Not enough hot water in the tank

[Fay Clough]

I have a Hercal coil indirect heater - up to 3 month ago it was producing enough hot water for all the family to have shower. But lately it is only enough for one and every time we want to have a shower I have to put the immersion heater on for an hour or so before hand. I keep reading about how boilers work on the internet, it seems to me that it could be the coil need replacing, but I am not a plumber and do not like to diagnose it. Please help me to find the problem and the solution. Many thank for your kind advice. Fay Clough

[Set Square]

Does the boiler also heat the central heating radiators? Is it only since the heating has been on that you have insufficient hot water? Do you have timers for HW and CH? If so, long long are they set to come on for each day? Is there a thermostat on the hot water cylinder (a square box strapped to the outside of the cylinder - *not* the one in the immersion heater)? If so, what is it set at?