I have run into this on two different houses in the span of a single week… A high-efficiency furnace combustion exhaust vent run up and through the middle of the old mid-efficiency B vent and out the top. There is the use of temporarily caulking where metal flashing is needed. Perhaps what the installers should have done is exhaust into the old B vent without penetrating the rain cap and then flashing/sealing from the underside? Or, is the only acceptable solution is they should have installed a new vent flashing? Anyone have a useful narrative about this situation?
This is what I think I should have said in the report and propose for the next time this situation occurs unless someone has a better narrative:
Flashing Defect at Furnace Exhaust Vent
I observed a high-efficiency combustion exhaust pipe run up and through the existing B-vent for an old furnace. This is a flashing defect and prone to leaks. Further evaluation and correction by a roofing professional is recommended.
The water heater exhaust pipe is sealed only with caulk and tape: materials that are insufficiently durable to prevent leaks. Strongly recommend these flashings be completely redone, so no caulk tape or seams are present in the path of water, using a proper manufactured roof cap. The installer must be careful to avoid gaps where CO could enter the house from the roof.
You don’t want to buy off liability for your preferred solution, which some plumber may mess up (for example by not sealing the gap between the plastic and b-vent, thus leading to CO flowing down the annular ring.
I really do not want to prescribe any solution. But, you brought up CO, which is a legit concern. Thinking my generic “leaks” could be interpreted as water or CO. Where you see high-efficiency furnace exhaust vents done well they have a normal plumbing vent stack type of flashing with rubber boot and then two to three feet of pipe sticking out above the roof. No chance for water or CO to become a problem as long as the rubber boot is in serviceable condition.
I think this is the best example I have found on the internet. The combustion exhaust is on the left and you can see the hot air rising. The combustion intake vent on the right has to be protected from rain water, hence the elbows, and perhaps should have been installed a bit lower (perhaps this install is in an area that gets a ton of snow).
Also, It is not just for rain. It’s to prevent exhaust being sucked into it. In the north, it is common to see a snowball on the intake because the intake is at a lower atmospheric pressure, causing the exhaust gas moisture to freeze at lower temperature.
As for your narrative, “Installation of furnace flue not installed to industry standards.” Is a short and sweet one.