The Vian Power C5 is a great starting point and being very small, compact and discreet and can fit virtually to any hot tub as long as there is enough space. We have installed many of these in the South of England and as they work down to -5 degrees will work all year round. We have great reviews on this one on our Checkatrade page. At 48db at 1m its pretty quiet. With its simple on/off system it works at full power to heat your tub quickly and efficiently. With great warranty and back up its no surprise this is a very popular Air Source Heat Pump for your hot tub and it will save you money on your hot tubs heating making those electricity bills a lot cheaper