We’ve had a few issues in our plumbing. In the upstairs bathroom it’s almost impossible to run cold or even warm water out of the sink - hot water also runs into the toilet, the shower in the same bathroom doesn’t have a noticeable issue.

In the other faucets in the house you’d typically get a bit of hot water 5 seconds into opening, which goes away.

I discovered today that in the basement, the cold water line that I believe is feeding the upstairs feels hot, but if you follow it back to the source before it splits for the different appliances it goes cold again. From what I can tell, this hot water is feeding back to create the problem where you get a short burst of hot water in the first floor faucets, and must also be related to the bigger problem upstairs, however I have no idea how the hot water is mixing into the cold water line, it seems that it must be happening after the pipe leaves the basement.

Does anyone know what can cause that? The plumber that has come by seemed perplexed by the issue, and changed the shower cartridge hoping it would address things (it didn’t).