I have a 5 year old house that we just moved into, the hot water in one of the tubs turns cold within 10-20 seconds, If you cut the water off and right back on it will do the same thing. I was thinking the dip tube, but it only does it in this one place, the other tub and shower, as well as all the faucets work fine. This house does have a hot water recirculating pump.