@squeakysim can you post their design layout?

@iancalderbank unfortunately not! I think they forgot to attach it. I'll be following that up with them.

They did attach the heat pump brochure for anyone interested:

Surely design should be based on "weather compensation" operation? It makes a huge difference to SCOP and therefore running costs. 

Mixergy tanks I believe work by forcing statification - hot water is pushed into the top of the tank, and there is a clear hot / cold divide line.  In contrast regular DHW tanks heat from the coil upwards, slowly raising the temperature of the entire tank.  

I have not seen an independent energy use comparison of mixergy versus conventional DHW tank.  My expectation is though that a mixergy will save energy when fitted with a gas boiler if less of the tank is chosen to be heated, however I do not think it will if the whole tank is heated, and it's unlikely to save energy with a heatpump.  This is because a mixergy will always operate at the lowest COP, while a DHW tank will at least start operation at a better COP.  Attached to help is our COP versus heatpump flow temperature - the graph shows all the datapoints available for January DHW.  We had a mean COP of 2.7 heating the DHW.  If we'd had a mixergy it would have operated at COP=2.2 the whole time.  You'd have to use a lot less DHW than we do to save with a mixergy.  You'd also have to be clairvoyant - so far as I see, its big selling feature is making tiny amounts of very hot water (rather than lots of lukewarm) - completely at odds with what a heatpump wants.

I suggest a big DHW tank, with lots and lots of insulation around it.

@robl thanks for this insight.

I agree, it would be good to see an independent comparison against a conventional DHW tank, and also ASHP system. The energy/cost savings they promote do seem to be focussed on gas boilers.

The main reason for considering mixergy was the ability to have hot water without heating the whole tank, particularly as there is only two people living in the house at the moment, with a low water usage. I don't really know how to figure out which is the best option for them without undersizing the tank for the house!

Also, space is limited in the airing cupboard so a slim tank is also being leaned towards.

Posted by: @tim441

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Surely design should be based on "weather compensation" operation? It makes a huge difference to SCOP and therefore running costs. 

I think MCS MIS3005 standard requires weather compensation to be enabled wherever it is an option on the system controls Tim. The design temperature is the maximum to achieve the required heating load. Should be no higher than 45degC in my view, and that's possible. Mine was designed for 50degC max but runs quite happily at 42degC max.

Posted by: @squeakysim

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@robl thanks for this insight.

I agree, it would be good to see an independent comparison against a conventional DHW tank, and also ASHP system. The energy/cost savings they promote do seem to be focussed on gas boilers.

The main reason for considering mixergy was the ability to have hot water without heating the whole tank, particularly as there is only two people living in the house at the moment, with a low water usage. I don't really know how to figure out which is the best option for them without undersizing the tank for the house!

Also, space is limited in the airing cupboard so a slim tank is also being leaned towards.

Have you consider a Sunamp unit instead?

@allyfish sounds good! Thanks for clarifying.

Hopefully installers will educate themselves to ensure they pass on information correctly to customers. Ours was installed without WC setup and flow temp a ridiculous fixed 55deg initially ... reduced in stages by me over last 3 winters ... finally moving to WC once i got to grips with settings and different way of using controls etc. Flow temps often running in range 35 - 40 deg on most winter days and now heading more 30 - 35deg as we come into spring.

@tim441 same with me! Installer & commissioning engineer left Climate Compensation disabled and fixed 50degC flow temperature. It was costing a fortune until I realised what they had [or had not] done. Apparently rumour is that this practice is quite common, so the ASHP supplier's help line is not bombarded with "my radiators are only lukewarm" phone call queries. In that case is disgraceful. Better handover and owner information & education is needed.

Posted by: @derek-m

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Have you consider a Sunamp unit instead?

I haven't really, they just seem pretty expensive! Do you have one? What kind of setup do you have? Why do you think it might be worth me considering?

I just had a quick look and it doesn't appear they have an option that includes both heat pump and solar pv input which is what we'd be wanting. You'd think they must have that option too!

Posted by: @allyfish

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Mine was designed for 50degC max but runs quite happily at 42degC max.

This is something I need to remember, because we can actually drop the flow temp straight away because most of the rooms temps have been designed 3 degrees higher than what the thermostat would be set at! Therefore if we do manage to get it designed to 45C flow, then the actual flow temp at the outside design temp should be closer to 40C which will be amazing!

Posted by: @squeakysim

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Posted by: @derek-m

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Have you consider a Sunamp unit instead?

I haven't really, they just seem pretty expensive! Do you have one? What kind of setup do you have? Why do you think it might be worth me considering?

I just had a quick look and it doesn't appear they have an option that includes both heat pump and solar pv input which is what we'd be wanting. You'd think they must have that option too!

I don't yet have a Sunamp, though I am considering one for the future. One of the reasons that I suggested that you could consider one is that I believe that they take up less space than a water cylinder, but can heat the same amount of water as having a larger cylinder.

From what I remember, they can accept heat energy from both a heat pump and solar PV if you select the correct model. I seem to remember that someone on the forum was either getting, or has now had, one installed.