Aredwood: Since you are now including a header tank, Just put a T junction in the wetback inlet pipe just before the wetback, and run a pipe from the header tank to this T junction. Make sure this pipe always slopes up to the header tank. If the pump fails, Steam will go from the wetback up to the cylinder, out of the vent pipe. And water will be supplied direct from the header tank. Instead of having to rely on flow through the radiators, or electronics, or moving parts to keep the system safe.
WanaGo: System installed and running beautifully.
Tested in both pump on and pump off situations. It thermo-syphons fine just takes longer for the system to warm, so is good for a backup if the pump fails. But when pumped, the fire only has to be on 5 mins and the radiators are warming up.
I put a probe on the outlet of the wetback, and another on the inlet, to monitor temperatures just temporarily. With the fire running normally, not stoked up high, we get about 70 degrees C out of the wetback, and its returning about 55 degrees.
Have 2 'side backs' in parallel. Very very happy with their performance.
Still yet to stoke it right up and see if I can get it to 80C. If I can then I may be able to install another radiator, but I suspect it might be too much for our little fire.
We made the side backs out of 4mm steel plate, sandwiched together with a 4mm gap inside and a 'M' shaped set of baffles inside so the water has to do a M pattern from inlet to outlet to maximise time in the wetback.
After doing reading, we read that just a standard 1/2" copper tube style wetback can be inefficient due to the water in the pipe being hot around the outside of the pipe, but with a flowing cold centre. One way was to squash the round tube, which apparently increases performance quite considerably. We took this theory and applied that to steel plates instead, with a 4mm gap to simulate a squashed tube. I couldn't be happier with the result. 3/4" steel nipples welded on top and bottom, top one one a slight angle to encourage the thermo syphoning effect. Both side backs paralled at the back of the fire, and hot goes up, and cold comes in from under the floor. Top tubes always on an incline, no flat spots anywhere.
Have 2 radiators installed at the moment, 2100W and a 800W, and the fire doesn't seem any smokier than it was before. House was beautifully warm today, I just cannot believe the performance, I am really happy.
So the system takes only minutes to warm up, which is warming up the 25L in the tank up in the ceiling, and about 10L total in the radiators, and then pipe work, so maybe 50L max. Would be very interesting to try and calculate the 'kW' rating of these side backs.
Ended up doing the setup I pictured last, http://www.taktik.co.nz/geekzone/OpenLoop-Idea7.png
We used copper 20mm pipe from the fire up to the cylinder, and then copper out of the cylinder into the pump. out of the pump goes into 22mm plastic PEX tubing with speedfit type push fittings, down through a wardrobe and under the house. It then T's off into 10mm PEX into the radiators in parallel, and the 22mm is capped off. Out of the radiators in 10mm PEX, T'ed back into another run of 22mm PEX and long to the bottom of the fire, back into 20mm copper, through the floor and into the cold side of the wetbacks. All lagged.
Got it running on Sunday, so 2 days of running, no leaks and a nice toasty warm front of the house. Bedrooms don't have heating yet, but even those are benefiting from the larger radiator, which brings the fire heat a bit closer.
Thanks to all who helped with info and input, no matter how small. All positively impacted the end result.