Bringing this back from the dead as 18 months in I have still not found a solution and was all but ready to abandon ship and just live with the forced air heating as the three contractors i have had come by will not touch it when my brother who is stationed in Vienna Austria had some plumbers stop by to service a 100+ year old hydronic radiator system in his building.

They pumped this goop into the lines for 6-7 hours to seal them and then purged the lines. he took a picture of the stuff they had and it was from a company called BaCoGa

http://www.bacoga.com/eng/bcg-td/

Anyone have experience with this stuff?

Only downside from my end is that I have already partly disassembled the system to make room in the laundry/pantry area. The furnace was scrapped and the pumps removed but the plumbing is still there except for one manifold which would have to be relocated anyway...

I also posted this over at greenbuilding advisor
http://www.greenbuildingadvisor.com/community/forum/mechanicals/28008/repairing-hydronic-floor-heat-leak-concrete

How expensive is the goop-sealer treatment? Can you even get it in the US?

I'd be a bit concerned about DIY approach using grey-market without knowing a whole lot more about the prescribed conditions for the treatment, but it wouldn't surprise me if such an approach really COULD work. If the pipes aren't so corroded as to be nearly occluded with iron oxides it's probably worth a shot if it isn't too expensive, but it might be important to temperature cycle it under pressure to verify that the seal was actually achieved before committing to a whole new boiler system, if there isn't a "plan-B" for what to do with the boiler should the repair prove unreliable.

Thanks Dana,

The goop is relatively cheap. My brother can ship it to me and it is about $300 for the goop plus the fittings I would need. I was considering starting with a descaler first to remove and flush out some of the oxidation. They also sell a product for this.

The big cost is the boiler, pumps, and control system. I would certainly pressure test the system first with air but it would be difficult to do any thermal cycling tests before biting the bullet and purchasing the boiler. i suppose it may be possible to loop through my domestic hot water heater (tank style gas unit) but I doubt i could get the volume of hot water needed for a good test.

You can get "Base Hit II - Hydronic System Sealant" for $140/gallon here in the US. But you may be better off just getting 3/4" of polyiso insulation on the floor instead of repairing the radiant heating system.