Silver Saints cover a massive range of plumbing jobs. One of our more specialist jobs is the repairing of faulty thermostatic showers and electric showers.
In fact we have repaired and fitted hundreds of showers. We recently had two showers at separate properties within a week which proved to have the same unique fault. Both showers appeared to only be getting hot water. On the lowest temperature setting no water would flow at all. We originally thought that there must be an airlock in the cold supply as both showers were been supplied by a tank in the loft. But after a bit of investigation we found that the non-return valves fitted to the cold supply within the shower valves had become stuck and were preventing the cold water from entering the shower valve.
The solution was to remove the non-return valves completely. As the cold water was tank fed there was no risk of contamination to mains water so these valves were not needed. Of-course if the cold was been fed by mains we would have replaced the faulty non-return valves.
On reflection we suspect that the problem with the non-return valves were caused because the cold water was tank fed while the hot supply was fed off a shower pump. Because the pump ran on for a few seconds after the shower flow was shut off, the hot water putting trying to flow into the cold supply, allot of pressure onto the non-return valve on the cold supply every time the shower was turned off. Eventually it just jammed closed.
In eight years of working with showers we have seen this exact problem only 3 times. So not exactly a common cause for failure, but an interesting problem none the less.