@webcmg 

From the pictures on your first post it appears that the DHW outlet and secondary return inlet are piped straight up into your ceiling space, no doubt that pipework then runs horizontally and then upwards to your taps / bath / showers upstairs.

I am not a plumber, nor a heating engineer, but I have recently had a similar piped system installed and I can see that I can have a 300l cylinder cool itself due to syphoning in a day, without any hot water drawn off for household use.

If you have your system programmed to maintain a certain temperature in the cylinder, or one or two times programs a day, you will be unnecessarily cooling the cylinder continuously with the secondary return which then needs to be reheated.

If you want to test this, ensure the DHW return pump is turned off and then close one of pump valves. Yes, you will have to wait longer for hot water at the outlets during the test, but I think you will find that your cylinder doesn't take as much energy.

@aceshigh thanks, that is helpful to know. Are there likely to be any risks associated with this?

@colin I have a small form factor PC on the way to get home assistant setup. The water board fitted a smart water meter to other day too, so should be able to get good visibility of a range of data

@aceshigh @colin I isolated the secondary return at the weekend and today the DHW has not had to recharge following yesterday's anti legionnela cycle. The temperature has remained above 50 degrees even though we've both had showers. It does take marginally longer to get hot water, but I prefer this if we're saving energy. So this is a positive sign. I wonder if this issue has impacted the ASHP performance for the heating. We had our DHW on constant and can imagine it would need to keep cutting in to recharge the DHW. When it did this perhaps the flow temp for heating dropped, incurring greater consumption to get back to desired temperature. Something to monitor but positive signs. I just wanted to say many thanks to you both for taking the time to help me. It is greatly appreciated. Thank you. 

@aceshigh Do you know if it is possible to plumb in a secondary return so it does not syphon? It might be nice to circulate on demand at particular times of day (which is what the pump with attached timer is supposed to do, but it didn't ever need the pump. A motorised control zone perhaps?

@colin it sounds like we have similar setups and houses (minus 2 children). What is your current daily electricity kwh usage for the house? I'm looking for a comparison because ours is still 6 kwh without any ASHP usage. I don't think we have several tvs on standby, two fridge freezers and a lot of Philips hue devices on standby. Not sure if 6 kwh is good or bad... 

@webcmg Glad you're making progress and happy to help. The screenshots shows my usage including approx 2kwh per day for hot water. That includes everything, oven, hob, Fridge freezer, computers, TVs, Starlink internet, Unifi PoE network, outdoor Pirs, alarm, garage door.

When the ASHP, oven, TVs etc are off, there's a baseload of approx 270 watts which is about 6.5 units per day. 

@webcmg

Our daily electricity consumption is normally around 6kW to 8kW, and this does not include DHW heating. Because we have a solar PV system, the water gets heated by Sun power and at the moment we only need to draw 2kW to 3 kW from the grid each day.

@colin @derek-m

Thanks, our usage is roughly inline with your values so I'm happier now. I definitely want to investigate pv solar and batteries.

Out of interest, what is your daily kwh usage in peak of winter for DHW?

@webcmg

In Winter we run the gas boiler, which provides heating and hot water, with a little help from the solar PV on sunny days. From Spring through to Autumn the vast majority of DHW is heated by the Sun.

@webcmg Below is a screenshot of my DHW heating in Feb. I'd only just installed the Vaillant Internet gateway & Home Assistant, I hadn't fully optimised things yet plus had a higher temperature set. DHW Consumption was averaging 3kwh per day. Being in an exposed area of Scotland it's also probably a little less efficient.

@colin that's really not bad at all, I was expecting it to be much higher. Thanks. 

@webcmg DHW doesn't use a lot. Heating the house does though. I was averaging 20-25kwh per day in Feb, posted screenshots so you can compare.

@colin

Could you please confirm the Total Yield value. If it is just for February then it seems rather high by a factor of about 10.

@derek-m Hi Derek, I thought the same when I first saw it but I just put it down to bad user interface design 🤭 . The yield figure at the bottom is the total overall for the heat pump (excluding DHW) since it was first commissioned, not for February.