Hi Phil
I’m sure it’s something you could do, there’s lots of different kit out there though - if you’re buying new is there anything you can use directly?
There are several small capacity (700W elec) air:watertank heatpumps, where the cold airflow can be routed internally or externally - you could use this, and duct the cold air internally (Vaillant aroSTOR):
https://www.theheatpumpwarehouse.co....gaAgVOEALw_wcB
Some multihead minisplits can have one outside unit, multiple indoor, and heat some indoor while cooling others. Is that helpful?
If you do change the heatexchanger, you’ll need to vacuum it down, braze, refill. You can do that with r290 and other non F gasses, all those gasses are a bit nasty (explosive or toxic or high pressure). I’ve made my own gshp from 2nd hand compressors with r290 by the way, and I’m very cautious with the gas itself.