Tankless water heaters need less maintenance than tank units, but they’re not zero-maintenance. Most owners should plan on a descaling flush once a year, a filter screen cleaning every six months or so, and a quick visual check of the venting. That’s the short version. Here’s what each of those actually involves, and where the line is between owner-serviceable and “call a licensed plumber.”
How a Tankless Heater Accumulates Scale
Cold water carries dissolved minerals, mostly calcium and magnesium. When that water heats up inside the heat exchanger, those minerals drop out and stick to the metal walls as scale. In the Bay Area and most of California, water hardness varies by city, but it’s rarely soft enough to skip descaling altogether.
Scale is insulating. Even a thin layer makes the burner work harder to hit your target temperature. Over time, this raises your gas bill, reduces flow rate, and can eventually damage the heat exchanger severely enough to require replacement, which is an expensive repair.
Navien units (the NPE series for pure hot water, the NCB series for combined space heating and hot water) give you an error code when scale buildup gets serious enough to cause overheating. But you don’t want to wait for that. An annual flush keeps things clean before any error trips.
Descaling: What the Process Looks Like
A descale flush pushes a dilute acid solution, usually white vinegar or a purpose-made descaler, through the heat exchanger in a loop using a small submersible pump and two service port hoses. The acid dissolves the scale. Then you flush with clean water.
The service ports are on the cold inlet and hot outlet lines. Most tankless units installed in the last ten years have isolation valves with built-in ports specifically for this. If yours doesn’t, a plumber can add them.
Owner-serviceable? Technically yes, if your unit has service ports and you’re comfortable working around the gas appliance. You need a small pump (a sump pump or a cheap transfer pump works), two lengths of hose, a bucket, and white vinegar or commercial descaler. Navien publishes a descaling procedure on their website and in their installation manuals; follow it exactly rather than improvising. Budget a couple of hours your first time through.
If you’re not comfortable around gas appliances, or your unit doesn’t have proper service ports, have a licensed plumber do it. It’s routine work and shouldn’t take long.
Inlet Filter Screens
Navien units have a filter screen on the cold water inlet. It catches debris before it hits the heat exchanger. These get clogged and reduce flow. The symptom is usually a drop in hot water pressure that doesn’t match the rest of your house.
Cleaning it is straightforward: shut off the cold water supply valve, release pressure from a nearby faucet, unscrew the inlet fitting, pull the screen, rinse it under the tap, reinstall. Check it every six months, or sooner if you’ve had any recent plumbing work that could have stirred up sediment.
Venting and Combustion Air
Navien units are typically direct-vent, pulling combustion air in through one pipe and pushing exhaust out through another. The termination caps on the exterior wall can accumulate debris, spider webs, or bird nests, especially the intake. A blocked intake causes combustion problems and can trigger error codes.
Once a year, walk around the exterior and look at the vent terminations. Make sure they’re clear and the caps are intact. This is a visual check any owner can do.
The venting pipes themselves, the joints and fittings, are a different matter. If you notice any discoloration around joints, smell exhaust near the unit, or see condensation where you wouldn’t expect it, stop using the unit and call a plumber. Combustion gas leaks are not a DIY diagnosis.
What a Licensed Plumber Should Handle
Some things on a tankless heater aren’t owner territory, regardless of your comfort level:
- Gas connections and pressure. If you suspect a gas leak or the unit’s gas pressure seems off (burner won’t stay lit, partial ignition), that’s licensed plumber and possibly gas company work.
- Pressure relief valve (T&P valve) testing and replacement. This is a safety device. Testing and replacing it should be done by someone who knows what they’re doing.
- Error codes you can’t resolve. Navien error codes can have multiple causes. Chasing the wrong one wastes time and can void the warranty. A plumber with Navien experience can pull fault history and diagnose accurately.
- Any work inside the unit. The heat exchanger, gas valve, flow sensor, igniter, and control board are not owner-serviceable parts on a live appliance.
When to Call
Call a licensed plumber if:
- Your unit hasn’t been descaled in over a year and you’re seeing reduced flow or heating issues
- You smell gas anywhere near the unit
- The unit throws a persistent error code after you’ve cleared the simple stuff (like the inlet filter)
- You want the descaling done but aren’t comfortable doing it yourself
- It’s been more than a year since anyone has looked at the T&P valve
Always verify that the plumber you hire is licensed in California. You can check license status at cslb.ca.gov. A valid license number and current standing matter, especially for anything involving gas appliances.
One More Thing on Warranty
Navien’s residential warranty excludes damage caused by failure to perform proper maintenance, and some models require written maintenance records upon request. If you’re doing the descaling yourself, keep a log with dates. If a plumber does it, get a receipt that specifies what was done. That documentation can matter if you ever need a warranty claim.
Maintenance on a tankless unit is genuinely manageable for most homeowners. The annual flush is the piece people skip most often, and it’s the one that causes the most expensive downstream problems. Start there.