I have a 13kW Grant ASHP, installed to the letter of Grant's instructions at the time.  50 litre buffer, 200 litre DHW cylinder, radiators (with TRVs which are never used), wired as classic S-plan.  After I rewired it to use the native ASHP controls since the installer followed the (very mistaken) Grant instructions, it works well for heating and hot water. The ASHP is maybe a bit oversized (deliberately, for noise reasons) but it seems to work fine - no short cycling. On mild days it'll run for a short time to heat the buffer and then circulate that heat for an hour or few around the house (ie the buffer is doing its job). It's set to run 'sniff' cycles every 20 minutes, ie run the internal pump to take the temperature of the buffer water by sending it past the outdoor unit's temperature probe and decide whether to turn on or not*. Here's a diagram:

(* I have actually wired a buffer thermistor so it can measure the buffer temperature directly, but it seems the only mode it'll use that is when it tries to keep the buffer at a constant temperature like a gas boiler - no weather compensation or anything, which isn't what I want. When I disable that mode the buffer temperature always reads 25C - seems using sniff cycles is the only option in more sensible modes.)

I'm pondering installing some fan coils for cooling.  Current thinking is to install them on a second zone (S-plan+) like an upside down underfloor heating system: UFH manifold, UFH actuators, and no valvegear in the fan coils (easier to manage condensation that way). Sketch:

Are there any reasons not to go with this plan?  I've tested the ASHP by switching the cables to the DHW and Z1 zones and running it in cooling mode - it makes a nice tank of chilled water (although the coldest water sinks to the bottom of the tank and the draw is at the top, so you can't actually chill the whole tank to 7C even if that's what it's making).

One thing I wondered was about the buffer when switching between DHW and cooling. If you've just made a tank of hot water for a shower then want to switch to cooling mode. What happens when you engage cooling is the unit runs its internal pump and the fan with no compressor to cool down the buffer circuit to outside temperature, and then runs the compressor to cool it below ambient.  But this is inefficient as every time you switch from hot water to cooling you have to cool that 50+ litres, and heat it again when you want hot water.  Typically I have it set to just run the hot water any time the water temperature gets a bit low, which can be at any point in the day. Let's assume that I just want a fit and forget option, ie I don't want to micromanage scheduling the hot water at 3am to avoid conflicts with cooling.  It seems less than ideal to have hot water and cooling fighting like this.

One option would be to move the DHW before the buffer, on the basis that there will always be a constant flow through the DHW circuit.  This would then move the buffer into the heating/cooling side only, and allow a switch between heating and cooling modes without having to shift the buffer water. Something like:

What would be the pros and cons of this approach?  I need to do the calculation of the ASHP's minimum flow against flow through the hot water cylinder coil, but let's assume that's ok. I'd have thought there would be enough heat in the DHW side to scavenge for defrost cycles when in DHW mode, and in heating mode there's still the buffer.  If one of the valves stuck presumably the ASHP would shut off to protect itself for lack of flow.  Any other things to watch out for, or reasons why this isn't a good idea?

Edit: maybe I'll need to move the expansion tank to before the Z1 and Z2 valves, to manage pressure when in cooling mode too?