We live in a 65 year old house in New Jersey. During the winter months, the hot water does not seem to last as it should. This seems to correlate with the heater running. If you can hear the heater running, then you're almost guaranteed 2 minutes of hot water before it turns cold. Eventually, if you're in the shower for 10+ minutes, the hot water will return again, but minutes 2-9 are cool if not freezing. We have an oil furnace, installed 2005. I don't see a hot water tank anywhere near the furnace, so is it possible we don't have one? We've experienced this problem ever since the new furnace was installed in 2005. During summers, we have gloriously hot showers, but as soon as we start running the heater, the cold showers return. This also happens in the kitchen, so the problem is not isolated to the upstairs shower. I doubt it has to do with pipe insulation because as I said, the first few minutes of winter showers are warm, then fade to cold, and eventually turn warm again.

Has anyone else had this problem? What could be causing it?