On a compact Navien combi install, a failed expansion tank can send system pressure climbing fast enough that the relief valve starts discharging.
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Quick answer
Navien's combi boilers are sealed hydronic systems, and like any sealed system, they rely on an expansion tank's air-charged diaphragm to absorb the volume increase that happens as water heats up. Many Navien wall-hung units in Metro Vancouver condos and townhomes are installed with a small internal or closely coupled expansion tank rather than a large freestanding one, and when that tank's precharge bleeds off or its diaphragm fails, system pressure can climb quickly enough that the relief valve begins discharging within just a few heating cycles.
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Wall-hung combi boilers are popular in Metro Vancouver's newer condos and townhomes largely because of their compact footprint, and that compactness extends to the expansion tank, which is often a small unit mounted directly on or inside the boiler cabinet rather than a large separate tank elsewhere in the mechanical room. Its job doesn't change with the smaller size — absorb the volume increase from heated water so the sealed system doesn't overpressure with every heating cycle.
Because these tanks are smaller, they can also be less forgiving when their precharge drifts, meaning symptoms sometimes appear a bit sooner on a compact combi system than on a larger conventional boiler setup with a bigger reserve tank.
Most calls start the same way: the pressure relief valve is dripping, or the system pressure gauge is reading higher than it used to. Sometimes there's a banging noise in the pipes as the system fires. Occasionally the boiler will lock out on a high-pressure fault entirely, particularly if the tank has failed suddenly rather than degrading gradually.
To check the tank properly, a technician isolates it from the system and releases system pressure at the tank so the reading isn't skewed by live water pressure pushing against the diaphragm. With the tank empty, the air charge is read at the valve stem using a standard gauge and compared against the pressure the manufacturer specifies for that model and your system's static fill pressure.
On compact combi units, access can be tighter than on a conventional floor-standing boiler, so this check sometimes takes a bit more disassembly around the cabinet than the equivalent job on an older-style system.
Once a failed tank is confirmed, the technician isolates and drains the affected section, removes the old tank, and fits a replacement matched to the boiler's specified pressure requirements — matching matters more on a compact combi system since there's less margin than on a larger conventional setup. After connecting the new tank, the loop is purged of air, brought back to correct static pressure, and monitored through a heating cycle to confirm the gauge stays stable rather than climbing.
If the relief valve has been discharging for a while, it's worth having the technician check whether repeated cycling has affected the valve's ability to reseat cleanly, since that sometimes needs addressing alongside the tank.
Because compact combi units are relatively new to a lot of homeowners compared to older-style boilers, there's sometimes an assumption that a smaller, modern system needs less attention than a big freestanding boiler did in a previous home. In practice, the expansion tank in a compact system still needs the same periodic precharge check as any other closed hydronic system — the smaller size doesn't make it maintenance-free, and if anything the tighter margins mean symptoms can appear a bit sooner once the tank starts to drift.
This is worth flagging specifically for condo and townhome owners who may not think about their combi boiler the same way they'd think about a furnace, since the mechanical room visit that used to happen for furnace filters doesn't always translate to checking a wall-hung combi unit's pressure system.
Most Navien combi units display system pressure directly on their control panel or a nearby analog gauge, and it's a genuinely easy thing for a homeowner to glance at periodically without any special tools. A reading that's meaningfully outside the manufacturer's stated normal range — either too low or climbing too high — is worth mentioning at your next service call even if nothing else seems wrong yet, since catching a drifting tank early is generally simpler than waiting for a relief valve to start discharging.
That said, a single unusual reading isn't necessarily cause for alarm on its own — pressure naturally shifts somewhat between a cold system and one that's mid-heating-cycle, and a technician can explain what's normal for your specific setup.
Safety note
If you smell gas or suspect a venting issue near your boiler, leave the home immediately and call FortisBC at 1-800-663-9911 from a safe location. This repair should be carried out by a licensed technician.
Often yes — many wall-hung combi units are fitted with a compact tank sized for the system, sometimes mounted on or inside the boiler cabinet rather than as a large separate tank, though the exact setup depends on the specific install.
A failed or under-charged expansion tank is one of the most common causes, though a technician will also confirm the system wasn't simply overfilled during a previous service.
It's the boiler's safety control responding appropriately to protect the system, so the lockout itself is a safety feature working correctly — but it does mean heat and hot water are interrupted until the cause is fixed.
That depends on whether isolation valves are present at the tank on your specific install — a technician can confirm this during inspection.
CANRO doesn't set a fixed interval, but checking precharge as part of an annual service visit is a reasonable way to catch a drifting tank before it causes a pressure problem.
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