I have the same boiler and had the same problem when I accidentally left my water treatment system unplugged for a couple of months.
When working properly, your water conditioner removes the minerals from the water before it enters the household hot water coil in your boiler. When the water treatment system is not working, the mineral rich water enters the hot water coil and the minerals get deposited on the coil walls as the water is being heated. Over time, this slowly blocks the flow of water through the coil.
To fix the problem, the mineral deposits must be recovered from inside the coil. This is done using acid. I used muaritic acid from the hardware store though other milder acids would probably work. Getting the acid into and out of the coil is the main problem. If the plumbing going into and out of the coil does not have access fittings and the appropriate valves to isolate the coil, you will probably have to call a plumber to have him cut into the pipe to be able to get the acid into and out of the coil.
The chemical reaction that occurs when the acid hits the mineral deposits is somewhat violent so be sure to wear a face shield and rubber gloves. I ran a hose from the output side of the coil into a 5-gallon bucket of water to minimize the acid spray. When the acid was poured into the coil, the gas produced by the chemical reaction made that water bubble violently for several seconds. When the bubbling stopped, I quickly poured fresh water into the coil to flush out the acid. After that, the coil worked like new with no flow restriction.
Bruce, May 2008
