Tankless Water Heater Repair in San Francisco and Daly City

Why has a tankless unit stopped producing hot water?

Tankless failures work differently from tank failures. There’s no reservoir of already-heated water, so when something goes wrong you notice immediately and completely. The usual causes, roughly by how often we find them:

• Scale in the heat exchanger. Narrow passages, high temperatures, and minerals in the water. Scale restricts flow and raises exchanger temperature until the unit protects itself and shuts down.
• Ignition failure. The unit tries to light and can’t. Gas supply, gas pressure, a fouled burner, a failed igniter.
• Flame loss after ignition. It lights, then drops out. Usually a fouled flame sensing rod or a combustion air problem.
• Flow sensor faults. The unit never knows you’ve opened a tap, so it never fires. More on this below, because it’s often not a fault at all.
• Venting or air supply blockage. Intake or exhaust obstructed, a failed fan, or condensate backing up in a condensing unit.
• Gas supply that was never adequate. A unit that has always struggled under simultaneous demand may have an undersized gas line rather than a fault. That’s an installation problem showing up as a repair call.

And one that isn’t a fault: the cold water sandwich — a slug of cool water between two hot draws, caused by the unit cycling off and on between uses. It’s a characteristic of how tankless works, not a defect, though a recirculation setup reduces it.

What do tankless error codes actually mean?

Code numbers are manufacturer-specific — the same fault is a different number on a Rinnai, a Navien, a Noritz and a Rheem, which is why searching a bare code number often returns the wrong answer. What’s consistent is the families the codes fall into. Knowing the family tells you how urgent it is and whether it’s worth touching yourself.

Ignition failure. The unit attempted to light and failed, usually after several tries. Check first that the gas is actually on and that other gas appliances work — this is the one family where the cause is sometimes trivially outside the unit. Beyond that: gas pressure, burner condition, igniter.

Flame failure or flame loss. It lit and then lost the flame. Most often a flame sensing rod coated in residue, which is a clean-or-replace job rather than a major repair.

Overheat or high limit. The exchanger got hotter than permitted. Scale is the leading cause, followed by restricted flow and blocked venting. Don’t keep resetting this one — it’s a protection device doing its job, and repeated resets on a scaled exchanger shorten the unit’s life.

Flow or no-flow detected. The unit isn’t registering water movement. Could be the sensor, could be debris, could be that flow genuinely is too low. See below.

Venting and air supply. Blocked intake or exhaust, fan failure, pressure switch. On a condensing unit, a blocked condensate line can present here too.

Temperature sensor or thermistor. Erratic output temperature, or the unit reading a temperature it can’t reconcile.

Maintenance reminder. Some units flag a descaling interval rather than a fault. Worth knowing before paying for a diagnosis.

Two things to do with any code: write it down before clearing it, and note what you were doing when it appeared. A code that only shows up when two showers run is a different problem from one that shows up on a single tap, even when the number is identical.

Flow sensors, and the unit that won’t fire at low flow

This one generates a lot of service calls that don’t need a plumber.

Every tankless unit has a minimum activation flow — it won’t fire until enough water is moving through it, typically somewhere around half a gallon per minute depending on model. Below that threshold, nothing happens, by design.

So a tap that only ever runs at a trickle, or a showerhead that’s silted up, or a scaled aerator, can leave you with a unit that appears dead at one fixture and works perfectly at another. Before assuming the heater has failed, check whether it fires with a bath tap wide open.

When it is the sensor, it’s usually one of three things: debris on the impeller, a failed sensor, or an inlet filter screen that’s clogged — the last being a genuinely common maintenance item that costs very little to address and is often skipped entirely.

Descaling in San Francisco versus Daly City

These two cities are on different water, and it changes the maintenance interval.

San Francisco draws overwhelmingly from the SFPUC regional system — Hetch Hetchy surface water, which is soft. Tankless units here scale more slowly than in most of the Bay Area.

Daly City runs a blend: SFPUC surface water plus groundwater from the city’s own wells. Groundwater carries more dissolved mineral than the surface supply, so Daly City’s water comes out harder than San Francisco’s, generally reported in the moderately hard range, with the exact figure moving as the blend shifts through the year.

Practically: a tankless unit in San Francisco can often go longer between descalings, while a Daly City unit should be on an annual schedule. If you’re not sure, the error history tells you — a unit throwing overheat codes is scaling faster than it’s being serviced, whatever the calendar says.

Manufacturers commonly make documented descaling a warranty condition, so this is rarely optional in practice. For how hardness varies further down the Peninsula, tankless installation in Palo Alto, Los Altos and Mountain View covers the wider picture.