Thermostatic Mixer bypassing

[TFGplumbing]

I've got a job to replace a Thermostatic Mixer, under a basin in a public premise. When the unit is turned on(service valves) you can hear mains water bypassing to the hot water side, I assume a check valve had broken inside.

Then a though occurred to me do the hw & cw supplies have to be balanced pressure? because the Hw side is fed from a cylinder with a head of about 0.8 bar and the cw is mains.

Thanks

 

[Chalked]

Some can work up to 5-1 pressure difference. But best on eaqual pressures.
Non return on the valve can’t be shutting.

 

[ShaunCorbs]

and also they dont work on gravity pressure (you can Sod faster or it doesnt get hot)