How a spunding valve work for pressure fermentation

The most important equipment when fermenting under pressure is the spunding valve. This is the item that will control the pressure in the vessel during fermentation, here is how a spunding valve works.

The core of a spunding valve is the overpressure relief valve (PRV) that will release the pressure from the CO2 production. Most valves are also equipped with a pressure gauge to dial in the correct setpoint and a connector for the brewing vessel.

Spunding comes from German and translates to English as “bunging.” To get a carbonated beer and still follow the German Beer Purity Law known as the Reinheitsgebot, the brewers had to bung the fermentor before fermentation was complete. The buildup of CO2 in the last part of fermentation would carbonate the beer.

How does a spunding valve work?

The most common overpressure relief valves are either diaphragm based or spring-loaded. Both use a spring to set the opening pressure, the force needed to open the valve. The difference is in the sealing mechanism. The diaphragm based uses a plate seal that normally is based on PTFE, Polytetrafluoroethylene, the same material as Teflon or Gore-Tex. The spring-loaded uses a small plunger to seal the gas vent.

The relief valve opens automatically when the inlet pressure is above the set force that’s needed to move the spring. Once the pressure falls back down to the desired setpoint, the valve closes.

If you want to read more about pressure fermentation and the benefits “benefits of pressure fermentation“.

KegLand Diaphragm Spunding Valve Pressure Release Valve
Kegland BlowTie
Exampel of commerical spundings valves.

How do you set a Spunding valve?


If you buy a commercial spunding valve it will come with a pressure gauge. The recommendation is to use CO2 to pressurize the vessel first to just over your desired setpoint of venting pressure. Close the spunding valve before connecting it to the vessel by rotating the PRV clockwise nearly all the way in. Attach the spunding valve to the vessel and rotate the knob counter-clockwise slowly until it begins venting.

What type of spunding valve is best?

Blowtie is the most commonly used spunding valve in the homebrew community. This is because of the reliative cheap price. Lookin’ into high-end and more control at around 3 times the cost of the Blowtie is the then SPUNDit 2.0. SPUNDit gives more control and comes with a blowoff to help to indicate the status of the fermentation.

I have used both and can say that I really like the spundit for the precis control but the Blowtie does the job.

Blow off for spunding valves

Its recommended to install a blowoff on your Spunding valve to better see the status of the fermentation. It’s easily done with, some DIY. Just adding a tube into a small bottle with your favourite sanitation solution. I use StarSan (Amazon). The bottle can be a PET bottle with a small hole in the cap or some people uses containers for PH-electrodes that already has a hole in the cap, like the one below. The functionality for detecting the fermentation will be, when the pressure is at the venting setpoint, just like a normal airlock indicated by gas bubbles.

If you get the SPUNDit 2.0 it will come with one already in the kit.

Blowoff Spunding valves

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