Wood Heaters, page-181

I can understand the details now. Great system and thanks for the interesting details on max temp. and why.

I was imagining washed gravel but can see that what you have used would be ideal. I wish I had that for my drive. I've had one load of crushed limestone that I spread with my tractor but to have something like your gravel on site would be very useful, save some money too.

All of this is very relevant to the thread. After all, it's taking wood heaters to the next stage and using them more efficiently. Imagine the tiny energy and emission footprint you have compared to someone who just turns on their mains powered heating and relies on that? You are using fallen timber that would break down and release similar emissions anyway and boosting that with solar thermal. Less work too because it's so much more efficient.
Those old water heaters with the flue going straight up through the middle of the tank were amazing. You only had to fill it up one load of stuff that would burn and could have a hot shower half an hour later. Win win, free hot water and no rubbish. They burned so hot smoke was a rarity.

That pump in tassie might well have been an Ajax too. There was another one for spares. Pre war piston pump with a fairly large electric motor driving it. They still do the job well but I doubt Bunnings carry any spares!

Those venturi cowls are widely available over here in WA.
I think you're in Tasmania? Surely the local wood heater shop would carry them or be able to get you one in?

Here's confirmation about the restricted flow from those capped off cowls. They show a parallel sided cowl but that means a much bigger opening to the rain. The venturi reduces back to the 6" flue diameter.

http://www.unley.sa.gov.au/webdata/resources/files/5DDec11.pdf