Originally Posted by Clifftall
One reason could be an outside shower faucet in the on position.
No, that would not do it, that would result in not-hot water but not low hot pressure...
What would do it... Answer gummed up check valve.
There are basically 3 water heater bypass systems
On the single valve system the valve is a 3-way that is installed in the COLD water line it feeds water (Cold of course) into either the water heater or into a bypass line, this line is connected to the HOT (outlet) line using a simple "T" fitting.
A check valve between the water heater and the pipe prevents backflow into the heater, and if it gets gummed up flow OUT OF the heater as well.
The two valve system uses two 3-way valves one accepts water from teh bypass or heater and passes it on to the RV Thus no check valve is needed... However....
Three valve system uses simple on-off Quarter Turn (usually) valves, one in the cold, one in the hot and one in the bypass line, the bypass connects to hot and cold lines on the non-heater side of the other two valves. so no check vlave is needed... Again However.....
However..... They often put check valve(s) in anyway, sometimes not only on the outlet but on the inlet as well (Absolutely no reason to do that)
The hot valve gets gummed up with mineral deposits and does not open in many cases.
And if you have the 2 or 3 valve version of the bypass. The top (hot) valve might not be fully opened to the water heater.