2.8kw cooling is actually a fair amount for a very insulated house in the U.K, assuming you have shading/overhangs. Our calculated cooiing load was <1kW from what I remember.
The thing is, and I didn't realise this at the time, those numbers are based on:
- Water temp of 7C which isn't particulatly efficient.
- Air temp of 28C
But post-MVHR air temperature is never 28C assuming you keep internal temperature under control. If external temperature is 41C, internal temperature is 21C and MVHR efficency 90% you are looking at closer to 23C. And, if post-MVHR temp is 23C and not 28C cooling power is only 75% of what is quoted.
On top of that, you can't really compare cooling power of Comfopost with cooling power of fancoil if you need run mvhr on boost to increase cooling, as it's cooling warmer incoming air, not air at existing room temperature.
A simpler approach to stopping first-floor heating up via MVHR on all but the very hottest days is to turn MVHR to minimum during the day on very hot days. Not as effective, but lot simpler..