Having studied in detail how ASHP's work, and how to use them most efficiently, I would not advise heating your hot water during nighttime, which is often the coldest part of the day. In the first place, many ASHP's may struggle to get water up to 60C, and would have a COP of little over 1 to 2. Even heating the hot water to 45C, with an ambient temperature of say 0C, would give a COP value of approximately 2.5, whereas heating the same water at an ambient temperature of 7C, would be at a COP of approximately 3.5.

A well insulated hot water tank losses approximately 2 kWh of energy per day, provided no hot water is used, obviously, the hotter the water, the greater the heat loss, but the difference is not that great. So the more hot water that is actually used, the greater the heat demand will be.

From the point of view of usage, the hotter the water, then the less hot water, you will need to mix with cold water, to produce the required temperature. From a heat energy usage point of view, there should be very little difference if the water is piping hot, or slightly cooler.

If you have solar panels and a diverter unit, it would make sense to allow the water to be heated by solar energy first, and then if necessary use the ASHP to bring it up to the desired temperature. Most days, the warmest part of the day is mid to late afternoon, so this may be the most efficient time to use the ASHP for this purpose. It is also after the Sun has peaked, so most of the available solar energy will have been used.

It is recommended that the hot water should be heated to between 60C and 65C for a period of a minimum 10 minutes for Legionella protection, at a maximum frequency of 7 days. Probably best overall efficiency could be achieved, by allowing the water to initially be heated by any available solar energy, then heated as high as possible using the ASHP, and finally being topped off by the immersion heater, if the ASHP cannot achieve the specified 60C.

I would agree with what Derek says.  Only heat your water at night if the unit rate is cheap enough to offset the reduction in COP.  E7 rates are about 2/3 of daytime these days.  There is likely to be little in it.  If you have one of the really cheap rates, e.g. the Octopus 5p one, it may be worth it. 

There is another factor though; ASHPs do HW or heating, not both.  HW normally takes priority; when the HW starts, the heating stops.  That means that your heating is off for 30-60 minutes or whatever, while the water heats.  The water temp in the heating circuit and your room temp will both drop and the ASHP will have to work harder to catch up when the HW stops. It might therefore be better to schedule your HW for a time when the heating is either off or not working too hard.     

How much of a temperature drop should I expect between the tank and tap . 

My system is Samsung gen 5 I think and the main controller is set to 51 degrees which is above the recommended 48 degrees , yet it only comes out of the tap at 42 degrees!

What if anything is wrong. This concerns the DHW system.

@alfapat there's a fair chance that the sensor is not reading the temperature in the tank properly, or there's blending that occurs. We get that from time to time. Water feels very different coming out the tap than what the system is reading inside the cylinder.

Ok , thank you . Isuspected something like that but didnt want to believe it. 

I had a problem where the heating valves and water valves were open to gether but wasnt affecting the temp too much , just the power it consumed was horric!

This I got sorted after finding someone who knew their job and the system was completely wired wrongly . I had this problem for some time as the company who installed the system went bankrupt and left me with a mess. Hopefully I can sort all this soon.

Best Regards 

PAT

We heat the water up to the maximum of 60c.

Timed for 3am @ 6.7p.

Takes about an hour to heat and that’s when my heating cuts in at 20c until 7am when the low rate ends and the good lady leaves for work leaving the house empty. Heating down to 18c until 4.00pm then back up to 20c. 
1 question..
I have turned off the legionnaires cycle as the water heating to 60c daily should take care of the legionnaires risk.

would I be right there?.

be interested in what people think on that front.

Yes I certainly agree. The chance of legionnaires being a problem under those conditions is very low, with the usual caveats if there is someone in the household who is vulnerable to this or if the system has dead legs which are unflushed.

@westkent 

Do you use the Airsource to keep your water at that level?

I didnt think this is possible to get it to stretch that far with out the immersion doing it ?

So I have been looking at the tank sensor and as I have a spare sensor I compared the resistance and are both similar values. So cant be the sensor, I stuck a cake probe into the same location and it came to 1.5 degrees of the system sensor . So the controller is telling me 48.5 and home probe 47.1. Both the heating and Hot water valves were in the same position, I wasnt sure if it is closed or open , I suspect closed as I withdrew the system probe the HW valve motorised , detecting environment temp.

What I have done and I dont know if it will reach it, is move the DHW Temp up to 52 degrees.

@alfapat .

I just set the water temperature to 60c via the systems MELCloud app.

As I’m still learning on these things how would I heat the water via the immersion if I’m not already??

@westkent OK , Interesting , Im surprised that Airsource only, is pushing to that level .