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 🙁