A bit confused by this one John? The solar gas boosted should be using a heap less gas than just an instant system. As it is essentially water that is already warmed up with the gas instant system only needing to top the heat up. It shouldn't be constantly circulating?
I have an evacuated tube solar hot water system with instantaneous gas booster setup and it's great (
300 litre Solar hot water system - North Solar Melbourne, Sydney). Doesn't use
any gas for 6 months of the year. So much so when we first moved in in December unbeknown to us the gas was still off at the main and it wasn't until I think April that the water in the shower was getting luke warm and I found out the gas wasn't turned on!