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Sounds like your place is already much more insulated than ours!
Glad you're getting a Vaillant, they have very effective control logic, meaning even though your 12kW will probably be oversized (the vast majority of heat pump installs are), it doesn't matter so much as it won't egregiously cycle on/off unlike most other brands, and bigger heat pumps achieve higher efficiency when heat demand is high in deep winter.
Make sure you ask whether your installer is planning to fit a buffer vessel or not; they reduce efficiency by a fair whack. Best practice is no buffer, no zoning or thermostats, entire house run as a single zone using weather compensation.
ectoplasmosis-
Nice tip on the buffer vessel. Will ask.
We’re zoning by upstairs and downstairs, so will have thermostats upstairs/downstairs (I assume!). Reason being they recommended a split system of gas/ASHP due to heat loss calcs. To make sure we could get off gas 100% we agreed that 18 deg upstairs, 22 deg downstairs would be fine. Given we’ve never lived in a house where we’ve ever had a consistent heat of 18deg anywhere and don’t like hot bedrooms that works for us.
andos
We're just outside of Glasgow. We have a local firm called 1314 Renewables who get very good rating round here and they seem to really know their stuff. We're getting a 12kw Vaillant Arotherm plus, full re-pipe and all new rads, water tank etc
If we managed to be getting what you are for ~£5 I'd be over the moon. Our place does need to be better insulated, but, we've put in double glazing (with 2 more windows to be done at triple), new doors, rooms in roof insulated as above, underfloor insulation and cavity wall insulation. The heating system is all over the shop and even spending £20 a day at the moment isn't getting most rooms above 16deg which is driving my wife insane.