Welcome to the forums @zoomuno.

This is a topic that has been discussed here:  https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/ashp-standby-electricity-consumption#post-8879

My suggestion, which is what we've done, is turned the heat pump off, and only turn it on when we physically need hot water. At the moment, with all the solar around, our iBoost has been taking care of that 95% of the time. That would be best workaround.

Thanks, Mars. That thread is interesting. So do you mean you switch off via the kill switch when only hot water is needed? If that's the case, doesn't it cost loads - potentially heating the tank from nothing or low - to produce hot water just for a shower or a bit of washing up? I don't have solar panels or battery (someday!), so hot water relies solely on ASHP. I'd willingly have cold showers to save money (and use the kettle for washing up), but not sure the rest of the household will tolerate that!

@zoomuno, on days where we have full solar, the ASHP stays off all day and all night as we have enough hot water to get through 24 hours. On days with no solar, I turn the heat pump on at 8am – it takes about an hour to reheat our water to 44C. 44C is our hot water set point and our hot water has been delivered at a COP of 2.3, which is OK. Since early May, we've consumed 239kWh on our hot water. My suggestion would be reheat to your hot water first thing the morning, then switch the heat pump off altogether and avoid the "standby" compressor and heat pump activity which for us was around 8kWh/day. That's a lot of wasted energy in my opinion.

The tank will loose around 1-2kw per day as natural losses, it is not replacing this?

@Mars - at 44 degrees, what is your procedure for legionnaires control? Does your iBoost or tank switch off at a set temp or can it get to a high temp, say 90'C?

@diverted-energy, our ASHP runs a legionnaires cycle every 10 days. We've capped our immersion (to which the iBoost is connected) to 50C. So we never get above that point. In my opinion, hot water over 45C has to be cooled, so you're wasting energy getting it to 50C and above.

We've not experienced any great temperature losses in our HW over a 24 hour period. 

@diverted-energy Yes, it could be that. Engineer today suspected so. Odd that sometimes it runs at 30p and other times £1.17, but I guess it's dissipated heat.

50 is a little low, HSE recommends 60'C, I used to work in United Utilities water quality labs.

Your iBoost, is it just taking surplus you'd otherwise have not used and would be lost to export?

Here's a thought, 4 years ago I came across 150 sets of 20x and 30x Tube Solar Thermal kits as bankrupt stock. I ended up at a lot of people's houses and learnt a fair bit. One question constantly being asked as most we're upgrading to double their heating capacity, they'd ask about secondary tanks.

The conclusion, if the energy is FREE, let the tank temp rise up to as much as 90-95'C. The are a number of reasons, the same volume of water can hold twice as much energy, water is guaranteed sterile and if it is cloudy for a couple of days, there is enough hot water after dilution to last to avoid that greedy ASHP waking up.

This water needs cooling, the energy would be wasted if you let it run direct from the tap at  60-90'C and very dangerous. So, use 2x TMV valves set to 55'C to 'dilute' by blending with cold water so less hot water is used. The reasoning for two, valves in series allows safety redundancy. The hotter the water, the less of it you use so lasts longer.

The same volume of water can hold around 50% more energy at 90'C instead of 60'C, and can give 3x more useful hot water over 55 degrees at the tap. Never have a cool shower again..

Yes, set the ASHP to give you 'just enough'(55'C)  as your paying for it or draining your batteries but let the iBoost run free to very hot. This effectively turns your tank into an energy storage with many benefits.

I had loads of feedback and they were all positive where temp was increased over tank volume.

Get the most of that FREE Solar and store more energy. I'm in the process of putting 500litres in (200+300) to run at 70-90'C. In Winter will be shut down and back to boiler. Legionnaires controlled by pumping tank water via heat exchanger rather than distribution of tank water. Hot water will always be 'fresh'. If tank too cold, diverter valve will send water demand via the boiler instead.

When boiler gets renewed, one with up to 70''C cold water input will be installed as this will decide whether to stand back or top up heat depending on temp of water entering it. 

50'C starts to kill Legionnaires spores but takes time, 60'C, ideally 65'C is the sweet spot to aim for.

@diverted-energy

I remember reading some time ago that heating water above 65C can cause limescale in hard water areas. This should be taken into consideration.

No different to running a boilers heat exchanger.

Tanks to be closed loop with pumped water to remote heat exchanger located at boiler to meet incoming cold supply.

Once I have my head around Arduino, I want to modulate the recirculation pump to only supply heat required.