We have a high output but very rarely installed GSW Envirosense water heater that stopped working six days ago on the May long weekend. I had two service companies, that I had used before, come out and both were baffled by the problem (with very little diagnostic work done) and both ultimately told me I needed a new water heater. Given no one could tell me what the specific problem was I tried a third company - Maple Home Services and the service tech (sorry I didn't get his name) came and for the lowest cost of the three spent 3 hours doing a thorough diagnostic on the water heater. He concluded that it was an problem with the air intake, but the air was coming through the intake pipe properly. We then put a shop vac on blower mode and blew air through the water heater intake and suddenly a lot of water and two pieces of balled-up electrical tape came out of the water heater exhaust port. The tape must have been there since manufacture or installation. without the thorough diagnostic by Maple Home Services I would have been spending thousands of dollars on a new water heater like the first two service companies recommended. Instead I paid a $95 diagnostic fee and the problem was solved. Thanks!!