Intake air filter

What it does

A sealed-combustion heater pulls its air from somewhere — outside through the concentric vent, or from the room. Either way, dust and lint come with it, and anything that reaches the fan and burner degrades combustion. Rinnai’s SENSEI RX has a serviceable filter behind the front panel at the top right of the cabinet, with a pull tab. Navien’s NPE units have an intake air filter too, and Navien names cleaning it as the owner’s remedy for three separate codes — flame loss, fan motor, and abnormal air pressure.

Why it matters more than it sounds

A restricted air filter reduces combustion airflow. Rinnai lists it as step 2 on its blocked air supply or exhaust code, right after the condensate drain. Navien puts it in front of the owner on E012 (flame loss), E109 (fan motor) and E110 (abnormal air pressure) — three codes whose other possible answers are vent work, a fan motor, and a gas-side diagnosis.

Cleaning it is free. That is the whole argument.

Rinnai’s instruction is also explicit that the heater must not be operated with the air filter missing.

Cleaning it

Power the unit off at the controller, remove the two screws and release the two latches on the front panel, pull the filter out by its tab, scrub it with mild dish soap and a soft brush, rinse, dry it with a lint-free towel, and refit. Our guide covers the whole sequence, including how often it is worth doing.

Replace the filter if it is damaged or if it still looks dirty after a second wash.

What a replacement costs

$20-60 for the filter assembly, or nothing at all if it just needs washing.