RussMac81

PS: regarding the question on source of fresh air, our old decommissioned oil boiler (before ASHP) had a flue going out roof which is still there. Although flue only 100mm looks like its encased in a second duct so whole thing 200mm which may be enough for my fresh air to avoid a further roof penetration. If not I'll have to core drill a gable wall which I'd rather avoid.

@joth FCU I am thinking of using is from Agility. It has 4 outputs each individually controlled. £1600 so not really much different in cost to buying one FCU per room, though has a commissioning charge on top. No MVHR. I don't need to worry on fresh air as I'm only doing it when we consciously want to bring in cool evening air to get rooms down in temperature without having to pay to run the ASHP. In winter ASHP will be via underfloor and rads, so FCUs not running.

Hi everyone Thanks for all the helpful stuff above, it was a god send to find the cheap coding resistor trick 😊 I’ve done a lot of research regarding cooling with Vaillant Arotherm Plus which I thought helpful to share – not least because any critique would be helpful too. Also really interested in how the fan coil mentioned above worked out for people. We live in a classicly problematic modern house – loads of insulation, loads of glass and all south facing. Cooks even on spring and autumn sunny days. Like many, we want to cool 4-5 bedrooms. Not fussed about downstairs. Got the 10kw or so cooling capacity out the Vaillant and reluctance to have another outdoor unit and all the faff that entails. Having spoken to various air source and air con people we’ve decided to go down the fan coil route. Advantages being that we can just run a dedicated cooling water supply from buffer tank to the fan coil in loft which I can insulation to high standard and then not have to worry about dew point and condensation issues as you would with rads and underfloor. We’re lucky in having a service space directly from boiler room to loft (from oil boiler / flue days). Also the fact that it’s much easier to get the 10kw out quickly via fan coil than it is through the underfloor or rads given limited temperature deltas, although I've spoken to others who have had a good experience with rads and the axial fans you can attach to them (there are even hidden ones) which effectively turn them into a fan coil. You can get fairly cheap ducted fan coil units (i.e. water rather than refigerant based) to go in a loft space (hung on straps from rafters for noise insulation) from where I understand simple 200mm flexi ducts would be OK for each room. The more sophisticated fan coils can vary fan speed/flow rate by room (google for multiroom by Ability as an example). You could have a single exhaust centrally in hall but you’d get less control and might confuse the system as it’ll be permanently pulling hot air up from downstairs, plus we want to control by room so we’re going to run an exhaust from each room. Anyway, the aircon guys I spoke to said not to be put off by the various assertions that everything needs to be calculated and what not and that in general terms: 200mm flexi duct in and out of each room, opposite corners would be fine. Simple diffuser on end of each flexi. Insulate cold ducting (no need to do that for exhaust) Aim for 28-30 cu m / min flow on high fan, with high static pressure, and with filter included Air going into room aim for 10-12 degrees About 2-2.5kw per bedroom normally fine We plan to go one step further and put simple duct actuators on each room if we can’t get the better fan coil with individual room control. Also, given that often our house overheats on days where it is perfectly cool overnight (but hard to exchange air into room given high ceilings etc), we’ll have a fresh air setup that doesn’t run cooling water and simply pulls air in from outside into the rooms. Because there shouldn't be the big lag issues others have experienced via UFH/slab, I'm figuring that a fairly simple zone control setup will work fine. Anyway, all theory thus far as we haven’t started the install. Interested in any feedback in case any of this is wrong, and I can follow up here once we know how it performs (likely now next summer tho!).