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

Updated 2021-03-22 08:00 GMT.
Solar hot water, low carbon. #solar #DHW #PV
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!

Will this cover all hot water use?

No. This will probably 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 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.