My hot water runs for about 2 minutes, then gets lukewarm and then eventually cool after a few minutes. This happens in all faucets in the house. I have a 5 year old gas water heater. Everything was fine until a plumber tapped hot and cold lines to run pipes to a bathroom that is being finished in the basement. The new lines are only stubs. I don't see how he could have cross-connected the hot and cold. We turned the thermostat up very high, but it doesn't seem to matter. If the water is off for a while (30 minutes) then it is hot, but again lasts for only 2 minutes. Any ideas?