This weekend our guest shower - which hasn't been used for 6 months or so - had no hot water. I took it apart and the cartridge is fine. I turned the water on without the cartridge and there is cold water out of the right - nothing out of the left. We have 2 hot water heaters piggy backed, both of which I flushed. No rusty water, no sediment at all. I tried to stick some weed-eater line down through the hot water pipe, but couldn't get past the first elbow. Our house is about 4 years old. I'd hate to start tearing out tile, but not sure what to do next?