I currently have my system set up to heat hot water at night, during my cheap Octopus time. Heat to 80%. Reheat from 30%. Else the solar PV heats the cylinder. 
I think H/W does take preference out of the box, but I’ve reset the Mixergy to only heat at night automatically. If we need more hot water during the day, will boost via the Mixergy app. I don’t want the ASHP to decide when to heat hot water 

Hi Julian,

That's the point I have been trying to make, what works well for one household will not necessarily do so for another. In fact, what works well during one period of the year may not do so during another period.

Which is why it is probably useful to collect and then analyse data concerning how ones system is operating, along with the prevailing weather conditions at the time.

Have you noticed any increases in the 'Agile' tariff? It would appear that the Agile pricing for my area (Yorkshire) was today showing the cheapest period to be 03:30 to 04:00 at 14.18p and for tomorrow to be 13.48p from 00:30 to 01:00. Neither of which appear to be particularly cheap considering that the price during the evening peak is the maximum 35p.

Personally, I would have expected that heating would take preference over hot water.

From the detailed studies that I have carried out on our central heating system, it only takes approximately 4 minutes from the boiler firing up and the water pump starting, to the radiators reaching the calculated temperature. This is when the desired water temperature is within the range of 27C to 45C. It then takes anywhere between 10 minutes (at 45C) and probably a couples of hours (at 27C) for the temperature of the radiators to fall a few degrees and hence start the boiler and pump again.

If our gas boiler were to be replaced with an ASHP I would expect very similar results, which would mean that even with an outside air temperature of -5C or below and a central heating water temperature of 45C, there would still be approximately 6 minutes out of every 10 minute period where the ASHP could be heating the hot water.

Because the quantity of water in a hot water tank, is much greater than the volume of water in a central heating system, it could take several hours to heat the water tank from cold up to temperature, during which time there would be no heating if HW takes precedence.

@derek-m, as @julianc says, I think that 'out the box' HW takes preference if both heating and HW are calling for heat. I'll ask the installer when they return to install MMSP next week.  

I wonder how much water is in the heating system compared with the 250l (or so) tank? Is it similar?

FYI I have my 250l tank HW on 24/7 at the moment.  That uses 5-6kWh of delivered power per day or 2-3 consumed. We use next to none of it (yes I know!) so that's mostly heat loss.  Not a lot really.  I'll have a play with the timer settings; I need to balance my cheaper E7 rate with better daytime efficiency of the ASHP. 

@derek-m totally agree with your statement about data driving decisions. 
We aren’t on the octopus agile tariff, but the Go tariff which delivers 4hrs at night at 5p/kWh. I have a separate app that tracks Go to Agile and with our current usage, we have saved 20%+ with Go. That could change over winter with the ASHP added. 
Go is targeted at EV drivers who can plan charging. And it balances our solar PV well. 
The advantage of the Mixergy cylinder is that it “learns” your hot water demand based on usage. Machine learning I think. 

Hi Kev,

You can calculate the approximate volume of your central heating system, by measuring your radiators and estimating the length and diameter of the pipework.

A double panel type 22 kudox radiator 600mm x 1200mm contains 7.28 litres of water. (If you know the manufacturer you may be able to find the information on their website). If not, just do an estimate based on size.

1 metre of 15mm diameter pipe contains 0.056 litres of water.

1 metre of 22mm diameter pipe contains 0.121 litres of water.

If you check the size and type of your radiators and the approximate length and diameter of pipework it should be possible to estimate the volume of your heating system. I would expect an average house to be somewhere between 50 and 100 litres.

Like you, we don't use vast amounts of hot water, which from Spring through to Autumn is normally provide by our solar PV system. In the Winter hot water is heated by our gas boiler with some assistance from the solar PV.

At the end of the day it is down to individual preference, do you want hot water available constantly, or do you set your system to heat the water in the most efficient and cost effective manner. 

I suppose that you could do some simple checks to see if you can identify the main source of heat loss around your hot water tank. Is all the pipework adequately insulated? 

@derek-m, it's all brand new so the tank and the pipes to the ASHP are very well insulated. It's as good as it gets already. It's only consuming 5kWh/day so even at a COP of 2 that's still only 30p per day or so.

Hi Kev,

Just to clarify, is that 5kWh of heat energy or electrical energy?

Sorry my terminology is wrong. 5kWh is the heat energy delivered by the ASHP.  The consumed electrical energy (what I'm paying for) is around 2kWh.  Does that make more sense?  These numbers are available from the ASHP control panel.

Hi Kev,

No problem, didn't want you getting a shock later on.

I worked on large projects with design offices dotted around the world. I quickly learned to be precise and clarify if there was any possible doubt.

We used our A2A ASHP for cooling for the first time today when the indoor temperature went above 25C. Don't worry we were using carbon neutral energy from our solar PV system, in fact we were still exporting electrical power, so were helping someone else to power their AC unit.

@derek-m ours is on all the time in the day. It's free cooling! Why not!!!