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How big is your house?
I'd bet big that your heat loss calc is overcooked, everybody's is. Ours came to an alleged 9.5kW total loss at -2degC, but the actual measured loss is ~6kW at -2degC. I had to fight to get the originally-specced 10kW Vaillant bumped down to a 7kW.
Zoning doesn't really work in practice, the warm rooms will always be losing heat to the unheated cold rooms causing flow temperatures to increase and the heat pump to work harder. It's also always more efficient to have as much water volume circulating as possible; shutting off radiators and reducing volume causes more on/off cycling which kills efficiency. Much more efficient and comfortable to balance the whole system properly and constantly heat the entire house as a single zone.
ectoplasmosis-
Last time I mention it I entered the GC thread 😂. Just under 200sqm, but this is rural-ish Scotland outside Glasgow, so GC prices need not apply.
The heat loss calcs are over cooked IMO. And, they were based on several factors that are now less of an issue such as increased insulation, soon to be replaced windows, a blocked up shitty/drafty door and so on. But they would only calc on the existing fabric so it was impossible to get any movement here.
We had 6 installers quote and all calcs were around the same…
We’re only 10 days away from install so I doubt there is much I can change but I’ll see what they say about zoning. I’d be fine with one zone but they were not confident it could be done purely with the ASHP specced without a split gas system.
andos
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.