I had a 2 zone open-loop radiant heat system heated by a Takagi Tk3 natural gas water heater which also heats our domestic hot water. The new system was put in four years ago, but we have only lived here 6 months. Because our climate (Pacific Northwest on the coast) is mild, the heating system didn't go on much when we moved in in the summer, but it has worked fine during the colder months. The problem is the domestic hot water. The tap water is supposed to get hot when we just turn the taps, but it only goes on when we turn up the wall thermostat for the radiant floor. We have the wall remote for the Takagi and it doesn't register the correct amount of flow (.5 gpm) to turn the Takagi on when we open any tap . We do get hot water when the radiant heat is working. I cleaned the little Takagi filter -very gunky), cleaned the flame sensor with a green scotch guard pad. Still Nothing. The unit has not ever been descaled so I ordered a descaling kit and will try that next. I put one of those Easy Water water conditioners on at the time of the system installation. I am also getting error message 111 on the remote: ignition failure. It would be easy enough to test the ignitor with a meter. So I'm thinking--descale thoroughly, test the ignitor, check the venting make sure they are clear, have the gas company check working pressure on the gas line. Can anybody think of anything else?? This is really a great forum!!