One way valve on hot water in shower

ktmrfs wrote:


another possible cause of the cold water problem is WH bypass systems with ONE lever, rather than the TWO lever systems. The one lever systems use a one way check valve that often has highly non linear characteristics. The result is very noticeable water temp changes with flow, very noticeable when water is down to a trickle then back up again. Solution, pull the check valve that is installed and install a WH bypass with two levers. That solved our issue along with two other camper friends complaining about the same problem.

This is exactly the issue. The water heater outlet check valve acts as a slight restricter to the flow of hot water. When the shower wand is shut off, it continues to trickle. Since the hot water flow is slightly restricted, it is over powered by the cold water flow. The hot water flow stops and cold water fills the shower hose. Cold water cannot back up into the hot water line, since the check valve at the water heater will not allow it to and hot water is not magically compressible by cold water. The cold water blast is from the cold water filling the 6 feet of shower hose as the water continues to trickle from the shower wand head.

There are three ways to solve the problem. Remove the check valve at the water heater outlet and use a 2 or 3 valve bypass, add a temperature controlled mixing valve to the shower lines or add a shutoff valve to the shower hose.