JohnMo

If you have the space put a proper UVC in. The mini store is unlikely to perform much better than your combi except immediate draw of would be quicker.

£1400 isn't modest in my book. Plus if used the way you are suggesting you would need a full safety group. You may have a few things to resolve. If water flows through the combi it will fire up, so use gas anyway. If heat store isn't heated, sun not playing ball, it would just cool the water already heated by gas boiler. So no hot water! I would install cylinder upstream of the combi, heat by solar, if water is above 45 divert around boiler direct to tap, if below 45 let it be heated by boiler. Saves boiling firing when not needed etc.

Really depends how it's setup. Would think every parameter you mentioned can be adjusted. But neither way mentioned sounds an efficient way to run a heat pump.

Or a dry trap? One or the other sounds contradiction, or your client has money to waste🤔

Where is your structural design? Should be zero need for such questions, if your building is designed.

So is your heat pump oversized to allow this?

Definitely found that with mine, certainly once you get cycle times below about 10 mins, you may as well give up.

So that is just the restart hysterisis. Doesn't the lower pump speed, just drive a lower output of the radiators, and then drive shorter boiler cycles, as there is less to take the boiler heat away?

If settings don't give the behaviour you expect set them back to default. What thermostat are you using? Is it an on off one or opentherm?

Read an article the other day saying electric prices in the UK are set to drop like a stone. As our price is directly linked to gas wholesale prices, which due to the glut of LNG on the world market is falling, and due to fall even further. Several countries are reducing their uptake of LNG due to more renewables coming stream and less reliance on imported gas.

How do you do your schedule?

Unless you couple that with a battery. Then low and slow wins.

That's a strange lunch your cooking up.

Here's a few things to read UFH-System-Design.pdf UFH output calculation.pdf

If they are quoting a buffer it isn't an expansion vessel. A buffer requires secondary pumps, a volumiser doesn't. A buffer provides hydraulic seperation and therefore needs a pump(s) for the heating circuit. Samsung Gen 7 schematic don't compel the use of a buffer, if your are doing an open system. They show a volumiser on the flow to heating circuit. Cylinder size is basically worked out as follows, 45L per bedroom, plus 40L. Reheat time assuming 300L and a 5kW output will be around 1hr and 20 mins to 50 degs, assuming bulk average temperature is about 30 degs.