Didn't you love those pilot lights?
Seriously, though, you don't usually want to heat up sourdough starter too much, as it can quickly get too acid, at least the loose type, or
Barm, and this can separate, and get acidic, as well as alcoholic, when left in the fridge too long, though it usually can be saved. This is why I switched over to a
firm starter, which doesn't get quite as sour, but is easier to control. Either type starts easier with some organic rye flour, then they can be switched to white (or WW, if you wish). Usually you will read about "only" using bread flour, which is better, because the acid in the starter degrades the gluten, but you can still use regular UB flour, since most of the gluten is added later - this is only a small percentage of the gluten.
The firm starter I make started with an equal weight of rye flour and filtered or bottled water (only 50 g each in a 2 c container, with lid), and mixed, and it sat, covered, for 2 days. Not much happens. Then the "refreshing" starts: 10 g starter, 25 g water, and 45 g bread flour, or 50 g, if using AP flour. Each time you roll this in your hands, until it is totally homogeneous, then put the ball into the container, put the lid on loosely, and let sit 2 days. This time, it should expand some, though it's still early. Refresh again (I always use the excess in some bread sticks, or something like that), and it might double or triple in just a day; if not, let it sit 2 days again, and next time it should take a day or less, and the last time should only take 4-5 hours.
The recipes calling for this firm starter only require 2-3 tb of it. Even after it has sat my the fridge for 6 months, it only takes 2-3 of those "refreshments", to bring it back to normal.