You mean a heat pump? They use them on koi ponds to keep them toasty during the winter months. Think 20,000 litres of water kept at 12degC in the coldest winter.
The issue is that the air heat pump is dependent on the amount of energy it can extract from the surrounding air. So above a certain temperature that's good, but as the temp drops, you need a larger capacity (more expensive operating) to keep that thermal capacity. I'd be tempted to run a boiler as a backup/augment. Only other issue is noise as they can be pretty noisy.
Still, worth looking at.