Noritz Tankless With No Water Flow
The Short Answer: A Noritz does not pump city water. No flow at the hot tap is closed isolation valves, a packed inlet screen, 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 complaint. The unit cannot light what it cannot pass.
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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 NRC, EZ, NRCP, NR, or NCC, not a burner problem. Same physics as Rinnai no water flow — different filter housing and, on some Noritz, a water-flow servo. Low shower pressure with some flow is tankless pressure drop and Navien low pressure. A puddle under the unit is Noritz leaking from the bottom, not this page.
What other guides get wrong
They tell you to “increase the water pressure setting on the Noritz.” Residential units do not have a booster. They descale a unit whose isolation valves are 90° closed. They replace the heat exchanger because the inlet screen 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 you have | Meaning | Next |
|---|---|---|
| Cold and hot both dead | Meter, well, PRV, main valve | Not the Noritz |
| Cold strong, hot dead | Path through the Noritz | This page |
| Cold strong, hot a trickle | Restriction — screen, scale, mixer | Steps 3–4 |
| One fixture dead, others hot | That cartridge / stop | Not the heater |
| No flow + leak under the unit | Isolation, filter cap, or exchanger | Shut down |
| Outdoor unit after a freeze | Ice plug or split | Do not torch it |
- What to look for: Hose-bib static. Write the number down. Under ~40 psi the unit may never wake; over ~80 psi you have a PRV/expansion problem, not a flow booster. Sister Rinnai low pressure if you have some flow.
- Required tools: $12 hose-bib gauge.
Step 2: Isolation valves — the 30-second save
- What to look for: Blue/red 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 NRCP so the loop is a brick. Quick-Connect / cascade pair with the secondary isolated.
- Required tools: Eyes. Hands on the handles.
Open them fully. Retest before you buy parts.
Step 3: Inlet screen — packed solid
- What to look for: Cylindrical cold-side filter housing on the service valve. Screen that is no longer a screen — grit, pipe dope, well sand, or solder. 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.
- Required tools: Channel locks with tape, needle-nose, bucket.
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, water-flow servo, and a mixing valve downstream
- What to look for: Hard water, skipped annual flush, unit used to flow, now nothing. Scale can close a Noritz exchanger like cholesterol — licensed isolation-valve flush or an exchanger, not vinegar in the house cold. Some NRC/EZ units use a water-flow adjustment / servo; a motor that failed closed starves the house while the board looks awake. 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.
- Required tools: Isolation valves + manufacturer descaler if scale is the bet. Contact thermometer. Do not screwdriver a live servo.
Hot taps bone dry with valves open and a clean screen: exchanger, servo, or mixer — hire it. 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.
Step 5: Freeze, recirc crossover, and when the board is already wet
- What to look for: Outdoor NR after a 15°F night with power off: ice in the waterway. 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. Cheap under-sink recirc pumping the wrong way. Check valve stuck. After rain or an outage, standing water in the cabinet is off — not a reset.
- 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.
When DIY Ends: Calling a Licensed Pro
You may open isolation valves, rinse the inlet screen, 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 servo, and any freeze-split. Heat-exchanger replacement and recirc design are licensed. A Noritz 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 Noritz tankless have no water flow? A: Closed isolation valves, a packed inlet screen, a scale-blocked exchanger, a failed mixing valve or flow servo, freeze ice, or the house has no water. The Noritz does not create pressure.
Q: How do I get water flowing through a Noritz 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 Noritz? A: Usually it stops the burner, not the pipe. If hot taps are bone dry with valves open, look at isolation, the screen, scale, a servo, or a closed mixer — then hire it. Do not jumper the sensor.