Plumber's Crack

There's something a bit off with our shower. Lately it's been offering tepid water or scalding hot water, no in-between. The water shoots directly out of the shower head even with the diverter thingy down, which should mean that it comes out the faucet. It also has a tendency to randomly switch to cold water in the middle of a shower, and then back to hot.

Because I pick the scalding hot option every time, I come out the color of a cherry tomato and walk around in a sweat for the next three hours. This is where you'd think that having a contractor for a husband would come in handy, but he's really booked up right now and he won't return my calls.