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Earth Notes: On Solar DHW For Our Home with UniQ and PV Diversion

Updated 2019-12-15 15:58 GMT.
Solar hot water, with low grid CO2 intensity. #solar #DHW #UniQ
under combi 20191215
If I were applying for government funding I'd call this a cross-vector hybrid time-shifting storage-based DHW carbon-reduction scheme. Which means that I shift some load from the gas grid to the electricity system (though probably not to the grid itself) while cutting CO2 emissions.

Summary Questions and Answers

Why install a Sunamp UniQ heat battery?

To try and reduce our carbon footprint, reduce flows of energy from (and to) the grid, and understand how different forms of storage play together. Basically we get to burn less stuff and stay comfortable.

Will a heat battery save lots of money?

No. I expect it to be roughly a wash on our bills. Every gas unit that we don't pay to import will be balanced by about one electricity unit that we won't get paid to export.

What are some other names for heat battery?

Thermal battery, thermal store, thermal energy storage, heat store, hot water tank. A hot water bottle is a (portable) heat battery!

What is the energy density of the UniQ?

The UniQ 9 unit is 365mm x 575mm x 815mm = 0.171m^2 = 171l, and stores 10.5kWh (see UniQ Heat Batteries Reference Manual V2.3). Note that approximately 1l of diesel holds the same nominal energy, but this excludes oxidiser and the machinery to extract that energy; energy can only be extracted not stored, and with a carbon cost. (See some other density calculations.)

2019-09-27: What, and Why Now?

In this article I describe the process of selecting and installing a (Sunamp UniQ) heat battery to take some load off the gas grid for domestic hot water (DHW) while trimming CO2. The aim is not simply to shift that load to the electricity grid. Instead this should reduce outflows from our PV geneneration otherwise spilled to grid.

This makes sense now that the typical electricity grid intensity in CO2/kWh is similar to or below that from burning gas in our boiler. Also, the Sunamp UniQ is considerably more compact than a hot water tank, and leaks much less heat too. If we can keep much of that DHW energy in the house everyone's a winner.

This heat battery has to play nicely with our other behind-the-meter Enphase AC Battery, and the off-grid storage.

Our boiler is a Potterton Performa 24 gas combi and does not accept pre-heated water. (Though must be good to ~25C since I've measured that at the kitchen cold tap.) Thus I would prefer a CombiSol type arrangement for auto fallback to combi for DHW so that we can never run out of hot water. I would like the option to tweak control settings to allow it to become a stand-alone top-up-at-night job if we do ditch the gas entirely.

I'd also like options to add extra control/override and monitoring, since the data and experience writeup is the primary reason that I'm doing this.

I estimated that PV diversion could cover most of our DHW needs (~4kWh per day) most of the year. That works better with storage of a more than a single day's demand. The storage can then carry heat into a less sunny day from time to time. (That implies at least a UniQ 6 with a bit over 6kWh of storage.) I'd expect us to be automatically almost entirely using gas for DHW Nov/Dec/Jan.

2019-09-28: Basics and Nice-to-Haves?

What are the basics needed?

  • Must not cause any import from electricity grid (unless otherwise overidden).
  • Must not hamper DHW delivery (eg annoy my family) even mid-winter. Ideally should improve flow rate and temperature stability while using UniQ.
  • Must not uglify the kitchen!
  • Must play nicely with the Enphase AC Battery.
  • Must fit into the slightly cramped space and non-vanilla (two generation feeds) system that we have.
  • Must allow the combi to be tested easily for servicing, eg isolate UniQ and allow combi to heat DHW directly from mains.

What would be nice to have, eg for better UX and grid integration?

  • A 'boost' button to manually allow us to top up enough for a bath, ASAP.
  • Indicator of how full store is.
  • Indicator of input and/or supplied energy to UniQ.
  • Ability to control/monitor remotely from RPi to gather stats such as DHW kWh and to try out more complex control strategies.
  • Ability to adjust 'spill' margin and delay before diversion starts.
  • Ability to adapt control to supply more DHW so eg ensures at least half full with wee-hours top-up from grid, maybe done slowly to reduce grid I2R (I2R) losses.
  • Ability to adapt control to supply all DHW if combi dies, so eg forced on when store nearly empty (maybe slowly at first to reduce grid I2R losses).
  • Autonomously stops diverting when grid frequency drops by near operational -0.2Hz limit, after a few seconds at most to be useful, to help avoid load shedding and other grid issues.
  • Ability to work alongside a possible future additional UniQ-based solution for central heating (CH) radiators charged at night.


  • It is not a goal to save money.

Some or all of the "nice-to-haves" are out of scope for an initial install. Avoiding making them hard later, if possible, is a goal.

2019-10-10: Space for the UniQ

kitchen boiler cupboard UniQ destination under combi

I took some bad photos with a tape measure in shot to establish the approximate depth (~60cm), height (~100cm), width (~55cm) available under the combi where the UniQ might live. It will mean giving up some valuable (goods) storage space (our house is small).

I don't yet have a design sketch. I did find some exciting reports of problems with UniQ 'e' models that won't recharge until either 50% or 90% depleted. Sunamp describes the reports as overblown. I am not troubled by them.

I can take a firmware update after install if need be. For my purposes it's not critical, it just reduces the utility of the store if it doesn't capture all that it could.

2019-10-19: No Design Sketch Yet

I'm still struggling to get any design sketches even though I have spoken to two installers and offered money up-front for the design itself.

2019-12-02: Design Sketch Ahoy!

James Ingram has very kindly produced some initial thoughts for me to digest. Possibly his friends Thermal Battery Solutions could help too.

One element of his suggestion to me is the following outline kit of parts and labour estimate:

System Estimate
ItemCost inc VAT (£)
UniQ 6 with element2100
Labour (2 men, 1 day)500

He also outlines some positives and negatives for the UniQ scheme:

  • Negatives:
    • High price relative to alternative: a similar main pressure cylinder setup would be cheaper (though fiddler to fit)
    • Weight: Uniq6 is 105kg, hence the need for 2 men
  • Positives:
    • Space saving
    • Low heat loss
    • New tech early adoption

James lists further ideas in his email (lightly edited), which I am working through the first part of here:

Sunamp UniQ 6 with electrical element.
Width 365 [mm]
Depth 575 [mm]
Height 605 [mm] (footprint doesn't change with model so upgrade to larger unit just requires additional height, that said if you've only 1m the 9 would be a squeeze at 815 [mm] as you'll need some room for pipework under combi)
Hot water cylinder equivalent 142l (volume at 40 deg C 185l) about 2.5 good baths or running your kitchen tap for very approximately 12 minutes (see https://www.mcdonaldwaterstorage.com/Hot-Water-Cylinder-Sizes)
Max flow rate 15 l/min
The UniQ controller is now situated in the unit itself (tech guide is out of date and lacking in a few ways).
There's now a power light and I think there may be a 'full' light, I'd need to check.
Minimum input pressure 1.5 bar but you should have that for your gas combi anyway. Might be worth checking at times of high demand, 6-8pm, also flow rate at that time.
That'll fit into your available space with additional plumbing, and should supply your DHW needs when fully charged (depending on usage, do you have data your DHW usage?)

In response to this I've said:

1) I don't think that we've ever had problems with water pressure or flow rate, not that I'd object to the bath filling a bit faster! Anything specific that I should check?

2) DHW usage is about 3 or 4kWh per day in gas demand, which I have going back some time, down to hourly resolution. We now only use gas for DHW and rads.

Eg here's a typical month (DHW gas usage by day) before the heating went on: http://www.earth.org.uk/data/16WWHiRes/gas/gas_daily_201909.csv

And here's quite a high demand day (over 6kWh): http://www.earth.org.uk/data/16WWHiRes/gas/gas_1h_20190928.csv

James replied:

1. It's probably fine as I mentioned the combi will have a similar minimum requirement. You could do an approximate cold flow rate at cold kitchen tap using a jug and time or flow rate gauge.

2. I was thinking in terms of litres as another way of looking at it but as we know the Uniq6 stores 7kWh of heat that shows it's suitable for your demand in terms if kWh plus giving potential dull PV day overlap.

Continuing to work through James' huge email, see below 2019-12-15...

Comments and opinions from others very welcome, as I work through this.

2019-12-08: Eddi Diverter API

The myenergy eddi looks like a good candidate for solar PV diversion that understands playing nice with a battery.

A few days ago I popped a query into myenergy's enquiry form:

Is it possible to extract data from the Eddi automatically over (say) Bluetooth or Wifi or USB, as I already do with an Enphase AC Battery and various other devices, for logging and analysis?

After some discussion, today I received a nice email thus:

Although we don't have an officially supported API at the moment, we do have an API which is in Beta and which a number of our users have managed to reverse engineer to very good effect.

Have a look on our forum at https://myenergi.info/api-f54/

And this thread in particular which will give you a good introduction to what they are up to https://myenergi.info/app-api-investigation-t53.html

In the future we do plan to publish and officially support the API.

I have asked some more questions:

1a) If I buy a unit before your API goes “official”, eg by Feb, will I nonetheless be able to tap into the device to get data, and is that likely to stop working eg when you go official?

1b) Is it possible to upgrade the unit in situ to your official API (I'm assuming that I can talk directly to the unit, not via your servers) later?

2) Is the hub or any other equipment required to use this (proto) API?

The myenergy forum is interesting, but from reading I see that comms would not be local as things stand:

Ah, I see from further reading that the API is to your central servers. I'd vastly prefer something local for a number of reasons, including connection and service reliability, and never imposing an unreasonable load on your servers.

I am able to locally poll my Enphase unit even when the Enphase central servers are unwell, and I don't feel bad about polling every few minutes.

Happily myenergy responded at 8am (on the 9th)!

  • You can access the data now using the (unsupported) API.
  • The only significant change that we envisage before supporting the API will be to require that user have a key which we will issue when they register
  • The API will continue to be developed with new API calls
  • Yes – you need the myenergi Hub
  • At the moment all calls are routed via the myenergi server. To provide local access directly to the hub requires additional hardware and firmware development
  • You are OK polling our servers every few minutes

2019-12-15: Design Sketch Continued

Continuing to work may way through James' email, see 2019-12-02 above...

Sunamp recommend a quality limescale inhibitor as a must and advise using a Hydroflow h38 (see https://theintergasshop.co.uk/scale-inhibitors/1130-hydropath-hydroflow-hs38-electronic-water-conditioner-hs38a.html).

Do you have a water softner or anything similar already?

We’d also need to fit a small expansion vessel/shock arrestor in the system.

So no, we don't have a water softener (our water is not hard).

The HS38 seems a little 'magical' to me. It seems unlikely that anything using 1W and on the outside of a decent copper pipe can do anything meaningful to what is in that pipe, and I'm not seeing linked reports on the product page that would make me change my mind!

under combi 20191215under combi 20191215


A photo of the cupboard space below the boiler would be useful.

James continues:

Linking it up to your combi boiler is straightforward via a solar-combi diverter valve. This will enable the gas combi to cut in once the store is depleted.

[There are a] couple of diverter valves available on the market. These pretty much fulfill all the functions in that basic schematic I sent you.

  • Grant CombiSOL
  • Instasol Combi

The Instasol outputs 28c to the boiler and I'm not sure if that’s adjustable, output is 35-55. The Combisol is 24c – 49c so perfect (that 49 will more than likely be adjustable). It’s also cheaper.

There’s potential to make up these valves from basic parts to keep cost down but the all in solution seems convenient.

new consumer unit going in during 2010 PV system upgrade

Then he says:

On the electrical side, without much research on the other PV dump options it pretty much looks like the EDDI will do a lot of what you’re after (at a price).

I’ve got an iboost+, it’s pretty basic and has a noisy fan, it's 2 output, does what I want but looks very basic compared to the EDDI functionality.

That’ll give you export only dump, boost, timed boost, input level control, a 2nd dump circuit and quite a bit more, also looks upgradable perhaps for future grid input/output activity.

All you need is a spare way on your consumer unit, for a new circuit to feed the EDDI and Uniq, worst case scenario we could use the combi boiler spur supply as long as it doesn’t overload that circuit.

What distance is it to your CU?

It is ~8m from the CU (there are two spare ways) to the boiler location, around the outside walls of the kitchen.

As to my possible expansion to cover space heat in future:

Expanding it to do your heating at some point.

It’s possible but depends a lot on your load, it would be pricey unless the heat batteries drop significantly in cost over time.

A mix of several larger Uniq units and a small electric boiler would probably be the way to do it. An ASHP using a Uniq unit as a buffer might well be more energy and cost efficient.