Our heat pumps can operate until -15°C (of course there won't be much calories to transfer into the pool).
For a pool of 14m3 such as yours, with a external air temperature of 5°C (as mentioned in your example) you can definitely reach a temperature of 28°C although you will need to cover the pool when you don't use it to avoid heat loss.
You will also need to oversize the heat pump to have enough power to heat the pool water under such conditions.
Ideally, we would recommend you to make a full study of your pool with your pool technician in order to know the right power to use.
The Pentair Team