Many boiler stoves available are still only rated for use on open vented systems, however a lot of heating systems are sealed or pressurised so you cannot simply join the pipes up.

Remember that you really don't need (or want) to pull all that much additional heat out of your exhaust, so you don't need all that much surface area to do it. These pipes are then all coupled to one pipe at the bottom part of the fire, this pipe then runs to a box where there are a few fans designed to blow room temperature air into the pipes which then run under up and over the fire blowing heated air out the top pipes. But as the others have said, unless you want your chimney to rain, knock off the MacGyver routine and invest in a new system. now with a class one brick built chimney you can get away with a 3 ft piece of standard m/steel rolled pipe to get the fumes into the chimney then let it do its job. It's much harder to get a good hot penetrating weld with a wire welder than a beefy old stick welder with some 6010 or 6011 rod.The old gas boilers would heat the water, and discharge the products of combustion to the atmosphere.

Purely incidentally, but somewhat along the same lines, the various devices used by some on wood stoves have somewhat similar problems, with the added problem of allowing the creosote from relatively incomplete combustion of wood (particularly in airtight stoves) to condense in the chimney. Since this thing is going to be pumping air into my work space, I wanted to be sure and remove all the Zinc to be on the safe side. The Typhoon heat exchanger connection diameter 150 mm and 6 pipes with damper sheet steel increases the heat exchange surface and heat output, saves energy costs and protects the environment. All bare water pipes have now been insulated but I'm thinking the biggest issue is with the boiler itself since if I hold my hand even a couple of feet away from the exit of the flue, it would burn.

If I built the heat exchanger, it would almost certainly be made of mild steel, a disadvantage in durability but an advantage in constructibility.