Is it normal for tankless intake and exhaust pipes to merge before the roof?

That fitting in your attic is a **concentric vent kit**, and your hunch about a pipe-inside-a-pipe setup is spot on. Installers sometimes call them bazooka vents in the field, and they are specifically engineered for high-efficiency condensing equipment like your Navien and gas furnace.

Inside that main outer sleeve, there is a smaller pipe running straight down the center. The hot exhaust gas travels up trough that inner pipe and vents out the very top of the roof termination. Meanwhile, the ring of empty space between the inner pipe and the outer pipe acts as the fresh air intake channel. Both air streams remain completley separated the entire time, so there is zero mixing inside the attic or the piping. Outside on the roof, the termination cap is designed specifically to prevent recirculation. The exhaust blows straight up out of the top nozzle, while the fresh combustion air pulls in from screened slots or a bell hood positioned several inches lower down. beacuse the hot flue gas has upward velocity and disperses fast, the vacuum from the intake only pulls clean ambient air.

Plumbers use them all the time because cutting one clean hole through roof decking and flashing a single pipe is way less prone to leaks than flashing two seperate penetrations spaced feet apart. As long as it is strapped securely to the framing so it does not sag, your setup is completely normal and doing exactly what it was designed to do.

Just make sure whoever hung the assembly maintained the required quarter-inch per foot slope back toward the Navien, because acidic flue condensate will inevitably collect in that inner pipe and needs an unimpeded path to drain back down into the unit’s internal trap rather than pooling near the junction.

Also take a quick look at the transition fitting to verify they actualy accounted for its equivalent length penalty in thier vent run calculations, as those concentric adapters typically add anywhere from five to eight equivalent feet of friction loss compared to standard straight schedule 40 pipe.

Totally normal setup, but one detail people forget to check is the exterior clearance above the roof deck. If you live anywhere that gets real winter weather, that lower intake collar needs to sit at least 12 inches above your area's maximum expected snow line. I've seen plenty of Naviens lock out on an E110 error during heavy blizzards becasue drifting snow packed right into those intake louvers, even while the center exhaust tip was still poking out into open air.

One thing I'll throw in—check that outside cap every spring. Wasps and mud daubers love building nests inside that fresh air intake ring since it's warm and protected. Had my unit throw wierd combustion codes a couple summers back, and sure enought, a giant wasp nest was choking the slots. If yours doesn't have a wire mesh screen over the intake opening, slap one on there so you don't end up with bugs getting sucked down into the blower fan.

While you're up there, give that pipe a firm wiggle test. When my house was built, the sub just rested the concentric assembly against a rafter with one loose piece of plumber's tape. First cold snap, whenever the water heater fired up full blast, the vibration hummed right trough our bedroom ceiling below.

I threw a 2x4 block across the rafters and clamped the outer pipe tight using a rubber-cushioned unistrut clamp. Killed the noise imediately and stopped the pipe from shifting around in the roof flashing every time the wind kicks up.

Quick update — huge relief knowing it's just a concentric vent and totally safe! However, after checking the slope like suggested, I noticed the horizontal run actualy sags a bit right before hitting that roof fitting instead of sloping back toward the unit. How critical is that slight dip? coud standing condensate there choke the airflow or corrode the PVC over time?