I have an issue with a Megaflow direct unvented hot water cylinder with the lower immersion heater powered by economy 7 and the upper one by a limited time booster switch. The booster was rarely required. It was operating satisfactorily up until I had a shower valve replaced. Since then it runs out of hot water after only a couple of showers. I have had an electrician check out the immersion heaters and they are both good. I have replaced the thermostat in the lower immersion heater but with no effect. The 2hr boost does not seem to produce much hot water. I have had the lower immersion heater wired into the day rate power supply and left on continuously. Although slightly improved it still runs out of hot water prematurely. Any ideas please?