Your tankless unit just finished its hardest stretch of the year. Long showers after beach days, guests, laundry running most weekends, and inlet water coming in warm enough that the burner cycles differently than it does in January. All summer it has been pulling Southern California water — typically 10 to 20 grains per gallon of hardness across the Inland Empire and much of the LA basin — through a heat exchanger with passages about the width of a pencil.
Calcium and magnesium drop out of that water when it gets hot. They stick to the inside of the exchanger. Skip the flush two or three years running and you get the classic failure: hot, cold, hot in the shower, an error code on the front panel, and a heat exchanger replacement that runs $700 to $1,400 in parts and labor on a unit that should have lasted 20 years. September and October are the right window because demand is dropping and you are not fighting for an appointment during the first cold snap.
What the scale is actually doing in there
A tankless heater has no storage tank, so there is nowhere for minerals to settle out harmlessly the way sediment does at the bottom of a 50-gallon tank. It all plates onto the exchanger walls. Scale insulates, so the burner has to fire hotter and longer to hit your set temperature, which lays down more scale. That is the loop.
You will hear it before you see it. A ticking or crackling during a long shower is water flashing to steam against a scaled surface. Then flow starts dropping at the hot side only, and the unit begins throwing ignition or overheat codes. The same minerals doing this are the ones quietly narrowing the supply lines behind your walls, so a heater problem is often the first visible symptom of a whole-house water quality issue.
The fall checklist
- Descale the heat exchanger (annually, or every 6 months on well-known hard lines). A pump circulates food-grade white vinegar or a commercial descaler through the unit for 45 to 60 minutes, then a clean-water rinse. This is the item everything else on this list supports. Homeowners with a service valve kit and a submersible pump can do it; most people would rather not stand over a bucket for an hour. A professional flush and inspection typically runs $150 to $300 nationally, and our tankless flush and service is $150.
- Clean the cold-water inlet screen filter. Small mesh screen at the cold inlet, usually a quarter-turn to remove. Grit from the street main — especially in older LA and Orange County neighborhoods with 1950s and 1960s cast-iron and galvanized mains still in the ground — collects here and mimics scale symptoms. DIY, five minutes, shut the cold supply first.
- Check the isolation valves for weeping. Those two red-and-blue service valves take the abuse of every flush. Look for green corrosion or crust on the bonnet. A dripping valve is a $150 to $300 fix now and a wet drywall repair later. DIY to inspect, pro to replace.
- Inspect the venting and combustion air path. Look at the exterior termination. Stucco dust, a wasp nest, dryer lint drift, or landscaping grown up against the vent will all starve the burner. Looking is DIY. Anything involving opening the cabinet, touching the gas line, or adjusting combustion is licensed-plumber work — there is no safe hobbyist version of that.
- Decide about a softener or scale filter. If you are descaling twice a year and still seeing codes, the water is the problem, not the schedule. Whole-house softeners typically run $1,500 to $3,500 installed; a template-assisted crystallization cartridge unit ahead of the heater runs $400 to $900 and needs media swapped every few years. Pro install, and it usually pays for itself in exchanger life and dishwasher life together.
- Log the date on the cabinet. Masking tape and a marker. Most manufacturers require documented annual descaling to honor the heat exchanger warranty, and nobody remembers whether it was last October or the one before.
What can wait and what cannot
A ticking noise and a slightly slower recovery can wait until you have an appointment that suits you. A unit cycling hot and cold mid-shower can wait a week or two. A gas smell near the unit, an error code that will not clear after a reset, scorching or soot around the vent, or water dripping from the bottom of the cabinet is a today problem. Tankless units are usually wall-mounted in a garage or on an exterior wall, which limits flood damage compared with a tank failure, but a leaking exchanger will not fix itself and running it dry ruins it.
Worth knowing before anyone quotes you a replacement: swapping a water heater in California generally requires a permit, and inspectors commonly add items to the job — correct venting, a seismic-rated mounting, gas line sizing for the unit's BTU draw, sometimes a condensate drain for a condensing model. Those code items are the difference between a $2,200 install and a $4,500 one, not the box itself.
Where a flush fits with the rest of the house
Fall is also when the rest of the hot-water side shows its age. Check the aerators on your kitchen and bath faucets; if you are soaking them in vinegar every few months, the heater is dealing with the same load. If you are heading out of town for the holidays, the same trip that gets the heater serviced is a good time to walk the shutoffs and supply lines before you leave.
And if you have a tank heater somewhere else on the property, a guest house or a garage unit, that one wants a flush and an anode rod check. In water this hard the rod is often gone by year three, and a tank heater is 8 to 12 years, not 20.
If you only do one thing
Descale the unit. Everything else on the list is maintenance around the edges; the flush is the thing that decides whether your heat exchanger sees year 20 or year 11. Do it before Thanksgiving, while the schedule is still open and a cold shower is still an inconvenience rather than an emergency.