Assuming my hot water tank heating coil is above the height of my tubes and the coil is suitable for gravity use, is it possible or advisable to gravity feed in this way instead of using a pump and the running cost that go with it?
A fantastic way of powering a solar heating system without using mains electricity is a PV-powered pump.
This has been used to great effect, and with the added benefit of the pump only engaging when enough sunshine to heat the solar tubes is hitting the PV panel too.
I’ve seen this used to great effect with a flat-plate system and will work with tubes too. However, I haven’t seen how they fix the PV to the tube system, so you’ll have to engineer a solution. Here is an example: