I have just finished building my mash tun using an insulated plastic barrel. Everything works great but when I pour hot water into the barrel and put a lid on, pressure builds up inside and eventually "blows up" the lid. My question how do you deal with this? Do you let the steam slip away (make a small hole or something like that) or do you just fix the lid so it won't budge?

2 Answers 2


Make a hole. There is no need for the mash tun to be air tight.


Are you applying heat to your mash tun? If so, don't. Your mash water should only be in the 150 degree (F) range so I doubt that's producing enough outward pressure to push your lid up unless it's made from foil and even then it would just condensate on the underside.

Here's what you do... heat the water in a separate container (we refer to this as a Hot Liquor Tank) and transfer the hot water to your mash tun. NEVER heat your mash tun.

  • Thank you for the reply. It indeed does happen if I put hot water inside (around 160 deg F) and close the lid immediately. I am testing the mash tun now, not actually mashing yet, it might not be a problem when I am mashing as I might not close the lid that fast. Anyway, I am not heating the mash tun but thanks for the info!
    – Kraken
    May 5, 2015 at 20:31
  • 2
    Never heat the mash tun? I do all the time, at every brew. Have been doing so for years. But don't forget to stir!
    – Pepi
    May 6, 2015 at 10:17
  • When you do step infusion mashes, you directly heat the mash tun. When you do decoction mashes, you add hot (boiling) liquid. Only when you do a single step infusion, you don't need to add heat.
    – Robert
    May 6, 2015 at 21:31

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