Yes the cylinder is often at 80 degrees or more but the water...

Yes the cylinder is often at 80 degrees or more but the water enters the floor at a max of 50 degrees, the max the concrete can stand. Water that exits the floor is re introduced via a mixing valve to maintain correct inlet temp.
Also, Calcium and lime separation begins at 60 degrees.
Yes, your description of the closed circuit is accurate, and yes we use a product called thermablend in that circuit to prevent corrosion.
The gravel we used is on the property and was described as "soapy" so it was fairly fine, its a white gravel so is a bit like a 50/50 mix sand and gravel. It goes down like concrete when compacted. We use it for everything here.
Ha ha the IR thermometer. I use a fluke 561 here daily........a tad more than $12,50 though!
Great way to exercise the dog...... without getting up yourself!
I have been lusting after the thermal imaging camera of a similar design. one day.
Also wanted to mention that my original point may have been missed and I wasn't trying to derail your thread.
The mention of the system was just to illustrate to fellow wood heating fans, that the suitable application of heater/boiler and associated heating system can cut the wood usage down by at least 40-50%, which obviously reduces labour, fuel etc, natural resources, and the associated emissions from the harvesting equipment as well the emissions from the boiler itself.
A wood heater with 50% of the normal work is a lot more attractive in my eyes.
We use an ajax piston pump to pump the water up to the header storage, (40000 litres) but with a modern motor driving it. Its the cheapest way to move it. A pump from the 1920s! We havn't come far as a race have we?
Yes windows are the equivalent of a hole in the wall but we have nice bush and water views so ours are a bit of a compromise. I think we ended up with 25% in the north facing wall which includes a sliding glass door.
Tried to find a venturi cap today, no luck so far so will probably make just one. I have some cone rollers here.