thoughts about adding hotter water to the mash to raise the temp rather than using the gas, or a combo of both? If you think doable, please help me work out the numbers for say a 500 litre mash :)

Equation something like:

250L @ 57C to mash in (for 10 mins).

Add 100L of hotter water @ X degrees C = 350L @ 66 degrees C (for one hour).

Add 150L of hotter water @ X degrees C = 500L @ 78 degrees C (for 15 mins).

I worry about ever seeing water in a mash go above 78C (sour flavours appear) so am reluctant to add anything even close to boiling water, although added slowly may dilute the heat enough....???

Thoughts appreciated!


  • 1
    Look up decoction first. The problem with adding hot water is that your mash gets thinner and thinner, and you need more and more water.
    – chthon
    Commented Nov 14, 2017 at 17:49
  • Totally off-topic, but if you got a burnt bottom, then you have to sit on the blisters :-p
    – chthon
    Commented Nov 14, 2017 at 18:34
  • Nice one! I will look it up. Many thanks for the lead.
    – user7268
    Commented Nov 14, 2017 at 20:19

2 Answers 2


John Palmer covers this in his How to Brew book. An old version is available online, and the equations you want can be found at:



You can add all the hot water you want as long as you don't go over the full volume of the water for your batch of beer. Many people do full volume mashes using the full volume of water they will need for the boil. Look up water ratios doing Brew in a bag or full volume mash in google. The other option is do do a decoction mash, pull out a portion of the mash and heating it up and re-introducing the hotter portion to raise the temperature. But, seriously, adding more hot water to your mash is just fine. Decoction mashes go briefly over 78c with no detrimental effects in this case sour flavors. It takes a while for that to happen and you'll be boiling before the sourness sets in.

  • Cheers Steve. Much appreciated.
    – user7268
    Commented Nov 14, 2017 at 20:19
  • It will not be sourness, but extra bitterness in fact. And for that to happen your temperature must be over 78° C, AND your pH should be higher than 6. If your mash pH was correct, this normally happens only when oversparging.
    – chthon
    Commented Nov 15, 2017 at 6:34
  • Decoction mashing you often boil the thick part of the mash extensively. As long as the mash pH isn't very high you wont extract tannins. It's perfectly fine to remove a part of the mash, bring it to a boil and pour it back to increase temp.
    – Mumble
    Commented Nov 15, 2017 at 15:08

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