BC - you're heating water and you're pumping it.
Both of those can make bubbles in the water. I imagine you might not have much "head" of water between the collection tank and the pump.
You probably know how water boils at a low temperature up a mountain..
And how carbon dioxide is dissolved in fizzy drinks..
One Atmosphere = 1 bar = about 10 metres pressure. If you have a 1 bar circulation pump the inlet side will go down by a chunk of that pressure so could easily be pulling air as well making water vapour. (See "Cavitation")
It's a serious consideration with showers which are pumped from "gravity" tanks.
They'll dissolve again in time but they collect - we always have air vents on top of standard UK unpressurised hot water heating tanks, to stop air locks.
So - check the pumping head of your circulation pump - probably marked on it, many of ours are designated in feet, like "30" is about 1 bar. If it's a 2 bar pump on a 5 metre head it'll suck air in through micro-leaks!
Check what the static head of the water at your pump is.
But mostly, check all the places that air could accumulate.
If it all used to be OK, I'd be suspicious of those anti-vac valves. As the ad says, they wouldn't last forever.
But as Tony says, they may not be good at letting air out anyway, they aren't quite designed for that.
I've spent a couple of months living on a farm in Mazabuka, not a trillion miles from you. A different philosophy on plumbing - "Ya for sure we like things leaking so we know there's water in, and we can see it squirt if it builds pressure when it goes wrong".
Went for a drive once, from Joburg via Maun to Etosha, and back through Cape Town , Port E and Durban. Abiding memory of plumbing from that is of glorified mud huts with solar panels and two-oil-drum water tanks on the wiggle-tin roof, and a satellite dish for TV powered by an exposed hot-wire from the distribution poles. Seems romantic now - well almost.
