Hello, I have a question as I'm stumped on what it could be, I have a rental that is an up and down unit, I had it empty and water shut off for about 4 summer months due to the main valve dripping. I needed to replace the main valve which I did, they did have a hard time at the street to shut it off. Once we turned the water back on it seems that the upstairs bathroom tub and sink gets hot for a quick second then get cold and does not get warm again, we drained the hot water tank but still nothing, we checked all the valves in the house, all are open, first floor really hot and good pressure, basement same thing, it's just the second floor. The kitchen sink upstairs does get hot as well but does not come out as fast as the first floor. Has anyone run into this problem and what it could be, I'm thinking the house is haunted.
Thanks,
Connie

Could be multiple reasons for this problem, like a bad mixing cartridge, bad check valve on a hot water return line

Have you tried turning off the cold water riser isolation valve to the upstairs and then running hot water in the bathroom sink… it is usually better to hire a pro to troubleshoot due to the complexity of some plumbing systems