It is because of this situation that code is in place to control this.
The unit is supposed to have a chimney liner the proper size installed in the chimney.
The flue gases are obviously condensing in the vent by lack of draft and temp. The contractor at the time was to have installed b vent and liner.
Nothing else will remedy this until the liner and b vent are in place.
It is not size...it is unconditioned space.
And a 5" clay in 5" is totally different than 5" round. And at 75000 btu I doubt you have a 5" vent pipe unless older 70 % efficient.
The fact you are condensing shows the flue gas is not exiting the flue in a timely manner and is condensing due to being stalled.