About a month ago, the hot water in my bathtub began turning cold about 2 minutes into my shower. It stays cold for about a minute, when it again gets hot. This week I noticed the same problem in the kitchen faucet. It happens every time I turn on the hot tap. The house is only a year and a half old and I have a Rennai tankless water heater. What might be causing this? The man who installed the plumbing thinks it might be build-up of chemicals/calcium in the pipes, but several others have said this is not likely in a new house, and that the water here isn't particularly hard. Thank you for any help!