It's going pretty well. I got the cistern cleaned out and poured a little mortar mix into a form on the top to level everything out, and sunk a few studs to hold the 2x6s down around the permieter of the tank. This weekend I'm going to lay up the block along the one wall to create the 1,000-gallon enclosure and put in the foamboard. Then I have to order the pond liner. Given the price of fuel, probably best to do that before shipping charges go even higher. I have some concerns about priming the pump and keeping it from cavitating when it's hooked up in a non-pressurized environment. I have a nice Grundfos 20-42 3-speed pump that I was going to use on the tank side of the heat exchanger, but now I think I'll just go with a $50 Taco 007 from Ebay and see how it holds up in a semi-oxygenated, possibly-cavitating application. I thought it would be fun to fool around with the speeds, but I have other places on the system where it will be just as fun if I put the pump somewhere else. My plan of attack on this project is to peck away at it over the summer, working on different elements of the piping as time and spare change allows. It should be ready to go by late July or early August, and that's when I'll have the boiler delivered. I'll post pics of my progress on the tank over the weekend.