JohnMo

Heat pump cylinders are generally big because they store water at 45 to 50. A typical direct cylinder could just as easily be at 70 degs, so effectively a much smaller cylinder holds just as much energy.

Can you not surface run it?

Get your finger out!

I would just a decent dMEV system.

That's the trouble with this country - lowest standards possible. And a general public that are willing to accept that. So a cheap 30kW combi that only modulates down to 10kW, thermostat in every room, boiler that lasts about 5 years because it cycles so often. That'll do

Nope. Fact any cycling requires a higher flow temperature to compensate. Example my install is cycling 4 degs outside, runs for 1.5 hours off for a hour. Flow temp max 29.4 degs. Overall CoP so far today is 4.55, but heat input is approx 500W higher than it should be (2.5 compared to 2kW). Steady state running flow temp required is 23-24, also need 2kW heat input, reduced flow temp increases CoP to around 5.2 to 5.5, reduced electricity input by 500W, so should yield something like a 30% reduction in running cost, just by a using a heat pump that covers full heating range by modulation. Apply that to 80% of the heating season it's not small change.

What make model - boiler or heat pump?

Depending on the controller it may be configured as the master thermostat unless it's just a time switch. Your sitting room maybe has no zone control it's just an open zone. The controller may be asking the system to do stuff based on what it sees in the garage?

The installation I am talking is simple. You will need a room by room heat loss calculation, then size emitters to room heat loss at same flow temperature. Direct cylinder with immersion timer. Set timer to any low tariff periods. ASHP, outside unit power cable via isolator switch. Two core screened cable between outside and wiring center. Power to wiring centre. Controller within wiring centre. No room stats. That's it no need for 2 port or 3 port valves.

Pipe sizes are simple for a heat load of say 5-6kW, I would just run 28mm until pipe splits for downstairs and upstairs then 22mm to each manifold. UFH pipes 16mm, and rads from manifold 15mm

But I would just delete all the zones, it's just going to make your heat source cycle and use more energy anyway. Zones add no value

Where is your boiler controller is it the sitting room?

So assume about 188m² (can't be arsed converting ft²) so 5kW sounds high. Would have expected 3 to 4kW. You can go grant, but by the time you're adding the MCS markup you will paying out as well. PV just couple to a 2x mppt string inverter. At a later date this could have an AC coupled battery added without touching much on the solar side. Or hybrid would replace the inverter. A 6kW Hiaer heat pump is £2100 including vat and you need to add a wiring center and controller for £550 Inc vat. It will modulate down to 1.6kW. (the 4kW down to just under 1kW) Sling in a direct UVC and run DHW from immersion (off peak), heat to 60 degs and install a smaller cylinder. Run UFH and radiators at same temperature, all one zone. Simple install, minimum materials, cheapest to run. Makes wonder - I would just ignore him and don't get him anywhere near your install. Don't bother with the grant, not worth the hassle and fighting over someone saying you need a 10 to 12kW ASHP.

All depends on modulation and demand. A heat pump that runs without cycling will always be more efficient than one that cycles. First a heat pump that doesn't need to cycle can run a lower temperature - mine I am looking at 5 degs cooler at 8 to 10 degs outside. That's a big chunk of CoP increase. Then there is the needless heating of big chunks of metal at the start of each cycle. So basically I don't agree, unless of cause the large heat pump can modulate down to meet the 7 to 10 Deg outside temp with undue cycling, then there is just capital expenditure and large pipework etc to accommodate it. Cylinder heating isn't a big issue, 210L from an indicator temperature of 6 degs only takes 50 mins with a 6kW heat pump to get to 50. Heat twice per day, no real impact on house temps, one heat is likely when your asleep anyway. I have actually found heating by immersion has barely changed overall electricity consumption. On paper ASHP heating gives me a CoP of 3, but in practice the immersion does a good job, maybe due to the long pipe runs on my install.

If you treat as vented you may not or wouldn't have an expansion vessel, inlet control group, or PRV. A unvented cylinder is just that, once installed you have zero control what the home owner will or not do. The rules at least make sure that at install stage, a basic set of rules are in place.