We had a new gas-fired boiler and indirect water heater installed about 12 years ago. All has been fine and we have had the system serviced annually and it has needed no repairs. A few days ago, after returning from vacation when we had lowered the water heater temperature as usual when we are away for some time, we increased the temperature and soon afterwards we noticed it was leaking from the overflow pipe. It was diagnosed as an expansion tank fail and a new #60 tank was installed. As soon as the water heated the leaking recommenced. The technician came back and said it was the other expansion tank after testing the pressure and today he installed an Amtrol ThermX and said he had never seen both go at the same time but we should not have any more problems. By this time we have spent around $1K but this is DC. We are not very happy that both were not diagnosed at the same time which might have reduced the around $400 per visit but figured if it was that unusual the technician, who we have had for years for both the heating and air systems and highly respect, could be excused for not testing both. Later today however we noticed the leaking again. We are very upset and the company got the technician to call and he said that the only thing it could be now is the valve that directly connects to the outlet pipe on the top of the water heater. Does any of his sound plausible? Thanks.