I've never really understood why any shower connected to a combi would need non return valves. Hot water is fresh, at the same pressure as the cold and how could hot water contaminate the cold if it's from the same supply?
Been to god knows how many "faulty boilers" where the hot is running luke warm only to find a stuck nrv on a shower, when the hot nrv sticks it allows cold mains to mix with the hot water system,
not all showers close off the hot and cold supplies as they enter the shower body but close off the outlet relying on the nrvs to prevent cross contamination,
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