I own a home built in 1955. I recently had a plumber install a new water heater (Navien NPE A2 natural gas tankless). The old water heater was a natural gas tank water heater. It had a Transite asbestos flue for the exhaust gas that passed through the roof. I read that the inner lining of this type of flue can over time flake off and sometimes obstruct the exhaust vent. The old water heater needed to be replaced, so I decided to switch to a condensing tankless gas water heater.
Unfortunately, the water heater is not near an exterior wall, so the plumber routed the vent pipes (2" PVC) up one level and then through the attic above the garage. The amount of condensation at the exhaust terminal is more than I expected. The horizontal run above the level of the garage initially had a slight downward slope toward the vent terminal. After reading that horizontal runs should have a 2% upward slope, I asked the plumber to adjust the PVC above the garage. He made a slight adjustment and there was some improvement. The slope is now roughly horizontal (still not a 2% upward slope). The plumber told me that this amount of condensation is to be expected. The exhaust terminal is above a brick pathway, so ice forms on the pathway when the temperature is below freezing, with some water/ice collecting near the base of the house (see picture).
It’s probably not ideal to have the exhaust terminal above a brick pathway, although the pathway does not get much use in the winter.
The length and number of turns is in the upper range of the manufacturer's specifications, but still within limits. Do you think that further adjusting the PVC would be helpful? A more upward slope would allow more of the condensation to flow back toward the water heater and out the condensate drain inside the house. Is there any problem with sloping upward more than 2%? Are there are other solutions to decrease the amount of dripping?
One option would be to send the exhaust through the roof. This would allow a shorter run but I have read this could create other problems, such as the possibility of a roof leak or potential damage to the roof due to the acidic nature of the exhaust.
I would appreciate any input. Thanks!