after installing a new heating and AC in the spring the heater failed

some pressure switch was xchanged the heater worked fine

two weeks later after heavy rain the heater failed again, the same switch was defective, this time the repair man stated that a wrong vent pipe caused the switch to corrode due to water condensation, the ceramic vent pipe needs to be replaced with a metal vent pipe this will stop the condensation, unless the vent pipe is changed the new switch will fail again

he quoted the bargain perice for the pipe of $250 and the labour another $250, so a whopping 500 bucks. My question :

is this true?

is the estimate reasonable?

can I do this myself?

any advice would be appreciated

Thanks