I do have plenty of South facing roof space and I am thinking of installing solar hot water panels to preheat the water for DHW and the heating system. 

if you preheat the oven, then pull off the knob and look at the back, loosen that screw, adjust the knob so the line will match up with the temperature your oven thermometer says, then tighten it back up.

The sun {pre}heats water for both domestic use and the hydronic heat ... the inline gas-fired heater/boiler is a little smaller than what Darren is using, but it has to contend with less lift since the source water temperature is higher.

There is more to that story than it appears , I watched the video , thats an old trailer , pre heat pumps .

The second ball valves may just need snugged a little ... that usually stops a leak from that location.The third looks like the joint below the valve is where the leak is/was.The first may just be the bleeder ... you may be able to snug it as well, but with that corrosion, I'm not sure.Those valves should have been checked shortly after installation (a few weeks) and then periodically as part of your annual, pre-heating season maintenance.

Even in our northerly (read long, cold winters) climate, a modest (8-10K installed) solar thermal system met 60% of the DHW needs of a 6-unit building and meant the boiler did not need to fire at all during the summer to heat hot water.Such a system can also be used to pre-heat the return lines of a hydronic baseboard/radiator heating system lessing the load on the boiler.  

I am sure that the energy savings with a new boiler, outdoor temp reset, domestic hot water pre-heat, etc. would qualify for the rebates.

So you need to get clear on your processes and definitions, you don't pre-heat the oven to 25 bananas then filter the blinker fluid into the chakra through the hopscotch, that's just word salad.Your choice of wording, lack of clarity and completeness made my antenna go up, that's why I seemed to come back curtly.  

In the summer, the only gas used is for the hot water heater, and the water going to the hot water heater is preheated by solar.

Step 3 - Pre-heat the oven to its "Frenzy" setting.Step 4 - insert the Pans into the oven and let bake for 3-4 "boom" years.