Homeowners would like to replace the current water heater (leaking) with a Rinnai tankless water heater. Shut off the water to the home and drained the system down. Remove and dispose of the existing water heater off site. Adjust gas as necessary in the home to adequately provide gas to the unit. Mount the water heater on the wall and run new water, gas, condensate, intake and exhaust to and from the unit. Install crossover tee in the home to provide for recirculation. Electrical installed new circuit and outlet for the water heater to operate. Pressurize and remove the air from the system and put the water heater into service. When relighting the furnace pilots lights the basement furnace would not light. Cleaned the thermocouple and it still would not light. It was below freezing outside and the furnace had been off all day while installing the water heater. The furnace is 30 years old. In an attempt to get the furnace operational I replaced the thermocouple, attempted to light the