I'm planning to get air con in my new house. Use cases are:

Cold snaps: Top-up heating in conservatory during day and kids' bedrooms (2 & 3) at night
Summer: Cooling in conservatory on weekends/holidays, cooling in office (bedroom 4) on weekdays
Heatwaves: Cooling in lounge & conservatory on weekends/holidays, cooling in office (bedroom 4) on weekdays, cooling bedrooms 1-3 at night

The plan of the house is here. We're going to extend the house to add 2 extra bedrooms above the garage in 2-3 years' time. We also may open up the conservatory, dining room, and kitchen into one large area:



I've had 2 quotes from Daikin installers that have different approaches and would like to hear your thoughts about which is better.

Installer 1
- 5kW single split (FVXM50A) for conservatory & dining.
- 4kW outdoor unit (2MXM40A) connected to 2x 2kW indoor units for kids' bedrooms.
- 5.2 kW outdoor unit (3MXM52A) connected to 2x 2kW indoor units for lounge & master bedroom connected to 1x 1.5kW indoor unit for office with pump (pipework through loft).
- An additional 2-way multi-split can be installed in future once extension is done.
- All 3 systems to be run off different ring mains.

Installer 2
- 5.2kW outdoor unit (3MXM52A) connected to 1x 2.5kW indoor unit for conservatory + 2x 1.5kW indoor units for kids' bedrooms.
- 9kW outdoor unit (5MXM90A) connected to 2x 2.5kW indoor units for lounge & master bedroom + 1x 1.5kW indoor unit for office with pump (pipework through loft).
- 2 further indoor units can be connected to the 5MXM90A once extension is done.
- 2 new dedicated circuits for power from garage (via loft).

Installer 2's approach of oversizing the outdoor unit so we can connect more indoor units later is an interesting one but I am not sure how this will affect efficiency, particularly if we only have, say, the office unit on during the day.

Installer 1 thinks 3 outdoor units will work better to reduce pipework and also thinks we need a much bigger unit for the conservatory due to the large number of windows and sky lights, plus it's south facing. If we ever open up downstairs this will help also. But again, for now will the efficiency be poor if the unit is 5kW?

Also, everything I've read suggests single splits are cheaper than multis but that doesn't seem to be true where I live (UK). Installer 3 recommended 6 single splits and wanted to charge 30+% more than the two installers mentioned above.

Based on this, which approach (if any) would work best in your opinion?

 

I do not have an opinion about what option you should choose and my units are Mitsubishi.

When I decoded this year to add a third zone to my 7 year old system I found out that current inside units are not compatible with the original outside unit. You should take this into account if planning for future expansion.

I ended up adding a single zone outside unit next to the original.

As far as I know the efficiency of a multi-unit running at less than capacity is not an issue in heat pump type units. However they cannot run heating and cooling in separate zones simultaneously. The first unit calling determines the mode of the outside unit.