DIWhy

Wow, great advice all, thank you. So for further information the house walls & floor are currently completely uninsulated. Double brick leaf with thin cavity & outside render. Loft is insulated with fibreglass. Concrete slab floor (unknown depth) with thin screed. On the plus side (as PeterW & Iceverge mentioned) the building has a tremendous thermal mass. On the negative side retrofitting insulation is going to be tricky. After looking at options any additional insulation will need to be external unfortunately; the initial thought is to insulate by extension over the coming years, then externally insulate whatever is left over. Because of this, as you mention, the estimated heating load is high. However even in winter the building is retaining heat well over several days; I suspect the base U-value isn't telling the full story here. Trying to spec the heating system for the worst case scenario. The thermal storage company I contacted responded to some figures about sizing the thermal store with "we're not heat engineers", so my current thought is to follow your advice and go for two separate unvented cylinders. My main thought with setting out was to keep this simple; two cylinders definitely does that. My main concern about ASHP was installation cost, unit lifetime, and therefore payback period. I'll make sure the cylinders have a redundant loop/coil that can fit one, and then run the sums again once everything else is up and running. ProDave; as far as I'm aware there is only one tariff like this. Average peak price per kWh (between 4pm and 8pm) over the past year is 20p/kWh to 25p/kWh. Overnight prices 5-6p/kWh, rest of the day prices 8-10p/kWh. Varies by region.

I've managed to find one (potential) supplier via a local heating engineer. Are there any particular ones you might recommend or know of?

Allo, hopefully you'll be able to help with this. Currently renovating a house with no existing heating system. Water underfloor heating is being installed; peak demand estimated at 5-6kw (across 80m²). The plan is to make use of the Octopus Agile tariff which offers very low electricity cost at night. Overnight (cheap) energy would be stored in a large (potentially very large) unvented hot water cylinder. Essentially use this as a thermal battery and charge it up with cheap electricity at night. The cylinder can be as large as it needs to be; the main requirement is to have enough stored thermal energy to survive through parts of the day that have much higher energy prices. I can find plenty of options for this if I want separate cylinders for the hot water and UFH supply. I'm finding it tricky to find an option that allows a single, large cylinder to supply both hot water AND an underfloor heating system. In the future it might also be useful to have a spare (redundant) coil in the cylinder for heat input from an external heat source, whether this be a boiler/ASHP/solar thermal. Does anyone know of a product or supplier that allows for a single cylinder for the above?