@boblochinver 

They may not be available in the UK, but here in Oz there are many options for an ASHP for only the HWS.  I've had a CO2 ASHP for my HWS for the past 2 years, and has been fantastic.  In my experience (5 bed house and 310L HW tank) it uses around 2 kW (total) per day to heat a 310L HW tank during spring-summer-autumn and 3 kW (total) during winter.  Their technical specifications state between 3-4 kW per day average.  The heating schedule option I selected was to heat during the day if you have a solar PV system (Setting from 10 am - 2 pm).  During heating it uses around 1 kWh so depending on your solar PV this may be covered for some of the year.   But if you don't have solar PV, or if your solar PV is not generating 1 kWh, then there are options to scheduling the heating during night tariffs etc.   

We see a lot of brands now offering similar systems in Oz now (mine is a Reclaim Energy unit), But I see many of the hot water supply companies (i.e. Rheem) now offering inbuilt ASHP to the hot water cylinder.  But for yourself staying with an EU manufacture/supported brand such as Mixergy (or Dimplex, etc) would be better options due to support.  

Just as long as you aren't thinking of ever using the ASHP for heating.  Although the CO2 ASHP are great options for HWS only, they are too small for a hydronic heating system.  Running a larger ASHP (R290 or R32) for HWS and heating would be better option if you need to future proof any heating (if you don't go down the A2A ASHP path for heating). 

Also we run A2A ASHP (in Oz referred to as reverse cycle air conditioners), and have used both single units (single split system; i.e. 1 condenser per unit in the room) and a full ducted system (16 kW cooling / 18 kW heating ASHP, with 11 ducts).  The COP for an A2A is less than you will likely achieve with an A2W ASHP, but most brands would have an inverter so will modulate down for using less kWh once up to temperature (as many of the A2W ASHP now do also). 

For a UK perspective 'The EV Puzzle' and 'Tim & Kat's Green Walk' are great YouTube channels discussing their A2A ASHP in the UK over a UK winter, if you haven't already seen their channels.  

@karloz I know this is delayed reply but I have just spent from May till just about 3 weeks ago doing a total renovation of the my home up here in the Scottish Northwest coast of the Highlands. I have a 5 bedroom house and work from home. Walls where taken down and a whole new kitchen was installed with all the appliances built in and a instant hot water tap for making cups of tea (overall more efficient than a kettle). The entire house had a complete electrical rewire (not done since 1965!!) and I have an Emporia electrical monitoring system installed into the distribution box. I have fitted new internal kingspan insulation for the outside walls which will help with keeping it warm (I point blank refuse to fill the cavity wall as the cavity is there for a reason and it will most likely cause issues with bridging here on the coast of Scotland with the winds directly off the north Atlantic with driving rain. 

I have had all new double glazing installed throughout the house and used it to ensure all drafts and leaks are sealed, it has made such a difference to the wind noise and keeping the warmth in. I replaced my oil boiler with a complete new A2A heating system (I have had lots of chats with Tim and Kat) which consists of 8 internal Toshiba Haori units and 2 external fans. which share the work of the heating. For my hot water system I went for the All in one Dimplex Edel 270 ltr ASHP water heater and that has so far worked well for me considering it is used for at least 4 showers a day and the odd bath. Since August the monthly kWh used to has Aug 72 kWh, Sept 94 kWh, Oct 90 kWh and Nov 52 kWh so far. I know that the rate will go higher during winter with the colder air but in summer will be much lower. I previously was just using the old water tank that the oil used to heat but when I replaced that I had to go to pure electrical element which was running about 150+ per month for heating just hot water for showers etc.  The Edel is performing to my expectations and I like that its app controlled and can see the current volume of heated water. It has never ran out of hot water. It is louder than a normal hot water tank due to the small ASHP being built in on top of it but as I located it in my garage the slight humming noise when its working is not noticed. I suspect if you had to place it internally in the house then it may need some sound proofing but not an excessive amount. I can choose to use the electric element to boost the heating cycle if required which obviously uses more electric but i have really needed to do that only when my plumber had to empty the tank and fit a safety value in the plumbing setup. I suspect in the summer I can set the time for it to heat water to suit the the solar output and I can charge it during my cheap rate COSY Tariff with Octopus but to be hones I leave it able to charge when it needs to heat up most of the time the only time I dont is i place it on standby during the high peak rate period between 1600 hrs and 1900 hrs. 

@wintergreen I did a reply on my progress in this group so hopefully you get some info from it 🙂

Thanks.  That’s really interesting and helpful.  Unfortunately, it’s also a bit late for me!

We have now installed a Stiebel Eltron WPL A 07 ASHP on a flat roof at our home in London.  We went for the Stiebel in part because the site is near an access hatch to a bedroom below, and most of the other ASHPs had refrigerants which were unsafe in that location.  This follows our installation of PV and batteries in early summer.

First, the good news.  The space heating of the Stiebel is great. Stable warmth in the main part of the house (which has been thermally upgraded).  Much less drafty, and terrific efficiency* - COP has typically been well over 4.   The machine is quiet too.  Second, the bad news.  I worry that the DHW energy use is really bad.  I went to the installer asking for an inbuilt ASHP cylinder such as the Arostor or the Dimplex model you have.  Stiebel were clear that that was not the right way to go.  Instead I should use their matched monobloc and internal cylinder.  I trusted their advice, and I’m very worried it was a big mistake.  The DHW is recorded as using between 5 and 8kWh per day.  That’s with 4 people taking a daily shower, and not much more.  It’s a big house, with lots of pipes, but the secondary return circulating pump is off, so that shouldn’t make too much difference.

The other worry is that the Stiebel machine seems to have a quiescent draw of around 200W.  Is that normal?  It seems to me to be a huge amount.  It means we’re using almost 5kWh per day before we’ve even heated anything.  In the summer that will take a big slice of our solar power.  *I am pretty sure that this power is not included in the efficiency figures.  Again, is that a typical approach?  It seems quite wrong to ignore it, but there is a consistent difference between the Stiebel consumption figure and the data I’m getting from my Emporia meter of around 5kWh.  I would be very interested in the views of other forum members.  

@wintergreen For comparison only: We have a Sunamp Thermino ePV210 heat battery for DHW and there are two of us showering and washing-up plus handwashing etc., we are retired and at home all day. I know the COP is just one for this but, our average daily consumption (according to the Eddi App is 3.75 kW/h. The unit sits in our airing cupboard so any ‘losses’ are always useful. Regards, Toodles.

Posted by: @wintergreen

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@boblochinver 

Thanks.  That’s really interesting and helpful.  Unfortunately, it’s also a bit late for me!

We have now installed a Stiebel Eltron WPL A 07 ASHP on a flat roof at our home in London.  We went for the Stiebel in part because the site is near an access hatch to a bedroom below, and most of the other ASHPs had refrigerants which were unsafe in that location.  This follows our installation of PV and batteries in early summer.

First, the good news.  The space heating of the Stiebel is great. Stable warmth in the main part of the house (which has been thermally upgraded).  Much less drafty, and terrific efficiency* - COP has typically been well over 4.   The machine is quiet too.  Second, the bad news.  I worry that the DHW energy use is really bad.  I went to the installer asking for an inbuilt ASHP cylinder such as the Arostor or the Dimplex model you have.  Stiebel were clear that that was not the right way to go.  Instead I should use their matched monobloc and internal cylinder.  I trusted their advice, and I’m very worried it was a big mistake.  The DHW is recorded as using between 5 and 8kWh per day.  That’s with 4 people taking a daily shower, and not much more.  It’s a big house, with lots of pipes, but the secondary return circulating pump is off, so that shouldn’t make too much difference.

The other worry is that the Stiebel machine seems to have a quiescent draw of around 200W.  Is that normal?  It seems to me to be a huge amount.  It means we’re using almost 5kWh per day before we’ve even heated anything.  In the summer that will take a big slice of our solar power.  *I am pretty sure that this power is not included in the efficiency figures.  Again, is that a typical approach?  It seems quite wrong to ignore it, but there is a consistent difference between the Stiebel consumption figure and the data I’m getting from my Emporia meter of around 5kWh.  I would be very interested in the views of other forum members.  

In the Stiebel manual there is mention of an Oil Sump Heater, which may be using some of the 200W, though it would be be best to ask Stiebel and/or your installer why you appear to have a constant 200W load.

Just to clarify, the 5 to 8 kWh of energy used each day to produce DHW, is this thermal energy or electrical energy?

Since you have solar PV, have you considered a power diverter for DHW production during Spring, Summer and Autumn?

That's interesting - I will investigate the oil sump heater with Stiebel.  The installer doesn't really know much about the system.

The 5-8kWh is electrical energy.  I would be delighted if that was our heat use!

I wasn't planning to get a power diverter.  I don't really see the attraction.  Why would one divert solar into an immersion heater, efficiency around 100%, when you can send it to the heat pump, efficiency around 400%.  Surely that's a better use of any excess?

Posted by: @wintergreen

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That's interesting - I will investigate the oil sump heater with Stiebel.  The installer doesn't really know much about the system.

The 5-8kWh is electrical energy.  I would be delighted if that was our heat use!

I wasn't planning to get a power diverter.  I don't really see the attraction.  Why would one divert solar into an immersion heater, efficiency around 100%, when you can send it to the heat pump, efficiency around 400%.  Surely that's a better use of any excess?

I suppose it depends upon the size of your solar PV array and the quantity of hot water used.

The diverter, as the name suggests, only diverts the excess solar generation that would normally be exported back to the grid, so if the excess is say 500W, only 500W is supplied to the immersion heater. It will still heat the water in the hot water cylinder.

On the other hand, running the heat pump to produce hot water may require 1.5kW of electrical energy, so in the above scenario you would be drawing 1kW from the grid. The water cylinder would heat quicker, but once hot, the excess solar generation would then be exported back to the grid.

Of course if you also have battery storage, then the solar generation could be stored and then used to run the heat pump.

A further benefit of a diverter is to perform the legionella cycle, to assist or even replace the heat pump.

At the end of the day it is a matter of what works best for you.

@wintergreen you don't say how big your cylinder is, but I have 4 people, 4 showers plus lots of pan washing up (we cook a lot), 300L cylinder, we cycle it in full every day. takes between 5-7kwh of electrical energy to put in around 14-15kwh of heat energy (target temp 55C). 4 is a really good number on the CH, but COP on water heating is never as good . so your number of 5-8kwh may be about right. Also a recommendation for DHW heating efficiency - if you are able to , let the cylinder go as cool as possible before you reheat it. the COP will be better.

But: 200W standby load seems very high. not sure there are many other Stiebel users on here to ask, but other heat pumps are a lot lower, sub 50W is where you want to be.

I think you should put time into chasing down that standby number, as you say its 5kwh every day, if you use a TOU tariff to charge your batteries you've eaten through one of them just with your standby load 🙁