Spunding valves are an interesting little tool for brewers that let you control the pressure in your fermenting vessel (almost always a keg for homebrewers) They are an adjustable blow off valve that will release at or above a certain pressure point. The benefits of using one are primarily that you can end up with carbonated beer straight out of the fermenter, and that they can allow increased fermentation temps (specifically in lagers) with fewer off-flavors. I don’t know if I have seen any proof of the second point, but I have certainly read it often.
The auxilliary thing I like about having a pressurized keg that is still fermenting is that it is VERY easy to take a sanitary sample using a picnic faucet, and there is essentially zero chance of O2 entering the keg, as the gas pressure is far more reliable than an airlock (that may require topping off.)
One down side to spunding valves is that some yeasts put out off-flavors in their gas, especially early into fermentation. If you’ve ever smelled cabbage or eggs coming out of a vigorous ferment you can see how disconcerting that could be coming off of a pint glass. Most commercial breweries vent off any natural carbonation so that they can inject the beer with pure co2 at bottling, and some people say that this gives a cleaner taste.
The primary way to use a spunding valve is to put it on when your beer is almost at final gravity. This article from BYO says to put the valve on when the beer is 2-5 points away from final. If you already have finished beer then I guess you could add 2-5 points of “bottling sugar” in order to get it back up to pressure.
Since almost all spunding valves have a knob that is disconnected from any useful scale it can be hard to know what your valve is set at. My approach is to connect my valve to my co2 tank in order to test it. Start with your spunding valve turned up above your ideal pressure and your regulator set to your ideal pressure. Slowly lower the spunding valve until it releases pressure. Once it is on the keg you can make small changes to the knob, but this approach will get you in the ballpark much quicker.
Building this is really simple. You need teflon tape, a bit of tubing, and the pieces below. Put a few wraps of teflon around the threads of each piece and then thread them all together. This needs to be very air-tight so keep the tape going in the correct direction (so that it doesn’t unwrap when you tighten it) and use wrenches to get everything as tight as possible. CAUTION – don’t use the gauge as a handle when tightening. Use a wrench on the brass part or else you will break your gauge.
Most other valves I have seen directly attach to the gas line. I chose to put a piece of tubing in the middle. The reason for this is two fold. It’s relatively easy for foam to get into the top of the keg, and then get forced up to your valve / gauge. If you get much beer in your gauge it will stop being precise very quickly. Putting a long piece of tubing in the middle will make that bubble’s journey impossible. The other reason is that I like to have my spunding valve at eye-height as opposed to down on the floor with the keg.
The shopping list is about as simple as the assembly will be. 4 parts, assuming you have the tubing to connect it to your keg already. Buy these pieces below and get to carbonating! Hit this button to load up your cart, or click through individually on the images below.
1/4″ mpt – I only picked stainless for this because it was strangely cheaper than brass. Feel free to swap it out for a brass one.