Dillsue

If you go for a larger than 3.68kw system but export limit it, you can draw what you want but just not export it. Save peeing around with a second off grid system and the headaches that could come with 2 differing systems.

?? You said you can export all your PV through one hybrid inverter so its AC output likely powers the house/grid and would have your FIT generation meter in the output?? Maybe you've got a FIT system inverter feeding the hybrid inverter via the FIT generation meter??? I seem to remember you've got a complex system??

Unlimited export?? Are you sure it's unlimited? Why wouldn't you need to involve your FIT provider if you've increased your TIC?

Depends is the honest answer. If currently on a combi then you'll likely need a new DHW cylinder. If your emmitters are predominantly radiators then there's a fair chance some or all of them will need replacing/swapping round or additional ones fitted. To get the best out of the ASHP system you want accurate heat loss calcs so the HP and rads can be sized correctly. Having excess solar is primarily a summer issue so will likely have little impact on the economies of running a HP predominantly in the winter. Finding someone who can design and commission a system for you may take some doing. My advice would be spend a good while researching how to get HPs to work efficiently and what constitutes a well designed/commissioned system before you start talking to suppliers. You'll be better placed to know if they know what they are doing!

For G98 compliance the inverter must be factory limited to 3.68kw and manufacturer certified as such. If it can do more but you limit it to 3.68kw then it's not G98 compliant. It then needs a G99 application plus a G100 supplement to cover the set up of the export limitation. My very limited experience of G99 is that it's straight forward and worth the effort to get consent for the max your local grid will support. Imagine export payments being £1 a unit and your neighbours had grabbed all the spare export capacity:)

There's a circulation pump for the secondary/cylinder side of your PHEX in the schematic you've not long posted????

Isnt that the same as heating an indirect cylinder via an internal coil?

No idea if cool energy provide this but on post 2020 LG Therma VS there's two inputs for thermostat control, one to call for heat and the other to call for cooling. Might be worth having a trawl through the cool energy manual to see if they provide similar

Thanks for that. Hopefully Dave can share how he's "powering" the cylinder side of his PHEX without a pump.

So PHEX I'd heating the DHW whenever the HP is running regardless of which way the 3 port is routing flow?? Intrigued to know how flow is achieved on the secondary side of the PHEX feeding the cylinder- gravity?? The coil we have available in our cylinder is undersized for our HP so looking at options to get HP energy into the cylinder without replacing it.

Doesn't the intelligent version of Go need Octopus to control your EV charger to get the very low unit rates? How would that work if you're adding your own, presumably uncontrolled, loads such as night time HP running?

It gets quite complicated and eveyones circumstances are different!! You don't automatically get paid for export as you need MCS or jump through a few hoops to get a non MCS system accepted for export. Either way there's ££s to pay. Without export payments then diverting to DHW could be beneficial. I specifically said intermittent PV as you can't run a heat pump on intermittent PV without a significant draw off the grid. Heating DHW overnight is fine if a TOU tariff works for you. We're on Octopus Go so we get 5 hours cheap overnight but pay a premium unit rate the rest of the time. We want to run weather compensation on the HP I'm putting in just now so for 19 hours a day we'll be paying premium rates to run the HP. If we went for Octopus Cosy then we'd lose the heavily discounted EV charging period and only be able to run the heat pump intermittently on cheaper eleccy. Again we'd be paying a premium outside the cosy periods and a big premium to cook tea! Buying a cylinder with a lower immersion boss defers making complex decisions now and gives flexibility in the future

If you're only going to heat DHW with a HP then you'll likely only need a pocket for a temperature probe. If you've got or might get solar PV then a second immersion lower down the cylinder allows you to divert excess solar into the cylinder when the solar is intermittent

Google "heat geek" and get a local guy to do detailed heat loss calcs for you. Youll need to pay for it. Be very wary of accepting a heat pump much larger than the Heat Geek calcs- running costs could be massive if you significantly oversize the heat pump

Octopus didn't ask us for proof of an EV but only wanted the model. I think its fairly easy for them to see the load profile during the night time cheap rate but whether they monitor that is anyone's guess??

If the meter is supposed to be read every half hour and they miss a few half hourly slots or a whole day/week then surely when they eventually capture a read everything will even out?? Won't show you half hourly figures but you'll not miss out on payments for total export?? Not sure if every smart meter displays it, but ours shows total export so probably best to take a reading every so often if you don't trust remote readings from Octopus??

An option to resolve all your queries would be to get a secondhand unit and play with it then either use it yourself or resell as a proven working unit. Ours is secondhand and I hooked it up to a 45 gallon drum of water to confirm it worked. Not sure what options you've got in Shetland but you can pickup HPs with controllers for a few hundred £. Maybe your installer would have a unit you could borrow?? Ours had been removed because the new householder didn't like it in their small garden and the installer who removed it and refitted a gas boiler wanted shut of it.

I'd guess as a backup for a failed HP a log stove with fireplace/chimney/flue/wood store/logs/back boiler etc isn't gonna be any cheaper than a boiler and oil tank/propane bottle?? Needing only a small genny to run either means another few hundred £ saved to use a boiler or WBS also as a back up for a power cut rather than a larger genny to run the HP.

Your 4 days trumps the max 3 days we've been without power. That 3 days was over Xmas a long time ago but it leaves a legacy of us always having a backup for essentials.

Maybe not for you but our 2.2kw honda unit is as much as I'd want to carry around when the lights go out. A backup heat source other than the HP also covers you for heat pump failure.

Whether you have back up heating or not is entirely down to individual circumstances. Unless someone's in the same circumstances their needs/wants are unlikely to be the same as yours. A heat pump requires a fairly beefy supply and would need a beefy generator to keep it running, an oil/lpg boiler wouldn't so a small generator and a few gallons of fuel will keep the heating and lights running for days. If there's a major storm which are likely to get more frequent and youre on a remote supply then you could be off grid for days which a typical battery bank won't cover. If your in an urban area sharing a supply with 1000s of others you'll be top priority for repairs to get you back on supply. If you're remote and share a supply with few other houses you'll be lowest priority to get back on supply. That's how the DNOs prioritise things when dealing with widespread storm damage. I don't know how the Ecodan interfaces to a back up boiler but the Therma V that I'm installing just now provides a volt free contact to enable a backup boiler to run. The configuration seems to allow the outdoor temperature value to be set that shuts the heat pump down and activates the boiler contact.....the manual isn't the best written but that seems to be what it says??

So are they all on their own DNO meter and have their own MPAN? How many flats are there?

Page 65 is within the section on eligibility criteria for FlT installations/payments and I read that as applying only to the FIT accredited part of your installation?? The section on extensions talks about accredited and non accredited installations and the tariff rates you are entitled to on p23 section 3.18. It gives 3 scenarios which I read as follows- Original accredited installation-MCS, entitlement to FIT tariff(80% in their example) Extension accredited installation-MCS, no FIT entitlement but entitled to SEG tariff(20% in their example) Extension non accredited installation- non MCS, no FIT or SEG entitlement. (20% in their example) You may get PPA from Octopus and Co., but no entitlement to anything. These 3 senarios suggest to me that self installed/non MCS extensions to a FIT system are allowable An inverter swap may impact DNC but not TIC which is driven by the panel capacity, as I read it! You'd be increasing the TIC if you added panels so wholly notifiable as an extension. From what's on our original MCS cert the G98 limit is what defines the DNC. Our FIT install has 4kw of panels and a declared TIC of 4 kw. The inverter is limited to 16amp so the DNC is declared as 3.68kw. Yep!!

I dont believe that MCS is required for any alteration or repairs/replacement to a FIT system. As an example, if you extend a 4 kw system by 4kw to give an 8kw system you get paid FIT rates on the original 4kw. If the 4kw extension element is MCS installed you can claim SEG on the extension. If the 4kw extension isn't MCS certified then you can't claim SEG but you could get paid under a PPA by the likes of Octopus. It's all in here https://www.ofgem.gov.uk/sites/default/files/2024-09/Guidance_for_FIT_Generators_V18.pdf

If your inverter supports it or you can change it for one that does, I believe you can DC couple within the FIT rules. If you can charge from the grid and potentially export it again through the generation meter then you need to replace your generation meter with a bi directional meter that gives a net reading so the net reading only represents the PV generation.