[Angel-la-la]

I live in a house that has 2 bathrooms; one in the basement and one on the 2nd floor. Everything seems to be fine with the basement but I've been having problems with the water temperature in the bathroom on the 2nd floor and it seems to be only getting worse.

Since about last fall, it's been really hard to control the water temperature in the shower. There seems to be no warm water or "midway" point with my Delta shower faucet (single handle that can turn 180 degrees from 12 to 6 o'clock). Its either very hot water or very cold even when the handle is set at 3 o'clock. Because of this I turned the hot water temperature down on my boiler (located in the basement). It didn't fix the problem but at least I didn't have to deal with scalding hot water water one second and then ice cold the next.

After a couple of months, I realized that in order to get hot water in the shower, I needed to turn on the hot water in the sink faucet first. And when I flushed the toilet, there was absolutely no hot water (even with the sink technique) until water level was refilled in the toilet.

Just last week, I realized I couldn't get any hot water in the shower at all. There was hot water in the sink but absolutely none in the shower. The water pressure also (especially for hot water) seems to have gone down. Especially noticeable when turning on the sink for hot water.

What the hell is going on with my plumbing? The house is about 50 years old and the shower faucet was switched a few months ago. Boiler is about 10 years old. Any help would be appreciated. Thank you.