I am tired of buying yeast all the time. is it OK to just repitch the trub from a previous batch?
Re-pitching of yeast is a normal method used by brewers around the globe. Although the practice is simple, there are a few rules that may make the method less than ideal for the typical homebrewer. The first rule is that the yeast should be harvested shortly after fermentation is complete and used within a short time period.
The most common method of yeast harvesting is by bottom cropping, because lagers are the dominant beer type in the world. Most commercial lager brewers these days bottom crop yeast from the bottom cone of cylindroconical fermenters and store it for short durations in a yeast brink (chilled and often agitated storage tank). Ale strains are sometimes bottom cropped and other times skimmed from the top of open fermenters, then stored in a similar fashion to lager yeasts prior to re-use.
There are a variety of ways to harvest yeast when homebrewing and the yeast can successfully be stored and re-used if you are careful. I suggest storing yeast in a flask and using cotton batting to close the top. Yeast stored in this type of container can be placed in a refrigerator without any problems for up to about 10 days before re-use. It is really preferable to keep this duration as short as possible since yeast viability and vitality decrease with time, especially as storage temperature increases. Ideally the storage temperature should be around 34 °F (1 °C).
Harvesting yeast for re-use at home has one major drawback and that is the fact that most homebrewers do not brew frequently enough to re-use for very long. Some brewers share yeast and are able to keep a culture going from one batch to another with short storage durations in between. This can work very well if you have a group of friends who are good, clean brewers.
Does the amount of time it takes to get the wort to a rolling boil have a negative impact on the brew itself, or not?
In a very general sense the time required to bring wort to a boil can cause problems when the time is too long. Holding hot wort for extended time periods leads to heat-related chemical changes, generally termed “thermal stress”. But in a more practical sense this is not normally associated with waiting for the kettle to boil for one very simple reason; evaporation rate.
Brew kettles are designed to boil and evaporate water from wort during boiling. Traditional, some would argue outdated, kettles are usually designed to evaporate about 8–10% per hour. More modern designs focus on reduced energy consumption and thermal stress during boiling, and the evaporative rates in these designs is usually around 4%. So how does this relate to kettle heating time?
In order to achieve these evaporative rates a certain amount of energy must be supplied to the kettle and this amount of energy is plenty to heat the contents of the kettle to the boiling point during wort collection.
In practice, brewers do want to get the wort boiling as soon as possible to save time and also not feel like too much time is spent looking at a pot of wort waiting for it to boil. It does help to have a burner that can be cranked up for the heating stage and then dialed back once the wort begins to boil. If the burner is too small to get the wort boiling within 30-45 minutes of kettle full I would look for a larger unit that can provide more heat.
One practical method used by most brewers during the brew day is to begin applying heat to the kettle during wort collection. If this is timed right the boil begins just about the time the kettle is full.