sharpener

Went to the Installer Show earlier this year, some notes here. Several mfrs inc Mitsi and Grant are bringing new R290 HPs to the market. I think any other technologies will be too niche or too far into the future for you. Presumably you have masses of PV and batteries then. A big thermal store might be cheaper per kWh than batteries though. How big an HP do you guesstimate? The new Octopus unit uses R290 but is only in 6 kW to start with. IIRC the new Mitsis go up to 9. Grant will be a twin fan so might be big enough.

Remote start might be useful but it hardly seems worth paying twice the price https://hyundaipowerequipment.co.uk/hyundai-hy6500sei-petrol-inverter-generator-6500w-pure-sine-wave. However it is also a good bit quieter.

It looks as though the 28A starting current would need as a bare minimum a 6kW genny and as @markc says they are notoriously difficult to size correctly. Modern inverter based gennys are more economical at part-load but have all sorts of protection in them which may trip. Suggest you hire one for the weekend and if successful buy the exact same model. Does your HP have a quiet mode or night mode which might draw less current? Another old trick is to get a biggish induction motor and run that in parallel. Start that first with a star-delta starter, then its rotating mass will help supply the HP on startup. But this would require intervention every time and is not easy to automate.

Yes I have been wondering about this if I keep my existing cylinder, as I don't think there is a spare pocket for it. Maybe using the existing stat as the call for heat will be enough, IIRC the Vaillant VR71 interface box has terminals for that. Otherwise since I will have a secondary circ pump I am hoping it will be OK if I strap it to the h.w. pipe where it leaves the tank - so long as I can arrange it that the pump keeps running for the entire programmed hw period and not just when the cyl is calling for heat. Else it might cycle once the pipework cools down, but maybe with enough insulation and big enough hysteresis it wouldn't be a problem anyway. Have now got a date for a site visit from Vaillant's area tech man so hopefully this is the kind of wrinkle he can resolve. I think he has been persuaded by @JamesPa's thermal model to accept the existing cyl (or he would not be coming), main worry atm is what he will think of the 15mm flow to the coil. Though I can if necessary run it in parallel with the feed to the TS as the times will be the same and then the load on the HP will be split between cyl and TS. Sorry to the OP for thread drift but it is all related!

Thanks. Didn't know about the Classic kind. Thinking of fitting my own upgrade rads for HP, Tectite Sprint look nice for that and as you say not constrictive unlike some cheap solder ring fittings e.g. Screwfix unbranded. Anyway am getting too old for lots of hot work (7 bigger rads will need min 28 elbows). Haven't tried the Hep2O but am a fan of Polyplumb, my rainwater system has quite a few as (unlike JG Speedfit) you can be confident they are done up but are easy to dismantle without tools e.g. to service the pump and controller. Nice looking job, I read somewhere that Ideal cylinders are in fact made by one of the major cyl mfrs but are nevertheless cheaper to buy from Ideal.

Fortunately my cylinder has the outlet pipe running downwards from it, after about 3/4 m its surface temp under the lagging is approx 30C. Airing cupboard is only faintly warm and all within thermal envelope anyway. I agree a heat trap might be esp useful with a vented cyl where the vent pipe typically runs straight upwards but I have never seen it done in any house I have had. BTW I note you use what look like Tectite fittings, what is yr experience of them? (I have only ever used the one but it is at mains pressure and has been fine.) However to judge from their pix of completed HP installations the professionals seem now to use crimped fittings instead and I have wondered why that is, it might be price but then the crimping tools themselves cost £00s.

How is it supposed to work? Doesn't prevent conduction. Might prevent parasitic convection currents. Also we would not allow a shut-off valve between tank and expansion vessel!

Yes, I was really only commenting on the order of magnitude involved vs usage. I imagine the standard methodology allows you to compare one make with another but not necessarily to have much relation to the real world, like EV range. My observations are probably not directly comparable, and are for when the house is unoccupied. They do not seem to vary noticreably from summer to winter which is odd.

Well, not a lot. By chance I was looking at Megaflo earlier today, their current 300l is quoted as 1.77 kWh. But is has got better, my OSO from 1996 takes about 2kWh after the battery is full and another 0.5 or so two hours later when some mixing/diffusion has taken place and the temp has fallen back by the thermostat hysteresis.

Well my plan for when the 12kW Vaillant gets installed is to charge my 210 l tank off E7 to 50C from 0500 to 0600, and again in the afternoons to capture any free surplus PV. This will allow 0600 to 0700 for the HP to recharge the thermal store and pre-heat the UFH. This mirrors our current usage on an oil boiler. We usually have 2x showers when we get up, but sometimes more if we have guests, and sometimes in the evenings as well or instead if we have been gardening. So 210 l is enough storage for us allowing for dilution to a comfortable temperature and I am a bit surprised by the amount you say you want though I suppose 3 kids baths might use 240l. What, hotter than 50C (see above)? A modern 300l tank has losses under 2kWh/day so it will not make a great deal of difference if it is left hot overnight or not. But I see no particular point in re-heating it earlier than you need to and I think the advice that it is less efficient is plain wrong. OTOH there is every chance even with a complicated schedule you will not achieve perfection, having it on just during the (entire) day may be the answer as you say.

Glad you got it sorted. I am still on my original FIT tariff with GE as have been told there is no reception for a smart meter. I don''t know what will happen when the BBC finally turn off the radio teleswitch service which controls my E7 meter.

(Sorry am a bit late to this thread.) It's the FIT export database. You have to de-register your FIT scheme before they will give you SEG payments otherwise there would be double counting. Ovo seem to have done this correctly. Your quarterly Good Energy payment notifications will like mine originally have shown the deemed export payments separately from the FIT and now they no longer do, I would have thought that Octopus (a) would accept this "before and after" documentation and (b) will anyway have access to the deemed export register so they can verify that Ovo have removed it.

Both Gledhill and Newark have a vast range and will also do custom cylinders with your own choice of aspect ratio, coil, tappings and immersion bosses.

As it happens I have this week received a revised quote qualified by the following wording Subject to a 2ndTechnical survey visit with Vaillant Technical to confirm existing cylinder is compatible as a replacement has not been quoted so maybe I am beginning to wear them down...

It's all relative. The OP is talking about an unusually small cylinder with a disproportionately large coil (15kW, presumably rated for a boiler though) and a small heat pump which can probably turn down to 3kW or less.

All that is true as per @JamesPa 's spreadsheet model. But a lot of this thinking is out of date; now that we can get higher flow temps with R290 refrigerant it is much less important than it used to be.

I took it that @Beelbeebub has no cylinder ATM i.e. a combi boiler but is planning to squeeze one in somewhere. I have had this experience in spades with three different installers. Am currently awaiting final confirmation from Vaillant that they will accept my existing 210l cyl on a 12kW Arotherm Plus (R290) so long as I fit a secondary circulating pump to stir the contents. (I sent a detailed perfomance model thanks to @JamesPa's spreadsheet). But with a 6 kW HP and the same coil area much less of a problem. You will probably find yr chosen HP mfr has a compatibility table. One installer came up with a "need" to increase my cyl to 300 l (for a 4-bed house) but I think this was only from some mfr's data sheet not any standard. 90l for a 1 bed flat sounds reasonable to me. Edit: see under. IIRC the MCS rules require that the HP provides the HW (though immersion top-up is allowed), so you will not get the BUS grant if it doesn't, someone will be quick to correct me if I am wrong. Octopus Cosy would be good for yr situation as there is also a 3 hr afternoon boost, so long as you can avoid the evening peak 1600 - 1900 i.e. don't cook your supper by electricity.

Don't forget to let us know how you get on!

But this is what you said in yr original post, is the HP output just sufficient or not? Perhaps we can reconcile the two statements if you don't consider 20C warm enough, in which case I would endorse the suggeston to put on another jumper. A former colleague of mine heated his house so he could wear shirt sleeves in the depths of winter. He now lives on the isle of Skye(!?!). I wonder what his fuel bills are like.

From what you say you are lucky enough to have an HP that is exactly the right size for your property. So you will benefit from the best possible turndown ratio, the least cycling, the highest efficiency and the lowest capital cost!

Sounds like it! If the house is warm enough in the coldest weather though I would not worry. If it bothers you maybe you can shift the zone timings so that everything is not calling for heat simultaneously? You could try changing it back but you will have to wait until it is really cold to see the result.

Do I infer that you have actually got one ot these things then?

Modern or what? DEC PDP-15 needed booting from paper tape!

AFAICS if you have an EV you can combine these two tariffs Octopus Intelligent (which is a better bet than their HP tariff Cosy if you don't need the afternoon boost) with their export rate Octopus Outgoiing which pays 15p/unit for exports and will even allow you to arbitrage between the two using your battery. Check the actual rates for your particular postcode though.

I don't recall there being any options for setting this. You could explore the simulator I linked to earlier.