Symmons Mixing Valve issue HELP!!

[Plumbingnewb80]

I'm having trouble keeping water temps between 100 to 116 degrees in my building. I've had professional plumbers out to solve the problem with no luck. I've cleaned the cartridge out myself and have used the back up cartridge and still having the same issue of water starting off being 120+ then immediately dropping to 94 degrees. The moment the water is turned off at faucet the hot water temp jumps back to 120+. Any ideas as to the cause and how to fix? I'm in a state inspection window and need this taken care of pronto before they show up.

 

[phishfood]

Is there a hot water fed air heating unit in the building? The check valve in these can malfunction allowing cold water to cross over to the hot side while the hot water is in use, and might cause such a problem.

A single handle lavatory or kitchen sink faucet? These can allow crossover in the same manner.

 

[Plumbingnewb80]

Is there a hot water fed air heating unit in the building? The check valve in these can malfunction allowing cold water to cross over to the hot side while the hot water is in use, and might cause such a problem.

Can the check valve be replaced/fixed or do I have to replace the entire unit?

 

[phishfood]

While I don't personally work on these units other than installing the piping to feed them, I do believe that the check valve should be replaceable. If that is not an option, a check valve could be installed on the piping outside of the unit that would serve the same purpose.