When an instant water heater stops giving hot water, shows an error code, or only runs lukewarm, scale in the heat exchanger or a blocked inlet filter is the first suspect, especially in Los Angeles. You can clean the inlet filter and, if the maker allows, flush the unit with vinegar. Faults with the flame, gas, ignition, vent or the electronics are for a plumber or the maker's service network, not a weekend job. This page covers the symptoms, the flush, the brands, and the point where you call. For the wider picture, see our water heater repair and replacement page.
What are the common reasons an instant water heater stops working?
The usual reasons are scale in the heat exchanger, a clogged cold inlet filter, a flow rate too low to fire the unit, a tripped breaker or no gas supply, a blocked vent or condensate trap, and a failed sensor or board. Scale and filters are the cheapest to fix, and they are the first to check.
A tankless unit starts heating only when water flows past its sensor at a minimum rate. A partly closed valve, a clogged aerator or a worn cartridge in the shower can drop the flow below that line, and the heater shuts off, then restarts, which feels like a cold burst. Check the faucet before you blame the heater.
For gas units, no hot water can also mean no gas or a vent problem. If you smell gas, leave the building, call SoCalGas on 1-800-427-2200 from outside, and call 911 if there is immediate danger. Do not try to work on gas parts.
What do error codes on an instant water heater mean?
Error codes vary by brand and model, and the manual lists them. A code about scale or flow often points to mineral buildup, while codes about ignition, flame, the vent or the condensate drain point to those systems. Look up the exact code in the manual for your model before you do anything.
As an example, Rinnai displays an LC code when scale is building up, and the advice on that is to flush the unit. A different code, 25, relates to the condensate trap, which is a separate problem. These are examples, not a list for every brand. If a code comes back after you reset the unit, write it down with the model number and give both to the plumber, since it saves a diagnostic visit.
Brand-specific repairs are covered in the manufacturer's service documents. Models from Rinnai, Navien, Noritz, Rheem, Takagi, Bosch and others share the same basic parts, but their codes and procedures differ, so always use the manual for your exact model, including propane and electric versions.
How do you descale or flush an instant water heater?
Turn off the power or gas and the water, connect hoses to the service valves, circulate white vinegar through the unit with a small pump for about 45 to 60 minutes, then rinse with clean water. Many makers say at least once a year, and more often in hard water. Follow your maker's steps.
Rinnai, for example, specifies about four gallons of undiluted white vinegar and says not to use chemical cleaners. A flush kit with a pump and hoses costs roughly $75 to $125, and the vinegar adds a few dollars. Without service valves on the hot and cold lines, the unit cannot be flushed this way, and a plumber may need to add them.
Los Angeles hard water is the reason this matters. LADWP classes water above 7 grains per gallon as hard. Its 2022 averages ran from 8.6 grains in the Valley and West Los Angeles to 16.3 in the east and the harbor area. Our page on removing hard water scale explains how deposits form, and a water softener installation slows them.
Why does an instant water heater give cold bursts or weak hot water?
Cold bursts happen when flow drops below the unit's minimum or when someone opens a second tap and the unit cannot keep up. Weak or lukewarm water usually means scale restricting flow through the heat exchanger, a dirty inlet filter, or a unit that is too small for the demand.
Try one fixture at a time and see if the problem disappears. If it does, the unit is working but is at its limit. If the problem shows up at every tap, clean the inlet filter and flush the unit, and if that does not fix it, a plumber should check the sensors, the temperature setting (units can deliver a range of temperatures) and the gas pressure or power supply.
Electric instant heaters add a different limit. They draw a lot of power, and a breaker that trips, or a house wiring that cannot supply the load, will cut the heat. If the breaker trips, do not keep resetting it. Call a plumber or an electrician.
If you have lost hot water throughout the house, our page on why you have no hot water sorts the causes.
Is it worth repairing an instant water heater, or should you replace it?
Repair makes sense when the fault is scale, a filter, a sensor or a board and the unit is within its expected life. ENERGY STAR and the Department of Energy put tankless life at around 20 years with maintenance, so an older unit with a failed heat exchanger may be better served by water heater replacement.
An exchanger that has scaled over badly or cracked can cost close to a new unit to fix. Ask the plumber for the repair and replacement prices side by side, in writing, based on the model and age of your unit. Our page on tankless water heater installation cost gives the published ranges, and installation of a tankless hot water heater shows what a replacement involves in Los Angeles. A switch of fuel or location needs a permit.
When should you call a plumber for an instant water heater?
Call when there is a gas smell, a vent or exhaust problem, a flame fault, a leak inside the unit, a code that returns after a flush, or when no hot water persists after you clean the filter. Repairs involving gas parts, venting or electrics belong to a plumber with the right California contractor license.
We can arrange a local plumber to come to your home, read the codes, check the flush valves and vent, and give you a written quote before any work starts. Send the details on our quote page, including the brand, model and any code. The home page lists what else we arrange across the city.
Common questions
How much does instant water heater repair cost?
It depends on the fault. A flush and filter clean is a small job, a sensor or board costs more, and a heat exchanger can approach the cost of a new unit. Published guides vary too much for one figure to be reliable, so ask for parts and labor on a written quote.
How often should an instant water heater be flushed?
Rinnai says at least once a year, and installers suggest more often in hard water, sometimes twice a year. Los Angeles has hard water in many areas, so annual flushing is the minimum. Clean the cold inlet filter once or twice a year too.
Can I repair a tankless water heater myself?
You can clean the inlet filter and flush it with vinegar, if the maker permits and the service valves exist. Anything involving gas, venting, the burner or the electronics should be left to a plumber, because mistakes can cause carbon monoxide or gas leaks.
Why does my tankless water heater show an error code?
The unit has detected a fault, such as scale, low flow, a flame or ignition problem, or a blocked vent. The code tells you which system to look at. Check your manual, try a power reset once, and call a plumber if the code returns.
Does an instant water heater work in a power outage?
Most gas tankless heaters need electricity for the fan, ignition and controls, so they stop when the power goes out. Check your model's manual. Electric units need power to heat at all. If you rely on hot water in outages, ask the installer what your options are.