Rinnai Tankless With No Water Flow
The Short Answer: A Rinnai does not pump water. No flow at the hot tap is closed isolation valves, a packed inlet screen, a seized flow sensor, scale-blocked exchanger, 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.
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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 heater, not a burner problem. Low shower pressure with some flow is Rinnai low pressure and tankless pressure drop. Dark In Use with some flow is In Use light not on. This page is zero or near-zero through the hot side.
What other guides get wrong
They tell you to “increase the water pressure setting on the Rinnai.” 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.
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 Rinnai |
| Cold strong, hot dead | Path through the Rinnai | This page |
| Cold strong, hot a trickle | Restriction — low pressure | |
| One fixture dead, others hot | That cartridge / stop | Not the heater |
| No flow + leak under the unit | Isolation, filter cap, or exchanger | Shut down |
- 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.
- Required tools: $12 hose-bib gauge.
Step 2: Isolation valves — the 30-second save
- What to look for: Yellow or 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 flush. Recirc return valve closed on a RUR so the loop is a brick.
- 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: Cold-side filter that is no longer a screen — it is a disc of 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.
- 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, flow sensor, and a unit that will not pass water
- What to look for: Hard water, skipped flush, unit used to flow, now nothing. Scale can close an exchanger like cholesterol — that is a licensed isolation-valve flush or an exchanger, not vinegar in the house cold. Water-flow sensor (turbine) seized so the board never opens the bypass/control path on some models; more often the sensor just refuses to fire — you would still have cold water at the hot tap if isolation is open. If hot taps are dry with valves open, think exchanger, isolation, or a closed mixing-valve stop.
- Required tools: Isolation valves + manufacturer descaler if scale is the bet. Do not screwdriver the turbine live.
A mixing valve at the heater failed closed will also give you no hot flow with a happy Rinnai. Thermometer on the outlet nipple: hot at the unit, dead at the house = valve downstream.
Step 5: Recirc, crossover, and freeze ice plugs
- What to look for: Cheap under-sink recirc pumping the wrong way. Check valve stuck. Outdoor unit after a freeze: ice in the waterway — do not apply a torch; thaw with power restored for freeze protection or wait. A freeze-split then a shut isolation is how “no flow” hides a leak. After an outage: Rinnai after a power outage.
- Required tools: Freeze history. Do not crack potable unions on a guess.
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), and any freeze-split. Heat-exchanger replacement and recirc design are licensed. A Rinnai 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 Rinnai tankless have no water flow? A: Closed isolation valves, a packed inlet screen, a scale-blocked exchanger, a failed mixing valve downstream, or the house has no water. The Rinnai does not create pressure.
Q: How do I get water flowing through a Rinnai 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 Rinnai? 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.