Geographic Data
Canberra  35 17 S 149 08 E  550m  650m 
Cooma:  36°12'S 149°08'E  800m  850m 
Bungendore:

35°16'S 149°27'E  700m 
Braidwood: 
35°27'S 149°49'E  660m 
Bega:  36°41'S 149°51'E  50m  100m 
Glycol
Glycol is useful in freeze prevention and for corrosion protection, but as well as being expensive, it has some nasty properties too. Read this article on glycol and its use in hydronic heating.
Efficiency of Evacuated Tubes
The Apricus website has a method for determining the efficiency of a set of tubes, but of more interest is the overall system efficiency. The system efficiency is what really matters  how much of the heat that lands on the roof can be collected and stored in the tank? In this very interesting article on evaluating the effficiency of evacuated tubes in Canada, the author measures everything necessary to do exactly that, and finds that the result is approximately 40%. Bear in mind that the location he is talking about is East Coast Canada  46N which is 10 degrees closer to the pole than we are in Canberra. The weather is a lot more favourable here.
Flow Rates  Pipes in the Slab
Water should enter the slab at 40 and leave at 30, so adjust the flow rates on the blue parts of the manifold accordingly. The flow rate needs to be between 2 to 3 litres / min. The higher the flow rate, the less time the water spends in the pipes, so the heat transfer is less.
Flow Rates  Solar Panels
Use a pipe size to and flow rate to get a fluid velocity of 1.2 to 1.8 m/s. Above the minimum flow velocity of 0.6m/s ( or 2ft/sec) water returning from the collectors will entrain bubbles and carry them back to the tank. More than 1.2m/s ( or 4ft/sec ) will cause noise in the line not to mention being inefficient.
For the solar loop, a good rule of thumb is 1 GPM for a set of 30 tubes, or 3.8 litres per minute.
At this flow rate, 3 sets of 30 tubes needs 3/4" pipe or otherwise the water velocity will be too high.
Dr Ben in
his article Determining the Proper Flow Rate for Solar Hot Water Systems recommmends
0.025 GPM/ft2 or 1 litre per minute per m2. For 6 30tube panels each with an absorber
area of 2.4m2, thats 14 litres per minute ( or 0.23 litres per sec). A variable speed pump is ideal  slower
in the mornings and afternoons, faster at midday.
Internal diameters for generic PEX pipe taken from "Modern Hydronic Heating"
Thermal Mass
Thermal Mass Properties (Source: http://www.yourhome.gov.au/passivedesign/thermalmass)
MATERIAL 
THERMAL MASS (volumetric heat capacity, KJ/m³.k) 

Water  4186 
Concrete  2060 
Sandstone  1800 
Compressed earth blocks  1740 
Rammed earth  1673 
FC sheet (compressed)  1530 
Brick  1360 
Earth wall (adobe)  1300 
AAC  550 
Drainback Systems
Watch this: Drainback systems compared to glycol
Pipe runs must be sloped at least 21mm per metre for effective draining.
If the vertical distance between the tank and the collector is more than about 4 or 5 metres, you may be in trouble. This is because the water will turn into vapour at the highest point( ie at the outlet of the collector, whick will break the syphon effect) To do your own calcs, look at this spreadsheet drainback system calculations You need to know the elevation of your location  you can check by going to Google Earth. Canberra ranges from 550 to about 660
Air pressure above sea level can be calculated as
p = 101325 * (1  2.25577 * 10**5 * h)**5.25588
where p = air pressure Pa(determined by your elevation), and h = altitude above sea level in metres
http://solarelectricityhandbook.com/solarirradiance.html
GeoExchange
On average, a horizontal geothermal groundloop system requires about 200m of trenching for every 10kW of compressor load. A typical system would therefore have 3 or 4 loops of piping each 200m long
Holiday Retreat
Why not book yourself in for a relaxing stay at the coast at Clark Bay Cottages?