Thanks for getting in touch, and welcome to the wonderful world of beekeeping!
You’re correct that some types of heather honey can be thixotropic. Thixotropic honey has a gel-like consistency when at rest and becomes more fluid when agitated or worked. This can make it difficult to extract using conventional centrifugal extraction, and it can also present challenges for Flow Frames.
One important distinction is that thixotropic behaviour and temperature are not quite the same thing. Warmer temperatures generally make honey less viscous and easier to move, but a genuinely thixotropic honey does not necessarily stop being thixotropic simply because it is warm. In other words, the pleasant summer temperatures you experience in your location may help with the viscosity of the honey, but we wouldn’t want to suggest that they will reliably make a strongly thixotropic honey flow through a Flow Frame.
Our experience so far has shown that thixotropic honey can be harvested when it is sufficiently diluted with other nectar sources. For example, when thixotropic honey has been blended with around 50% honey from other floral sources, we have found that it can be harvested by repeatedly opening and closing the Flow Frames.
The important consideration in your situation is therefore the actual floral mix your bees are collecting, rather than simply the presence of heather in the area. Bees rarely collect exclusively from one plant species, and the honey they produce will generally reflect a mixture of available nectar sources. However, if your colonies are situated in an area where heather/fynbos nectar makes up a very large proportion of the honey crop, we would be more cautious about relying on the Flow system for effortless harvesting.
It may be worth speaking with local beekeepers in the area about exactly what sort of honey they are producing and whether it is strongly thixotropic. That local experience would probably be the best indication of what you can expect from your particular apiary.
There are also advantages to Flow Frames with honeys that have a tendency to crystallise. Because the honey can be harvested while the frames remain in the warm hive, and because you can harvest small amounts to check whether the honey is ready, you can sometimes harvest before crystallisation becomes a problem.
So, in your particular situation, I wouldn’t recommend assuming that Flow Frames will automatically solve the harvesting problem if your bees produce predominantly strongly thixotropic heather honey. If the honey is a mixture of fynbos and other nectar sources, however, the situation may be quite different.