Earth Notes: On Solar DHW @Home, UniQ and PV Divert

Updated 2022-01-17.
Solar hot water, low carbon. #solar #DHW #PV
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 in summer. 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. In winter, charging the thermal battery from the electricity grid will be significantly more expensive than using gas.

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!

Will this cover all hot water use?

Nearly. I hope to fairly aggressively charge at night in the winter when grid intensity is lower than direct use of gas (which should get easier over time), and so get almost all our (~1MWh/y) DHW use without the combi firing up.

(As of 2020 this was a "no". This was intended to cover DHW for about 75% of the year, not by coincidence when electricity grid carbon intensity is low, lower than burning gas directly. (This aims to shift ~800kWh of ~3200kWh per year of demand away from gas.) The upshot is that in winter our combi will still be providing our space and water heating, but should otherwise get to slumber...)

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 2018-07-19 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 from this diesel not stored as new diesel made from my PV electricity, so diesel has a nasty one-way carbon cost. (See some other energy 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 generation 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 the 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.
  • 2020-01-26: Ability to monitor various water flows, eg to see how much DHW from UniQ and how much from combi. Would help with estimating when top-up useful too, and actual energy efficiency.


  • 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 element 2100
CombiSol 240
Hydroflow 160
Eddi* 400
Sundries 100
Materials 3000
Labour (2 men, 1 day) 500
Total 3500

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
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:

And here's quite a high demand day (over 6kWh):

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 of 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 myenergi 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 myenergi'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

And this thread in particular which will give you a good introduction to what they are up to

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 myenergi 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 myenergi 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

I'm continuing to work my 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

Do you have a water softener 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).

According to the Thames Water tool for our postcode, we are in area "KINGSTON SOUTH" and our water is in fact hard at 257ppm of Calcium carbonate (CaCO3).

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 20191215 under 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 James points out:

Only a short run then (and potentially easy if behind kitchen cabinets?). It would be good to put it on its own 16A MCB or RCBO.


The Eddi could potentially go anywhere along [a new] radial run.

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.

2020-01-03: Avoiding Scale Build-up

While I didn't think that our water was hard (though it seems that I'm wrong), I am aware that we do have the odd scale mark (eg in the bath tub), and our boiler was very different after its heat-exchanger was descaled once!

Given my uneasiness about the HS38, I asked on buildhub if anyone knew of anything to support its functionality, or for alternatives.

For salt-based softeners, Harvey and TwinTec were recommended. The Harvey is of the order of £900 to buy, it adds sodium to the water so probably best not used for drinking or cooking, requires regular replacement of salt blocks, and takes up a significant fraction of the space of a UniQ. Costs may be partially recovered in increased heating efficiency for hot water, and in the dishwasher and washing machine if downstream. Soft water may gradually undo any scale already laid down. The Harvey does not require electricity.

An alternative suggested is the Combimate which is a phosphate doser, so does not soften, but stops more scale being laid down. At ~£170 and small and with a single annual maintenance step, this seems more in line with my available budget and space. Again, I wouldn't route water to the kitchen cold tap via it. This does add polyphosphates to the water, which, like the salt of (say) a Harvey, has an environmental cost and probably makes waste-water treatment harder.

2020-01-05: DHW After The Combi

Ruminating on what we might do to replace the combi entirely, considering only DHW for the moment, and excluding the possibility of AHSP for the moment (cost, planning and logistics complications)...

I found an instantaneous water heater buying guide which suggests a number of power ranges to support different DHW uses. (It notes that these depend on incoming water not being very cold.) The article's scale ranges from 3.7kW to supply a hand basin, through 5.5kW for a kitchen sink, up to 11kWh to supply a bath. Our current combi is notionally 24kW, and a bath can be run at reasonable lick.

The ~3kW (2.8kW at 230V AC) element in an empty UniQ running as a surrogate instant water heater is clearly going to struggle to do a good job even for the kitchen sink. And running a bath from that would be miserable. So the aim would clearly be to anticipate demand and (eg) top up overnight on low-carbon electricity in the winter months (or otherwise) when diverted PV won't cut it. The 3kW of the Sunamp will not strain our electricity supply. 11kW might, especially if other big users are running at the time such as the oven or kettle or dishwasher.

Looking at this as a fallback, for example the 9.5kW Powerstream Eco Instant Hot Water Heater costs under £120 2020-01-05 (RRP apparently ~£290) and is 160mm x 370mm x 84mm (H,L,D). (Manufacturer's page says A single Powerstream will supply 1 or 2 hand wash basins, or a basin and shower, when installed with the optional RM1 manual mixer accessory. They are not suitable for kitchen sink use, other than for hand washing purposes, and should never be fitted to supply a bath and the manual state The unit MUST NOT be fitted to any type of THERMOSTATIC mixer valve or tap.) Something of that ilk is potentially a cheap backup to provide at least a decent hot shower with no storage losses, if storage runs empty. A device like this may also accept a hot feed direct from the Sunamp, simplifying the plumbing considerably.

Maybe better, at 11kW, though still not claiming to be enough for more than a shower with ~3l/min flow at a lift of 30°C that we might want in winter, Strom SEIH11KTS1 Electrical Touch Water Heater Instant Water Heater 11kW is ~£250 on 2020-01-05, size 385 x 260 x 88mm. This one still not suitable for pre-heated water or thermostatic mixer valves.

Aquahot responded to my enquiry to suggest a model that will accept pre-heated water Maximum inlet temperature of 70°C is suitable for use with pre-heated water from solar heating systems: Zip CEX-U Undersink Instantaneous Water Heater 6.6 - 8.8kW £320 2020-01-06 294 x 177 x 108mm (H x W x D), though again not suitable for thermostatic mixer valves downstream. In this case it would not matter as the UniQ output could be routed directly through with no TMV required. Any UniQ-warmed water that was in the unit would however cool to room temperature, so this is an extra source of loss.

Also worth considering is the Stiebel Eltron DHC-E 8/10 instant water heater, see a thread on BuildHub.

No additional instant heat may be required in any case, given low losses from a UniQ 6 of 700Wh/d, ie max ~250kWh/y, or ~25% of total DHW demand, or roughly £50/y currently. In any case, with some extra feedback from the UniQ, eg the <50% full sensor, it may be possible to do without the combi with the simplest version something like:

  1. Divert spare PV generation until full. (Basic functionality.)
  2. Top up for up to ~1h in the small hours to put up to ~3kWh in the store. This will force some imports even in summer, but ensure that some DHW is available every morning first thing. No enhanced UniQ feedback required; doable with Eddi programming. Better: extend top-up time in winter and/or top up otherwise until 50% full, thus no top-up at all if low use and/or diversion sufficient.
  3. Manual 1h boost at any time. (Can be done from Eddi front panel.)
  4. Top up when <50% full outside 4--7pm peak. (Would need UniQ status output.)
  5. Top up when <50% full entirely locally forced by UniQ controller. (Would need UniQ control and an extra DPDT relay, but nice and simple.)
  6. Top up (slowly — 1kW max?) when near empty, at any time, if combi removed (or grid intensity low), to always have some 'instant' DHW. (Would need UniQ status output.)

Without extra UniQ feedback to force a top-up when in danger of running out, or UniQ local ability to force direct top-up at <50%, the overall system would be considerably less pleasant than the current combi 'always available' behaviour. (Auto top-up at <50% would allow a full bath to be run on demand, with recovery time of somewhere over an hour, which would be fine for 16WW; this would limit PV diversion to ~3.5kWh/d, ie the 'top' 50%.)

(When diverting PV, the system could dynamically — and smoothly! — adjust the amount spilled to grid before diversion. From (say) 50W minimum near noon when the grid may be full of spilled PV, or when all is good and the UniQ is empty to avoid fighting the Enphase AC Battery, up to (say) 500W when the tank is nearly full and/or grid carbon intensity is high to help out the grid when short of RE.)

It seems as if it would be useful to run several parallel mains cables from the CU/diverter to the UniQ in one go to save extra work later:

  • 240V 16A+ cable for UniQ control (plus possible local top-up supply, plus space-heat circulation pump, etc) if existing boiler supply is not suitable.
  • 240V 16A+ cable for diverted power to UniQ 6 for DHW.
  • 240V 16A+ cable for future diverted power to separate UniQ for space heat.
  • 240V lower-power cable primarily for signalling between DHW UniQ and diverter, eg a "getting empty" feedback to force diverter 'boost'.

2020-01-20: Schematic c/o Sunamp

Unvented cylinder and Combi pre feed with solar Schematic 002 p2 SA
Schematic for diverter, UniQ and gas combi boiler in correct configuration for 16WW. (Extract from Sunamp PDF, with permission.)

2020-01-26: Likely Formula

I am now reasonably convinced that a solution to do the core opportunistic DHW diversion from PV can work and should be based on the formula:

Eddi + UniQ6 + CombiSol = ~75% of DHW from PV

2020-02-03: Simple Gas Space Heat Replacement

A thought experiment, once the DHW is souped up not to need combi fallback, so I could rip out the combi and disconnect from the gas mains:

  1. The council is determined to pull our place down in a few years, for redevelopment.
  2. We have already greatly reduced gas consumption, from ~9MWh/y (2007) to more like 3.5MWh/y (for a family of four now) of which ~1MWh/y is DHW by my estimates.
  3. Thus there is ~2.5MWh/y of space heat demand over winter.
  4. It would likely be a tremendous pain getting an ASHP installed for reasons, including planning. I'd like to do a heat pump if I sensibly could for many reasons.
  5. Highest mean recent gas demand was 28kWh/d over Jan 2017 with a HDD (12C base) of 248, the highest over several years. Probably 24kWh/d was space-heat.
  6. I want to stop burning stuff, eg gas. I'd like to be grid friendly, eg move demand away from peak.

Why shouldn't I replace the combi gas boiler with a 3kW Willis heater (or possibly two in parallel, one set to a much lower temperature, mainly to help with eg morning start) and circulation pump, guaranteed NOT to run 4pm to 7pm (would probably be enabled 6am to 4pm and 7pm to 10pm ish, called on demand by individual Radbots) and switch to Octopus Agile or similar? Carbon intensity probably up a bit in the short term, OPEX costs probably similar to Ecotricity (eg matching SVT locally, no gas needed any more), CAPEX costs very low. System complexity fairly low. Uses more electricity than strictly necessary, which is a clear downside. But upfront (embedded) carbon of ASHP might not be recoverable before house is pulled down anyway.

The rough mean GB grid CO2 intensity for December and January full months was ~220gCO2e/kWh. A little above the notional 190gCO2e/kWh for burning gas. But my official gas boiler efficiency (SEDBUK) is only ~80%, so probably very comparable if I were to put together the Willis system well.

(2020-02-05: an interesting Twitter thread on GB electric heat, EVs, demand diversity, and how the grid will cope.)

(2020-02-07: in response to my considering an electric space-heat system with/out storage and with/out HP, it seems that current pricing rewards just avoiding 4--7pm rather than soaking up the wee hours. We're not rewarding ToU #exergy and #storage yet? a tweet from Greg Jackson Octopus founder: ... sadly the way the energy system works makes "sensible" structure uneconomic. There’s currently no "discount" to us for using empty distribution network at night. Solving this would be HUGE.)

2020-02-09: Dear #EnergyTwitter, for the last ~5 years of the life of this house would it be better to keep the current ~80% efficient gas combi for ~2.5MWh/y of space heat, or switch to low-capex direct resistance, assuming a heat-pump with CoP ~3 is not reasonably do-able?

Return Willis MVP

2020-05-21: rather than completely replacing the combi, it seems to me that I could put upstream of it, ie in the rads return pipe, a Willis heater (~£40) set to a slightly higher temperature than the combi's radiator flow temperature. Not so high as to risk damaging the combi. This should then prevent the combi from ever firing if the Willis is on and supplying enough heating power. This gets the benefit of the existing combi's pump, and an immediate automatic fall-back to gas if needed.

The Willis would only be allowed to be on when electricity intensity was lower than gas intensity, and outside of peak electricity demand times, and when there's actual demand being called by Radbots. Maybe the boiler would fire briefly before Willis-warmed water would get to it. Maybe I could even generate a separate advanced call for heat to the Willis so that that effect would be minimised.

This could be cheap and simple enough to do alongside the DHW work, and give me an option to shade over from gas at least in part, whenever good from a carbon point of view.

The Willis could be supplied from the second heating element output on the Eddi, which could also limit power if other big electrical loads are running, such as cooking. This could then all be enabled/disabled remotely from my RPi for the carbon-intensity and grid-peak logic, and even the Radbot heat call. That should also allow tracking of the space heat kWh covered by the Willis.

Taking a sample of plausible heating hours in December 2019 (6am--10pm, excluding grid peak 4pm--7pm) with grid intensity under 190g/kWh, suggests that maybe 10% of all hours (~18% of heating hours) would have worked:

% gzip -d < data/FUELINST/log/201912.log.gz | awk '$1 ~ /T((0[6789])|(1[0123459])|(2[01]))/ && ($2 < 190)' | wc -l
% gzip -d < data/FUELINST/log/201912.log.gz | wc -l

Sample of first eligible slots in 2019-12:

2019-12-06T06:00Z 184
2019-12-07T06:10Z 171
2019-12-07T06:20Z 172
2019-12-07T06:30Z 174
2019-12-07T06:40Z 179
2019-12-07T06:50Z 180
2019-12-07T06:55Z 182
2019-12-07T19:40Z 187
2019-12-07T19:50Z 185
2019-12-07T20:00Z 182

(GBF thread.)

2020-03-31: On Hold

Given the coronavirus lockdown this project is on hold for now as 'non-essential'. It may not happen for a year unless the lockdown is as short as 3 months, in order to be able to catch some of this summer.

The vote for regeneration of the local estate was 512:190 in favour, so this house will almost certainly have been pulled down within a decade. That means any sort of heat-pump is unlikely to be financially viable — or a lifetime carbon saving — as a replacement for the combi.

2020-05-03: Year-round Top-up Thought

In order to minimise DHW carbon footprint through the year while allowing fallback to the combi when the UniQ becomes empty, I could force an electric top-up to the store whenever:

  • grid carbon intensity is significantly less than burning gas directly,
  • the UniQ is (say) less than half full,
  • the grid is not around peak demand (eg 4pm--7pm or 'red'),
  • house demand is not currently high (eg from cooking).

That 'half-full' threshold could be cranked down if lots of PV generation is anticipated soon, so as to leave space for it, or cranked up when little PV spill is expected such as in mid-winter.

If this causes some net imports in summer this will still have saved carbon.

This could attempt a slow top-up (eg 1kW) most of the time, and crank up towards max (3kW) as the UniQ empties, to reduce the need to revert to gas, while keeping grid load low and slow when possible.

If tuned right this should mean that the combi almost never needs to run, paticularly as the grid gets greener.

This may not be the best use of green grid energy until we have too much of it (eg regular negative pricing or similar). The algorithm could change to top-up whenever pricing is zero or negative instead, to help absorb excess.

2020-09-18: No Green Voucher For You!

It seems that none of this work is likely to be eligible (at least as primary measures) for the government's £5000 Green Homes Grant voucher scheme.

2021-03-21: DC Dump?

I'm sure that there's something like 500Wh to 1kWh per day in the sunniest six months going unused in the off-grid system from the ~550Wp of large wall-mounted panels.

Perhaps, when the off-grid battery is full, or for any other reason they are being allowed to float to Voc (open circuit) ~37V by the solar controller, that could be tapped.

If the raw output from the panels were to be fed to a nominal 240V 3kWh heating element (~19Ω) it would draw ~70W or ~2A. For 12 hours that would be ~800Wh.

Something as simple as the low voltage dropout that I currently use for my NiMH battery charger, but adapted to trigger just above the maximum power point (Vmpp) of the panels (~28V), could do the trick.

That could then be (say) fed to a UniQ3 upstream of the main UniQ6.

But considering the likely energy available, maybe a well-insulated Willis heater either side of the UniQ6 would work.

Maybe even more simple, would be to power something like a Nikou Car Kettle- Portable 750ml 24V Travel Car Truck Kettle Water Heater (~£26 ex-VAT as of writing) with Auto cut off power when water is boiling or Atyhao 12V / 24V 420ml Car Electric Kettle Travel Tea Mug Water Heating Cup Bottle Holder Vacuum Flasks Thermoses (~£21 incl VAT) to keep water hot for tea and coffee off-grid.

I could experiment by monitoring how often such a raw ~28V dump triggered, and how much power it drew. I could add a voltage monitor for the raw PV DC, analogue to an ADC, or capture it via the Morningstar controller.

2021-07-11: Brain Dump

Still not installed. Still planning to get on with it.

Given the observations in Do people with PV panels consume more electricity?, especially the point about energy stores getting full before noon, I might do all of these before (say) 11:00Z and/or when grid intensity is high:

  • Widen the spill to grid (say) from 50W to 250W before diverting.
  • Set a maximum level over which the store should not fill.
  • Set a maximum power at which the store can fill.

All of those help with grid management and making best use of PV generation from a grid point of view. All can be gradual rather than binary. High intensity can be 'red' and/or when over the 190g/kWh for burning gas.

I note that @Zapaman says:

Before the entire Sunamp(s) setup it was ~4.5kWh/d gas. Last summer with Sunamp iPV but no ASHP it was ~1.65kWh/d gas. So yes consistent with a fairly poor boiler efficiency I would estimate.

This was, from other figures, replacing gas kWh with electricity kWh roughly one for one.

2021-09-16: Waste Water Heat Recovery

This month I have replaced my off-grid battery storage and a second grid-coupled Enphase AC battery.

Having gotten those out of the way, I'd like to restart the planning for the UniQ installation.

I may wish to try putting the phosphate dosing device in first, to separate out that complexity and effort and experience.

I am on a webinar today hosted by the SEA (Sustainable Energy Association) presenting results for WWHR (Waste Water Heat Recovery), particularly considering how this interacts with heat pumps.

So maybe I should include WWHR on the existing boiler feed right ASAP to recover heat from shower use (ie when the boiler will be drawing water while hot water is flowing away).

Quote from the session: We tend to shower at about 41°C and get uncomfortable below that.

Things to consider for viability fo WWHR:

  • Estimated fraction/kWh DHW for showers for us, possiblt recoverable
  • The likely remaining lifetime of 16WW as a building
  • How easy it may be able to get to the bath/shower waste to install
  • How easy it may be able to get to the bath/shower waste to install
  • Capital/install costs of WWHR

We are not a two-long-showers-per-person-per-day household! It seems that between us we maybe only take ~5 showers per week. (Some of us prefer baths for example.) A suggested figure of 2.5kWh/shower (pulled up from the Intarwebs) would imply ~650kWh/year on showers; not all would be recoverable. Note however that avoided gas consumption with the UniQ in pure diversion, assuming ~4kWh/d DHW covered for 6 months of the year is ~720kWh, so WWHR should not be written off prematurely. WWHR should be effective all year and may possibly be more effective in winter with a higher differential temperature between waste water and incoming mains helping heat exchange.

Note that sust-it suggests lower energy per shower, with mid-table and a shower per day coming to ~550kWh/year.

For reference the Showersave Vertical System (QB1-21 / QB1-21C / QB1-21D) seems to retail at somewhere around £500 excluding VAT. Claimed around 60% efficient, assumed to mean 60% heat recovery.

2021-09-18: Carbon Diversion

I just did a quick and dirty check to see when grid intensity has been lower this year than gas per kWh, ie when it would make carbon sense to divert PV generation to a heat store via immersion rather than let it spill to grid and reduce emissions elsewhere.

Picking out samples from 10:00Z to 19:59Z (when we might have some sun), and GB grid intensity <190gCO2e/kWh from the start of 2021 gives:

% cat data/FUELINST/log/live/2021*.log | awk '$1 ~ /T1/ && ($2 < 190)' | head
2021-01-16T10:10Z 162
2021-01-16T10:15Z 165
2021-01-16T10:25Z 169
2021-01-16T10:35Z 172
2021-01-16T10:45Z 173
2021-01-16T10:55Z 177
2021-01-16T11:05Z 178
2021-01-16T11:15Z 177
2021-01-16T11:25Z 174
2021-01-16T11:35Z 174
% cat data/FUELINST/log/live/2021*.log | awk '$1 ~ /T1/ && ($2 < 190)' | tail
2021-08-13T17:55Z 179
2021-08-13T18:05Z 179
2021-08-13T18:15Z 180
2021-08-13T18:25Z 181
2021-08-13T18:40Z 185
2021-08-13T18:45Z 186
2021-08-13T18:55Z 187
2021-08-13T19:05Z 189
2021-08-13T19:15Z 188
2021-08-16T10:05Z 189

So not since mid-August at all given the UK's long wind drought!

If I'm happy to pre-charge the heat store from the grid when intensity is low, things are marginally better!

% cat data/FUELINST/log/live/2021*.log | awk '$1 ~ /T[^1]/ && ($2 < 190)' | tail
2021-08-31T03:45Z 185
2021-08-31T03:50Z 185
2021-08-31T04:00Z 184
2021-08-31T04:00Z 184
2021-08-31T04:05Z 184
2021-08-31T04:10Z 183
2021-08-31T04:15Z 184
2021-08-31T04:20Z 185
2021-08-31T04:25Z 187
2021-08-31T04:35Z 189


% cat data/FUELINST/log/live/2021*.log | wc -l
% cat data/FUELINST/log/live/2021*.log | awk '$1 ~ /T[^1]/ && ($2 < 190)' | wc -l
% cat data/FUELINST/log/live/2021*.log | awk '$1 ~ /T[1]/ && ($2 < 190)' | wc -l

2021-09-27: Progress

Some progress!

Following recommendations from PeopleWhoKnow (I'll save their blushes) I put together a list of a few recommended installers. Thank you both!

I'm on the waiting list to be considered by one installer from a few days ago. Another one called me this evening and is happy in principle to do it. He may visit informally for a pint and a discussion soon.

2021-09-29: Estimate

More progress! I have an estimate in today, along with some practical ideas. Not a million miles from the Before Times estimate...

2021-10-04: Combimate

I'm going to order and have installed ASAP a phosphate dosing (not softening) Combimate. We can get an idea of any pros or cons independent of the rest of the works.

It took many attempts to get exactly one of the right thing in the basket! Someone really needs to fix this UX for a 2-SKU site... Combimate Limescale Prevention Device and Combiphos Starter Pack, 15mm (COM1), £146.00 + £29.20 VAT (£175.20 total before delivery). (Delivery charges are mentioned in very small faint type...) I finally gave up with the on-line system and phoned. There's an extra £10+VAT for delivery.

I am told that a Combimate is made up to order and should be with me next week.

2021-10-11: arrival and fitting

20211011 Combimate during installation under kitchen sink

Combimate has been delivered: neither huge nor heavy!

Our favourite sensible builder has arrived to fit it ! I have paused the dishwasher mid-cycle while the fun happens, and indeed it has been powered down and moved out of the way...

Note that everything except the kitchen (cold) tap is being routed via the Combimate. So the kitchen tap remains direct from the mains, as it is where almost all our drinking/eating water is drawn.

Fitting all the pipework in the confined space under the sink took hours, about 4.5h in total! We also have a belt-and-braces pair of in-line cut-off values and a bypass valve, so we can take the Combimate out of line if we decide that we don't like it for any reason.

Meantime we can keep an eye out for our water behaving less 'hard'.

To-do: the dividing tray was left in place (as if for single-appliance use). That will need to be removed and the full 800g of Combiphos loaded...

2021-10-13: Visit

I had a visit today from a possible installer. We worked through the scheme and checked a few measurements, and have not found any huge barriers yet. This chap can't do the Part P electricals, so I may try to get those done in advance.

My visitor very kindly did a water hardness test at the kitchen tap (bypassing the Combimate) which suggested 220ppm Calcium carbonate (CaCO3). At the bathroom tap (via the Combimate) it was maybe showing 220ppm. Though i don't know if anything that the Combimate does should make a difference to this test. (The test involved titrating 10cm^3 of tap water with a drop at a time from a reagent bottle, at first pink, then going blue when enough is added, 20ppm per reagent drop.) Thames Water's on-line tool suggested 257ppm, so this is in broad agreement.

I may be seeing some signs of Combimate changes, eg soap lathering better in the bathroom. But nothing dramatic.

2021-10-17: Control Thoughts

I continue to ruminate on how best to manage the heat battery.

External variables that I may wish to watch include:

  1. Time-of-day (eg for peak demand times ~4pm to 7pm weekdays).
  2. Grid carbon intensity (current/forecast).
  3. Weather/insolation forecast.
  4. Mains water temperature (from monthly data) to estimate lift energy needed.
  5. Current SoC (State of Charge) of other energy stores (Enphase, maybe off-grid) to more effectively prioritise.

Control parameters that I may wish to set for the Eddi to manage in real-time, or slower via my control software (with a suggested range):

  1. Maximum power delivered to UniQ ([0,3000]W).
  2. Maximum SoC of the UniQ to allow further UniQ charging, slow via software ([0-100]%).
  3. Maximum total power drawn by house to allow UniQ charging ([0,4000]W).
  4. Minimum spill to grid power to allow UniQ charging ([50,500]W).

For example, in the 4pm to 7pm peak demand slot, max UniQ SoC or max power to UniQ could be clamped to 0. (And possibly also the max power drawn by the house, for good measure.)

Being slightly more clever it may make sense to actually boost UniQ max SoC just before the peak-time window, though maybe keep the whole-house power low, to encourage topping-up when grid demand and wholesale cost are often reduced. That helps avoid the grid having to ramp generation steeply. The Enphase Envoy-S on firmware D5.0.34 will not try discharge to support loads for ~4h before 4pm (with a peak time tariff set from 4pm to 7pm), so preloading the UniQ from grid then would not steal Enphase charge.

The initial intent is to set the system up 'vanilla' to heat the UniQ when a default (~50W?) spill is exceeded, up to the maximum 3kW UniQ load if the spill/excess would allow it, without watching any of these external variables.

After tweaking parameters for managing diversion so as to minimise carbon, but still relying on the combi as a fallback, the next step may be to start partly topping up from the grid when intensity is (much) lower than direct gas burn, eg in the wee hours. That will be expensive, with each gas kWh being replaced by an electricity kWh costing ~5x more. It would be good to prevent the Enphase discharging at these times, but that is not yet possible. An alternative workaround might be to charge the UniQ at maximum rate (3kW) to minimise the fraction drawn from the Enphase. Another might be to only pre-charge the Enphase overnight when the Enphase batteries are already empty.

The last stage in this madness could be to manage the UniQ so that the combi is never called on, ie the UniQ is kept full enough to cover reasonable demand. The no-combi version of the peak-time lock-out above might be:

  1. Maximum power delivered to UniQ: 1000W to only allow slow top-up and low demand from grid or reduced spill to grid.
  2. Maximum SoC of the UniQ to allow further UniQ charging, slow via software: 25% to be enough for washing up or a quick shower but not a full bath.
  3. Maximum total power drawn by house to allow UniQ charging: 2000W to basically cut out UniQ charging if there are other big loads such as cooking to limit grid load.
  4. Minimum spill to grid power to allow UniQ charging: 150W to dynamically default to spilling more to grid to reduce grid intensity.

2021-11-26: Thermino

Now that the Thermino is on the horizon (same price and physical size, greater storage capacity), I'll likely be going for one of those rather than a UniQ.

The Thermino PV 150 seems to be the equivalent of the UniQ 6, but I might be able to squeeze in the new intermediate size 180 instead.

2021-12-13: Yet More Thinking

I treat burning gas for heat as 190g/kWh, though it is likely higher for us.

Taking December so far there has been plenty of electricity below that (27%), and enough well below that (~11% below 150gCO2/kWh) to cover any storage losses. The latter is enough to capture ~4kWh (half the Sunamp capacity and a day's average use) at ~2kW. That would reduce I^2R losses, and is well below the maximum that the 3kW element allows.

(I'd like to have the Eddi stop importing to top-up at somewhere below 3kW to save some I^2R, but enough above or below 2kW to allow the top-up load to be distinguished from (say) the dishwasher.)

% cat data/FUELINST/log/live/202112??.log | awk '$2<190' | wc -l
% cat data/FUELINST/log/live/202112??.log | awk '$2<150' | wc -l
% cat data/FUELINST/log/live/202112??.log | wc -l
% cat data/FUELINST/log/live/202112??.log | tail
2021-12-13T16:45Z 278
2021-12-13T16:55Z 278
2021-12-13T17:15Z 279
2021-12-13T17:25Z 279
2021-12-13T17:55Z 278
2021-12-13T18:15Z 285
2021-12-13T19:35Z 293
2021-12-13T20:00Z 296
2021-12-13T20:25Z 301
2021-12-13T20:35Z 301

Most of it is in the wee hours.

So a simple carbon-saving top-up-from-grid algorithm (mainly for winter) might work, ie when all the following are true:

  • if the time is between approx 0:00 and 06:00 (or grid demand is in the bottom quartile?)
  • if the tank is less than half full
  • if the live current grid intensity is available and below 150gCO2/kWh
  • if the Enphase AC-coupled battery is empty (to avoid stealing/churning energy from it, and also as an indication of poor solar generation yesterday and thus likely today)
  • if there is a poor solar forecast for today

2022-01-06T16:00Z: an oddity: the grid has just gone relatively 'green' now, which is unusual for entering the 4pm to 7pm peak-demand period.

For the record: Effective grid carbon intensity for a domestic user is currently 190gCO2/kWh including transmission and distribution losses of 7%.

As of the evening of 2022-01-17 I note that there are a decent fraction of the wee hours this month that have had low enough grid carbon intensity to bank:

% cat data/FUELINST/log/live/202201??.log | awk '/^2022-01-..T0[0-6]/ && $2 < 150' | wc -l
% cat data/FUELINST/log/live/202201??.log | awk '/^2022-01-..T0[0-6]/ && $2 < 950' | wc -l

Note here that I am looking at the first 7 hours of each day, to 06:59, UTC.

But there are many days in a row with no such low-carbon overnight spots, eg none after the 9th up until today the 17th, so 8 days out of 17, under 50%:

% cat data/FUELINST/log/live/202201??.log | awk '/^2022-01-..T0[0-6]/ && $2 < 150 {print substr($1, 1, 10)}' | uniq -c
     42 2022-01-01
     43 2022-01-02
     43 2022-01-03
     43 2022-01-04
     41 2022-01-05
     43 2022-01-07
     35 2022-01-08
     42 2022-01-09

Assuming that storage losses would be outweighed by our gas boiler inefficiency allows us more gas-free days, 12 out of 17:

% cat data/FUELINST/log/live/202201??.log | awk '/^2022-01-..T0[0-6]/ && $2 < 190 {print substr($1, 1, 10)}' | uniq -c
     42 2022-01-01
     43 2022-01-02
     43 2022-01-03
     43 2022-01-04
     41 2022-01-05
     11 2022-01-06
     43 2022-01-07
     43 2022-01-08
     42 2022-01-09
     18 2022-01-11
     42 2022-01-16
     13 2022-01-17

Allowing the battery to fill (to 100% rather than 50%) when the following day is (predicted to be) low sun and high carbon might gain a couple more days, ie get about 14 out of 17 or 80% days' DHW without gas, and reduced CO2.

2022-01-02: The Year It Gets Done

After years of delay, it's all going to happen this year, sure thing!

To maximise the chances of this happening smoothly, I'm simplifing things a little.

Here is a summary of where things are:

  • A Combimate has already been installed to deal with hard water.
  • David S, who knows his Sunamp stuff, has informally agreed to do the Thermino installation and plumbing round our combi.
  • Dave M will project manage, including organising electrical works.
  • Sunamp is putting together a Thermio 150 (or 180 if available) ePV, with data port available for additional monitoring.
  • Sunamp can provide other elements such as expansion vessel, NRVs if required, myenergi Eddi + Hub, from stock, possibly to all arrive on the same pallet.
  • The TMV that can slowly bleed heat from the Thermino into the combi if not hot enough to send directly to the taps, can be a Combisol or Intasol I believe.
  • The basic system schematic is as above.
  • The Sunamp unit will need to be installed under the combi within the cupboard unit, preferably retaining some of the shelving.
  • The Sunamp will need to be installed so that the %-full indicator lights are visible easily, eg by opening the cupboard door.
  • The Sunamp will need a local control supply (~3A max?) (and I may want to install an extra Raspberry Pi to talk to the Sunamp); it may be possible to use the existing fan/boiler spur, else a new supply cable will need to be brought round from the consumer unit.
  • An Eddi and Hub will need to be installed near the consumer unit and wired for power.
  • The Hub needs physical ethernet to the Internet router at all times, and connects to the Eddi through wireless (not WiFi).
  • The Hub could be deployed in the cupboard next to the router, so no wires need be routed through walls for it.
  • A cat 6 Ethernet (or RS423 serial cable) will need to be brought from the existing Raspberry Pi / Internet router to near the Sunamp unit.
  • A 4kW-capable cable will need to be brought round from the Eddi output 1 to drive the immersion heater in the Sunamp unit.
  • It may make sense to bring a second 4kW-capable cable from Eddi output 2 to the boiler cupboard to allow future expansion.
  • It has to be possible to safely isolate the Sunamp so that the combi can be tested independently eg for DHW delivery on demand even when the Sunamp unit is charged and able to supply DHW directly. This should also allow the Sunamp unit to be removed if necessary, while allowing heating and DHW to be provided from the combi.
  • All these works (Sunamp, cabling, etc) should not be visible in the kitchen unless looked for, eg by opening appropriate cupboard doors.


Physical dimensions of some key items:

Sunamp Thermino 150 ePV
h x d x l: 640mm x 365mm x 575mm, weight 139kg; 180 would be slightly taller and heavier; manual [PDF]
myenergy Eddi
220 x 205 x 87mm (excluding wall bracket), weight 4.3kg (excluding wall bracket); note antenna at top
myenergy Hub
100 x 100 x 38mm, the packed weight of the Hub (including ancillaries) is 200g as packed

Eddi to Sunamp Electrical Wiring

eddi wiring
Simplified partial wiring diagram (excerpt from Myenergi Eddi manual: thank you):
  1. Eddi
  2. Mains supply (230V AC from B16 circuit breaker in CU)
  3. Sunamp heat battery
  4. Sunamp heating element (actual rating 2.8kW at 230V)
  5. Isolator (20A double pole)
  • The Sunamp needs its own 3A supply (via 2-pole isolator) from local boiler spur, or brought round from CU.
  • Cable from Eddi to Sunamp heating element should be rated for 16A continuous. This is in the long cable run around the kitchen external walls.
  • Cable (also to be 16A continuous rated) from the second Eddi output is not to be used yet, but may be fed to a second Sunamp unit's heating element in future.
  • The cat 6 Ethernet cable should probably be alongside the 2x Eddi cables to the boiler cupboard for the majority of the route.