Washing yeast makes sense to most homebrewers. Yeast is one of the most important ingredients in your beer, and sometimes one of the most expensive. If you must purchase a new packet or vial of yeast for each beer you make it gets expensive. One way to reduce costs is to reuse your yeast. If you don't have a lot of trub in your fermenter, you can pitch another wort on top of the yeast cake immediately after transferring to secondary or to a keg. For some this is not a problem. For others there is just too much "gunk" in there and they don't know exactly how it will impact the flavor of their next batch. For them, washing yeast makes sense.
There will always be some trub in the bottom of every fermentation. If you use a conical fermenter, trub removal is easy. Open the bottom valve and drain off the trub before the yeast begins to flocculate. The bottom portion of the flocculated yeast should be discarded also since it contains a high percentage of yeast which flocculates fast and attenuates little. The middle layer of yeast is what you want to harvest. It should be creamy in color and texture with no trub. It should have no off odors and have no off tastes (yes, some people do taste yeast). Discard the top layer of yeast because the yeast in this area flocculate the least. You want to harvest the "average" yeast in the middle. Your "average" yeast will give consistent reliable results that can be predicted. If you harvest the top or bottom extremes and propagate these you have changed the properties if your yeast. It will be difficult to predict what it may do.
The principles of harvesting yeast from a conical also apply to harvesting from the bottom of your primary. We still want the "average" yeast. Since we will harvest from the primary fermenter instead of the secondary, we are favoring the most flocculant yeast. You won't be able to be as precise as with the conical, but you should be able to get the yeast you want. As mentioned before, there will be a lot of trub in the bottom of the primary. This will consist of hot and cold break material that made it into the fermenter. We use the techniques of washing yeast to harvest the cleanest yeast we can. The way homebrewers wash yeast should not to be confused with what the commercial breweries do when they "wash" their yeast. They lower the pH to just above 2.5 which kills off most of the unwanted bacteria. But, since wild yeast is still yeast, you can't remove them by washing. For the average homebrewer, it is just simpler to buy another packet of yeast than to take the chance of damaging your yeast by altering the pH using acids.
For the rest of us, washing involves adding sterilized water into a slurry of yeast and trub, chilling the slurry, separating the yeast from the trub, and repeating if necessary. This is one of those times when you must be very careful with your sanitization.
The items you will need for washing yeast are:
The steps involved in washing yeast are as follows:
It will take you a few times of washing yeast to get the process down. But eventually you will figure out what works best for you. Another idea I've heard is to use a portion of the original packet or vial and pitch an extra starter. Store it in a sterilized jar, covered, in your fridge for a couple of months. I'd step it up prior to using it with another starter. The advantage of this is that you can continue making an extra starter each time you brew. As long as you can manage to keep the yeast viable until you need it again, you'll be perpetuating the original generation of yeast indefinitely. Of course, your mileage may vary