sharpener

A gas or oil boiler would have failed too. Admittedly to run a heat pump you will need a bigger battery to be useful (or @JohnMo's generator. I have got an interlock which turns my HP off automatically on grid failure, this gives me time to switch it into low power mode or decide to do without it. Also have kept the oil-fired AGA, and for the real extreme case I have one of my grandmother's Aladdin oil lamps.

Not sure I understand the rationale of this. Don't the mini-SoC targets mean that the battery is eventually charged to 100% mostly from PV at 15p per unit not exported rather than the grid at 12.65p (at present Cosy rates)? Why is this better? The battery is still cycled from 100% SoC so the depreciation is the same.

Told NGED (ex WPD) over the phone I had had more panels and a heat pump fitted and wanted the inverter power limit relaxed so it could drive the HP, and the export limit relaxed so I could export any surplus. They agreed subject to a new G99/G100 application but it took a long time to process and much chasing.

Before I managed to get the DNO to remove the export limit I considered how to do this, IIRC it involved holding off recharging the battery until midday so that when there is max generation there is room in the battery to take the excess. This runs the risk however that the predicted sun does not arrive. I could not work out exactly how to automate this but I think it must be possible on a Victron inverter with NodeRed, you might find it easier with your HA setup using solcast or similar. For me the DNO made the problem go away in the end, I assume in yr case that the 3.6 kW is a hardware limitation though.

I think it's more to do with the efficient area of operation of the compressor. It's easier to design a gas burner to work over a wide range of output. The lowest speed the Vaillant 12kW will run at is 27.2% which gives a turndown ratio of over 3:1 and that is typical of better HPs. A preference for matching the behaviour of a (20kW) gas boiler is unrealistic. In most cases it would lead to an extremely oversized HP, with attendant capital cost and inefficiency at low outputs. A longer coil will not help (much). However you do not need to limit yourself to a single tank, you can cascade them e.g. with a feeder tank in the loft space. If you were planning to scrap the existing one doing this might get you more capacity more or less for free. It will not save you capital cost now. It should break even on running cost now, and assuming the govt eventually does something about the cost structure will likely give you worthwhile cost savings in the future. As the cost of solar PV and battery systems continues to fall you will increasingly be able to run an HP off your own power plant. Given the demands on the grid, at some point in the future you may be refused an HP connection by your DNO so best to get in before the rush. As with many incentive schemes the BUS grant may not stay at £7k5 once its initial purpose has been served. You will be able to bask in the glow of the brownie points.

This may well result in infeasibly big rads whatever their construction. But the fancy column radiators do not have as much emitting surface area per unit frontal area as a conventional Type 22 or Type 33. Our installers designed for a flow temp of 45 and used a (conventional) rad size uplift factor of 1.9. This is a compromise with the partial ground floor UFH set to 35C, but a mixed system is going to have very big rads indeed unless you sacrifice some efficiency. As were were migrating from an oil system we are happy enough if it costs no more to run.

OP reads consumption off his solar inverter and 2.5-3.5 kW there would equate to maybe 7.5 - 14 kW thermal output depending on assumption about CoP so is broadly consistent. Can't think of anything other than that the fancoils are undersized so cannot get rid of the heat/coolth. It would be good to know how they were sized, and the floor area and construction of the house.

As others have said if you are already planning to get a bigger one it is perhaps not fair on attributing that requirement to the heat pump! The equation you need is 1 kWh = 860 litre-degrees. So it takes 10 kWh to heat a 215 l tank through 40 degrees. Or a 5kW HP will do it in 2 hrs etc etc. An MCS approved installer (not much of a qualification but you have to use one to get the BUS grant) will use the MCS sizing guide which is 45 litres x (no. of bedrooms + 1) so if yr house has 4 bedrooms that would mean a min of 225l, and with your usage I would go for at least 300. We have a 210 litre OSO cylinder and at 60C it will supply decent showers for two normal ppl plus 2 guests in quick succession. We usually have the HP flow set to 45 which gives just under 40 at the outlet which is enough when there are no guests. There are various claims for fancy tanks such as Mixergy or the new Heat Geek combi boiler substitute but they are mostly spurious and you will do best IMO in spending the money on a bigger conventional tank with a large coil. Have you tried the aerating shower heads which apparently give a full shower experience while using less water? You can download quite good free heat loss s/w from www.heatpunk.co.uk and there is also a quick heat geek cheat sheet which from your description would suggest maybe 65-85W/m^2 is appropriate but you need a more formal method than that. There is a new Heat Geek Zero Disruption process which may be suitable for yr house particularly bc of the existing 15mm pipework. The planning restrictions @SteamyTea refers to have been relaxed so you can put a HP within 1 m of the boundary and do not need PP for even a big twin fan unit like the Vaillant 12kW (which are slimline and extremely quiet). I would not want to install a HP above ground level, though it can be done subject to the manufacturer's requirements about safe access for servicing. HTH

Victron inverters have a Dynamic ESS setup that does something very similar which I tried for a while. As reported by many other people on their forum it had behaviours which had no obvious rationale so I gave up in favour of the very simple strategy upthread. There is not enough difference between Cosy's cheap rate import and 24h export tariffs for arbitrage to be worthwhile so I think KISS applies here. I will have another think when the heating season starts but I can't see anything significantly better atm.

At the present rates of exchange I think this is still the best strategy. It takes too much explaining to have OH run the washing m/c and dryer overnight if sun is expected the following day but ?not otherwise which (I think) has a very slight advantage in theory. I also have a zappi EVCS. With the Eco + setting and a boost timetable configured for the Cosy cheap periods it will then also charge at other times if there is a surplus being exported. At a marginal rate of 15p this is disadvantageous, so it is maddening that it involves remembering to set it to OFF before the battery is full in the morning.

We had two bc the first installers changed the spec on me and put the price up. There is a guidance document for installers on the OFGEM web site. From my copy of the May 2023 edition it seems applications may be made in retrospect, in which case yr installers should simply be able to do that and get the money.

Stand your ground! Unless the failure to give further information was due in any way to your not responding they will not have a leg to stand on if they take you to court. In my case OFGEM wanted to evidence that the wood burning stove was not part of the central heating system but were easily satisfied. My impression was that they struck a reasonable balance between the consumer and protecting the public purse. Did they tell you what information they were trying to get from the installers? Perhaps you can show it was already in their possession so they had no need to revert to you before replying to OFGEM? Also maybe ask OFGEM if they will entertain a new application as the old one has lapsed through no fault of yours. My boss once told me most disputes are won by the party with the better filing system.

I don't think you will get a useful amount of gravity flow through a HX. But 7kW HP will probably be OK with existing coil (based on my experience with a 12kW in Noise Reduction mode). You will get some improvement - if needed - by circulating the (potable) water with a bronze pump, so buy that first and try it before shelling out on the HX. You will need to fit a NR valve as well otherwise there is a backflow path through the pump when it is off.

If you install i.a.w. Vaillant's spec as above I don't think you will have a problem. Work by a third party will not invalidate the warranty if they are Vaillant approved installers - in practice most of their MCS installers are. I would go for wall bracket mounting though and I am not sure this is advisable for the 10 and 12 kW twin fan units as although very thin front-to-back they are quite tall and rock a bit on the isolating mounts. But they are extremely quiet.

Link upthread already. This takes you to the conclusions Nothing "goes wrong". The flow temp will get very high, but the HP does not object - though the CoP will suffer. As @JamesPa says, you can take quite a hit before that justifies the capital cost of fitting a different cylinder.