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Outside cylinder, – Virginia you say “We chose a [solution] as we could put it outside”, and there is not room in the house. I would likely have chosen the same as you for the same reasons.

Thoughts about other options that might suit in similar situations: An electric califont? May put people on a more expensive tariff and not work well with coinciding with peak load, (and also may be wiring issues/need 3phase if you want higher flow etc) but also may use the least energy, if most hot water is being supplied when it’s cold outside and the heatpump is working at low efficiency/using the boost?

Since your system has a 320l tank outside, I’m not aware of any fundamental reason why a resistively heated cylinder couldn’t be put outside? Of course the practical reason is probably that such outside cylinder products don’t exist? But the need to save space inside is common so maybe it’s a gap in the market?

I’m interested in the practicalities of heating a 320l hwc during the day (say if people have solar), then having a bath at night (quick check tells me a bath takes between 70l and 200l), and still having a decent shower in the morning? Anyone doing this? The water should be stratified with a hot layer at the top, but of course there will be some mixing/heat transfer overnight from the hot water at the top to the cold at the bottom. Is is possible to heat the hot water to a higher temerature initially (e.g. 75+) and still have a warm enough top layer in the morning for showers?

Interesting stuff. Sorry I can’t offer practical help with your actual situation.