Hi @saf1973 did you have a play around with the WD DHW by any chance? I have a Daikin system as well and have wondered about this setting - it seems to make no sense to me as I want my shower to be the same temp all year round, and have the DHW set to 48C as a consequence. I'm experimenting using the reheat setting as I've read that is meant to be the most efficient setting but I'm finding after a week it's consuming more kWh than just having a scheduled time for DHW (in my case, at lunchtime to take advantage of PV). Anyhow, just wondering if you'd managed to figure out WD DHW and how it was performing.

And see last few posts in this thread:

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/disappearing-hot-water/paged/3#post-17280

Hi @marzipan71

I did look into this and did some experiments and came to the conclusion that it wasn't something I would use.   I thought i posted and update (maybe that was on Facebook).  From what I recall the DHW WD will raise or lower the DHW tank temperature depending on how hot or cold it is outside, so the colder it is outside the hotter the tank temperature and vice versa.  The theory being that in summer the cold water being mixed at the outlet (tap/shower) is not as cold as it is in winter so you do not need the hot water to be as hot.   You can only set one temperature (comfort), which from memory it uses as a reference point and does some calculations to achieve that at the outlet.   For me, I prefer to know what the DHW tank temp will be and let the shower thermostat and mixer taps bring the temp down.

The most efficient use case is to set the DHW to the temperature which provides enough hot water for you, if you have an undersized tank you will need to raise the temperature and then mix it down at the outlet with cold water.   Mine is 48c on schedule at 1pm as that's the best Solar PV period and the warmest part of the day.  I also have an Eco (38c) set before the comfort heating time in the morning so there is hot enough water for first thing in the morning.   This then has the advantage that the circuit is warmed up for when the Heating comfort is called.

Reheat is actually inefficient as it will be opening and closing the three way valve all day, which has a cooling effect on the heating circuit.

Worst thing to do is to heat hot water during the so called "economy 7" period in the middle of the night if the main usage is in the evening, as the tank would have cooled so much it wasn't worth heating in the first place.

Having just read through the other thread I see your usage is morning and evening.   If you have an economy 7 period then you could use that time to warm up your DHW for your morning showers and then boost it again at the best Solar PV time for the evening showers.  Else, the most efficient time to heat your water is as close to the usage time as possible so that you are using the water before tank heat loss.  Don't use reheat, just use schedules.  An installer telling you anything different is just trying to reduce call backs and wont care about your electricity bill.

Hi @saf1973 thanks for the update, very sensible. I've previously used schedule heating for DHW but since I turned off our space heating last week, the DHW is flying solo so I thought I'd see how reheat affected consumption. I think so far, comparing with October last year when the space heating was off also and we had similar daytime temps and solar production, the reheat is causing a 25% uplift in overall grid consumption and I'm not sure that's a great return. We have a 500L Daikin thermal store.

I don't think our unit has the ability to set multiple set point temps for the DHW - your eco setting - interesting. I think I'll switch back to scheduled heating during PV hours. I was thinking of two approaches which are kind of specific to our somewhat odd setup, since our solar production feeds only our ASHP - firstly, set the scheduled heating to a longer period - say, 10am to 4pm, during which time we have lots of solar - thinking that this would be similar to using the reheat function, but only during that time window and fed by solar (when available); and secondly, to increase the DHW set point to something higher than 48C (current setting) such that any heat losses after 4pm may leave us water warm enough for showering first thing without the need for any additional grid-fed DHW heating. Take the point about economy 7 but we are in Italy and the difference between daytime and night-time prices for electricity are minimal (17c v 14c per kWh). Scheduled only seems the way to go, then fine tune for efficiency and to match usage patterns.

A solar diverter would probably be a good option here but we don't have an immersion element installed in our thermal store and Daikin will only allow use of their 400+ euro element, plus costs of the diverter, installation etc; and that's part of the way towards us paying for the Daikin solar thermal kit that our thermal store was designed to have but our installer dropped from the design for some reason.

Anyhow, thanks for the information - always useful to hear from other Daikin owners who have managed to make sense of the manuals!