Navien Tankless With No Water Flow

The Short Answer: A Navien does not pump city water. No flow at the hot tap is closed isolation valves, a packed cold-inlet filter, a scale-blocked exchanger, a failed mixing valve, freeze ice, or a house that has no water. Gauge cold at a hose bib first. Trickle is a different article. The unit cannot light what it cannot pass.

Search Intent & The Real Problem

You opened hot. Nothing. Or a cough of air. Cold taps still blast. That is a path problem through the NPE-A2, NPE-S2, NPN, or NCB/NHB combi, not a burner problem. Same physics as Rinnai no water flow and Noritz no water flow — different filter housing and ComfortFlow buffer on A-series. Low shower pressure with some flow is Navien low pressure and tankless pressure drop. Ice-cold with a flame icon and a code is not getting hot, not this page.

What other guides get wrong

They tell you to “increase Navien water pressure” on the remote. Residential units do not have a booster pump. They descale a unit whose isolation valves are 90° closed. They replace the heat exchanger because the inlet filter is a brown cookie of Teflon and solder balls from the original install. And they skip the cold-vs-hot test, so a whole-house PRV gets blamed on the tankless — low pressure after install.

Step-by-Step Troubleshooting

Step 1: Split house water from heater path

• What to look for: Hose-bib static. Write the number down. Under ~40 psi many Naviens will not wake cleanly; over ~80 psi you have a PRV/expansion problem, not a flow booster.
• Required tools: $12 hose-bib gauge.

Kitchen next to the unit dry, master bath also dry, cold hose bib hammering: the Navien path. One sink only: that stop or aerator.

Step 2: Isolation valves — the 30-second save

• What to look for: Service-valve handles not parallel to the pipe. Both cold and hot isolation must be open; a closed hot-side valve passes nothing to the house even if the unit would fire. Valves left 45° after a “handyman flush.” Recirc return closed on an A-series ComfortFlow so the buffer loop is a brick. Cascade pair with the secondary isolated.
• Required tools: Eyes. Hands on the handles.

Open them fully. Retest before you buy parts. If your install has no service valves, that is an incomplete job — hire the flush, do not unscrew the house shutoff and hope.

Step 3: Inlet filter — packed solid

• What to look for: Cold-side inlet screen packed with sand, well grit, Teflon, or solder balls. Brand-new installs do this. Pull it only after cold isolation is shut and a hot faucet has relieved pressure. Cap not seated after the last “flush” is how you get a leak and no flow at the same time. Navien documents this screen as customer-cleanable when the isolation kit exists.
• Required tools: Isolation valves shut, bucket, needle-nose.

If the screen is a brown cookie, you found it. Rinse. If there are no service valves, pulling that filter is a licensed add-on plus a wet floor.

Step 4: Scale, mixing valve, and a seized flow sensor

• What to look for: Hard water, skipped annual flush, unit used to flow, now nothing. Scale can close a Navien exchanger like cholesterol — licensed isolation-valve flush with Navien’s kit, not vinegar in the house cold. A mixing valve at the heater failed closed gives you no hot flow with a happy burner. Thermometer on the outlet nipple: hot at the unit, dead at the house = valve downstream. A flow sensor that is seized more often kills the burner than the pipe; you would still have cold water at the hot tap if isolation is open. One fixture only at the kitchen is no hot to the kitchen sink.
• Required tools: Isolation valves + manufacturer descaler if scale is the bet. Contact thermometer. Do not screwdriver a live sensor.

Hot taps bone dry with valves open and a clean screen: exchanger or mixer — hire it. Do not jumper the flow sensor to “prove” it.

Step 5: Freeze, ComfortFlow, and when the board is already wet

• What to look for: Outdoor NPN after a 15°F night with power off: ice in the waterway. Indoor garage that hit 15°F with no freeze kit. Do not apply a torch. Restore freeze-protection power or wait. A freeze-split then a shut isolation is how “no flow” hides a leak — leaking from the bottom. A-series buffer air-bound after a drain. Cheap under-sink recirc pumping the wrong way. After rain or an outage, standing water in the cabinet is off — not a reset. Condensate overflowing onto the PCB is condensate drain.
• Required tools: Freeze history. Do not crack potable unions on a guess.

Manifold gas pressure is irrelevant to a dry hot tap. Do not purge a gas line because the shower will not fill. NG ~3.5–7 in. w.c. / LP ~10–11 in. w.c. is a licensed manometer job after you have water — confirm the rating plate.

When DIY Ends: Calling a Licensed Pro

You may open isolation valves, rinse the inlet filter, and gauge the hose bib. Stop at no service valves, a flush that does not restore flow, a leaking exchanger, static over 80 psi (PRV work), a failed mixer, and any freeze-split. Heat-exchanger replacement and recirc design are licensed. A Navien with no flow on day one was piped with a closed valve or a crushed flex — that is an install defect, not a warranty board.

Frequently Asked Questions

Q: Why does my Navien tankless have no water flow? A: Closed isolation valves, a packed inlet filter, a scale-blocked exchanger, a failed mixing valve, freeze ice, or the house has no water. The Navien does not create pressure.

Q: How do I get water flowing through a Navien again? A: Open both service valves fully, shut cold isolation, relieve a faucet, pull and rinse the inlet screen. If that fails, stop and hire a licensed plumber.

Q: Can a bad flow sensor stop all water through a Navien? A: Usually it stops the burner, not the pipe. If hot taps are bone dry with valves open, look at isolation, the screen, scale, or a closed mixer — then hire it. Do not jumper the sensor.