Lukewarm shower: Crossover in Pressure Balance

ABSTRACT Crossover allows the unintended flow of hot water into the cold water system, and vice versa, and may have a significant impact on energy losses in the system, particularly in central DHW systems in multifamily and commercial buildings. In a CEC funded study of over 100 multifamily buildings throughout California, over 50% exhibited crossover failures in their central DHW distribution. This occurs most often when a cartridge in a shower or faucet’s mixing valve wears out and is no longer able to effectively control the mix of hot and cold water at a fixture or point of use. Many times these issues are asymptomatic and the effects not readily felt by the user. 

[aceee.org] 

The Eatherton method, which is properly named after Mark Eatherton, the plumbing engineer who created this particular procedure, consists of testing each fixture individually, beginning with the shower valve in the housing unit that is experiencing hot water issues The faceplate of the shower valve is removed, leaving the shower’s mixing valve exposed. The hot side of the shower valve is heated by turning on hot water until the user gets the hottest water and then turning the shower valve back off. A nearby lavatory hot water faucet is then turned on, while the user feels the hot side of the exposed shower’s mixing valve. If there is crossover, the shower mixing valve will rapidly cool down because the lavatory is pulling cold water through the shower mixing valve, thus rapidly cooling down the hot side of the shower mixing valve. If no crossover exists, the shower mixing valve will remain hot, because cold water is not crossing over, and thus the shower valve will remain heated. 

Pressure balancing failure crossover: PRICE-PFISTER SHOWER VALVE!!!!! 
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Is it possible for a faulty shower to pump hot water back into the cold water supply line? 
Yes this is possible if the pressure balanced valve or thermostatic valve goes bad. A bad valve could create a hot and cold crossover which will force hot water back into the cold water line. 

A plumbing cross over is a condition whereby cold water is allowed to flow into the hot water system. The plumbing system in a home is divided into two sub-systems, the hot water supply and the cold water supply. ... A plumbing cross-over can cause a complaint such as 'water not hot enough'. 

Crossover is a term that is used when the hot water “crosses over” to the cold or the cold water “crosses over” to the hot. The frequency of customers calling in with these symptoms is increasing. Having some hot water mixed with your cold is not necessarily an urgent situation, but it does indicate that there is a problem and it is certainly not energy efficient. Having cold water crossing over into your hot water lines often results in a lukewarm shower and that usually gets everyone’s attention! 
[www.bcpsi.com] 

Single handled shower faucets have a mixing area inside valve where hot and cold come together before going into sink. 
This mixing valve can have crossover where the hot and cold can pass through to each other even when faucet handle is off. When single handle faucets get old, this can happen. 

How it happens: 
For example another faucet is turned on inside house, and pressure differential between hot and cold causes hot water at shower handle to pass through mixing area and into cold water line. or vice versa 
[www.fixya.com]

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https://www.plbg.com hot water only lukewarm - faulty valve?