I have recently had a thermostatic shower and singleended (positive) shower pump installed in my flat. The property has a small combination cylinder - not sure of the capacities -with an immersion heater. The 2.1 bar shower pump will only supply hot water to the shower for about 3 minutes. I believe the problem may be that both tanks within the cylinder are too low capacity. In which case the solution would appear to be replacing the existing cylinder with a bigger one. Either that or possibly removing the pump and going for an unvented tank. Any advice would be appreciated.