@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. 

Posted by: @webcmg

@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... 

Sorry for the delay, notifications for this forum go to my spam folder.

The installer I used suggested fitting a motorised valve, linked to the same timer that operated the DHW return pump, he did warn that while it would solve the Syphon problem it would likely cause the pump to fail prematurely or cause noise as the pump would be running immediately while the valve was still opening.

I looked at this and if I went ahead with a valve I would actually make use of the control wire (just like if it was being used by central heating) such that the timer operated the valve, then the valve operated the pump once it had opened.

However like others have suggested earlier, a London loop can be used, I have attempted to get more information on this but my Google-fu is failing me; I need to figure out whether a downward loop is required on only the outlet or return of the DHW pipes or both.

Thanks

@aceshigh I wonder if a control valve and no pump would work? Surely it would simply syphon when the valve opened? 

@Aceshigh

The thermosypon effect will be from any hot water store, where a pipe exits and heads upwards, or horiz then upwards.  It's most likely just the top of the store that's the issue, as the return pipe at the bottom is generally cold preventing the effect.  So I would expect the issue arises if you have a hot tank, with the flow pipe at the top of the store going upwards or up + horizontal - then inside the "up" sections there will be circulating water flows, efficiently stealing the heat from below and moving it upwards.  The water isn't flowing all the way around the whole circuit, it's just local convection in that section of pipe.

So - check the pipes from the store - the ones that are hot long after they should have cooled, they're the ones which are thermosyphoning.  It's likely to just be the DHW out pipe, but it could be more of them, no reason why not.  These ones could do with this "london loop", which I think is an added (minimum)10cm down + 180 bend + 10cm up section of bent pipe.  

Linky has pic of solution:

https://www.diynot.com/diy/threads/preventing-thermosyphon.539906/

@robl thank you. It's definitely the complete circuit (DHW out and around to the DHW return) - exasperated by the Mixergy technology with their pump on the tank to take hot water from the top and blend it with cold water at the bottom to get the tank hot throughout.

If I close the DHW return pump valve the siphoning stops and the tank stays very hot for a couple of days.

I'll look to add the downward loop to both the DHW out and return and hopefully that cures things. 

@colin do you know if you can set the legionella cycle to trigger at the same time as the scheduled water charge period? 

@webcmg Yes, I do this with already, I have everything to start when the daily electricity tariff is 5.5p.  On the flip side, if you have it set at different times for example you had the the legionella cycle set to 7pm and it finishes at 9pm, and your daily heat schedule is set to 10pm, then the 10pm cycle will just skip because the temperature threshold is already met.