Hot tub pump and heating design

[LouiseSJPP]

I'm designing a wood-fired hot tub installation. I'm hoping to use solar panels to drive the filter pump and for excess panel power to add a little background to the water. (For the ecologically-minded, water will be harvested from the house roof or from our spring, which supplies the house. We live in a forest, in northern Spain, and fallen or culled timber provides for all our heating needs.)

The hot tub is for four people, capacity 1200 litres of water. There are no jets, the pump is for filtration only.

Should I be looking for a sand or cartridge filter, or both? What power does the pump need to be, and how long should it run, given a likely usage of once per week?

Can anyone point to an inline heater which could put maybe 1 kW of heat into the circulated water, or suggest a different way of heating the water from excess PV generation?

The alternative to solar PV would be to use direct solar heating. Can anyone comment on the relative effectiveness of heating via PV, proper water heating panels or simply coiling black plastic tube beneath a sheet of clear plastic?

 

[Why Not Indeed]

If you want to use excess solar PV, then I think you should consider a solar diverter. These are usually intended for heating an immersion heater but the principle would be the same for a hot tub.
Direct solar heating is always going to be more efficient than PV + resistive heating, but if you already have PV then how many years of hot tub heating will it take to recoup the costs of installing another system?
Have you looked at bespoke solar hot tub heating systems, eg:

Solar hot tub heating — SolarisKit

The SolarisKit solar hot tub heating solution lets you harness the energy from the sun to heat your hot tub, saving you money while lowering your carbon emissions. Enjoy your hot tub without breaking the bank with the SolarisKit solar hot tub heating solution.

www.solariskit.com