How to find and fix the plumbing crossover, when the hot water is not hot enough or water heater doesn't produce enough hot water.
Every house, either new or old, has two separate water lines; hot and cold. When the cold water is heated inside the water heater, it produces the hot water from where it is distributed where it is needed; hot water taps, washing machine, dishwasher…
The water temperature inside the heater is controlled by the thermostat, and if properly set, the water temperature should be between 120F (factory set) and 140F (maximum recommended).
If hot water is not hot enough, it is lukewarm or even cold, something is wrong either with the water heater or the plumbing.
There are several reasons why the water heater is not producing hot water; and some are described in the troubleshooting guide.
If everything is ok with the water heater, the reason for the problem when hot water is not hot enough or there is not enough hot water, might be in the house plumbing, and the most frequent problem is the plumbing crossover.
This is the situation when the cold water mixes with the hot water; for some reason it flows into the hot water system, which shouldn't. The location for the plumbing crossover (assuming the plumbing was done correctly and there is no plumbing cross connection) is usually inside one of the faulty mixing valves (such as those found in washing machines) or single handle faucets.
Locate the shut off valve installed on the cold line and close to the water heater and close the valve.
Usually, inside the laundry room or even garage there is a faucet with the two separate, hot and cold spigots. Open the hot tap and look what’s happening. The normal situation is when the hot water stops running completely after some time. If it doesn't and it turns cold, you have a situation with the plumbing crossover where the cold water enters the hot water plumbing system. If the water stays hot and continuous running with the slower flow rate, the problem is with the defective shut off valve that allows cold water to enter the water heater tank. The faulty shut off valve must be replaced.
Once you figure out that there is a problem, you have to locate where the mixing occurs, which might be time consuming. This is one of the ways:
First, check is the plumbing cross connection happening in the washing machine. Shut off the service valves to the washer while the hot water tap is open. If the water stops flowing at the hot water tap, the mixing valve on the washing machine is the cause.
If the washing machine runs properly and the mixing valve is not causing any issues, check every faucet that has the single handle, by touching the pipes that runs to that tap. If the pipes of one fixture are both cold than that faucet is the cause of the plumbing crossover and must be fixed.