Venturi (gas-air mixing)
The narrowing that mixes gas into the combustion air stream in the right proportion. No moving parts on most units, which is why it fails by getting dirty rather than by breaking.
What it does
Getting a clean burn means mixing gas and air in a narrow ratio, and holding that ratio as the unit modulates from a trickle to full output. The venturi is how it is done: a constriction in the air path that drops the pressure and draws gas in proportion to the airflow through it.
Because the geometry sets the ratio, the venturi is matched to the gas type. Rinnai’s SENSEI generation uses a coloured orifice to mark it — red for propane, blue for natural gas — and a unit running with the wrong one will not burn correctly no matter what else is right. It is one of the first things to check on any heater that has been converted between gas types.
How it shows up in the codes
It has its own numbers on the newer Rinnai generations — code 15 for a venturi control fault and code 17 for a venturi blockage — and neither exists on the older non-SENSEI units, which handle gas-air mixing differently.
Navien numbers it too: E060, an abnormal operation of the dual venturi, for which Navien’s owner-facing instruction is simply to contact technical support. There is no homeowner step, and that is the manufacturer being honest rather than unhelpful.
Why it fails
Not mechanically, usually. It gets restricted. Dust, lint and insects reach it through the air intake, and on units drawing combustion air from inside a laundry or a dusty basement it happens faster. The symptom is a unit that burns badly rather than one that stops outright, which is why it is often found while chasing something else.
Keeping the intake air filter clean is the maintenance that matters here. How to clean the air filter
Why this is not a DIY part
It sits in the sealed combustion path. Disturbing it means breaking seals that keep combustion products contained, and refitting it wrong changes the gas-air ratio on a live gas appliance — which is the definition of the work that needs a licence.
What a replacement usually costs
$60-400 for the assembly, and it is often replaced together with the gas valve rather than alone. Installed, $200-1,100 depending on whether the valve goes with it.