I have the same problem

i heat at night on cheap tariff but one shower and temp drop from 48 to 18

@mikefitz

I suppose that the first thing would be to quantify how much electrical energy is being used during the DHW heating cycle, since this should provide a measure of how much thermal energy is being supplied.

If the electrical energy usage is low, then check the position of the temperature sensor fitted in the side of the hot water cylinder, also ensure that the sensor is making good thermal contact inside the sensor pocket.

If electrical energy usage is high, you will need to discover where the thermal energy is going.

Do you have a anti-stratification pump on your hot water cylinder? If so, is it running whilst the cylinder is being heated?

Where is your mains water supply pipe connected to the hot water cylinder? When you draw off hot water from the cylinder the outlet pipe should be at the top of the cylinder, with the mains supply being located near the base. Is this correct?

@derek-m attached a screen shot of what happens when someone goes for a shower. Takes about an 1 hour and 15 minutes to heat the tank on average from the heat pump. Not sure how much energy was used.

We have a Temp sensor in the lower part of the tank. There is only one place for this to go in. Goes in all the way and seems to be reading correctly. 

Hot water comes out the top pipe and the cold water plus if on the recirculated water goes in the bottom of the cylinder.

@ambaz79

The fact that the temperature sensor is located in the bottom section of the tank will be the reason why the reading plummets. There should still be plenty of hot water in the rest of the tank.

@ambaz79 You can query Melcloud under Energy Usage and specify just one day and see pie charts of the energy consumed and generated for heating and hot water. It only gives figures for consumed energy, though there are ways to see other figures using MelCloud in a browser window. 75 mins seems a reasonable time to complete a reheat, especially on a large tank.

Unless it is an optical distortion you seem to have a portion of the tank above the hot water draw, the hottest water you can never use! Try putting your hand on the various pipes entering the tank and see how warm they are - you have a number of uninsulated pieces of copper connected. A couple of hours after the HW cycle has finished you would hope those flow and return pipes would be cool as they are uninsulated but if they are still warm then this might show they are conducting heat back out of the tank.

Possibly check the cold supply pipe too, though in this case I wonder about the effect of the recirculation loop that joins it. Even though you are not now actively pumping the hot water round, which will destratify the tank and make it all increasingly (decreasingly?) tepid, it is still a loop through which the hot water within could cool and sink into the bottom of the tank drawing more hot water out of the top. I am not certain but I think the pump will still allow water to flow through even when it is not actively pumping.

Turns out my son was going for a shower for over 20 minutes after school. This was emptying the tank. I've kept the Circulating Pump off and might just remove it. I've set the tank to warn once at night and once in the day. Also districts my sons shower to less than 10 minutes. Not sure this is being fully followed though! We are getting through 24 hours with just 2 cylinder head ups. Thanks for all your help on this.