LEARNING CENTER WATER HEATERS
Tankless water heater problems: error codes, the ‘cold water sandwich’, and the flush that fixes most of it
The short answer
Nine times out of ten, a tankless water heater that’s throwing error codes, swinging between hot and cold, or losing hot-water flow is dealing with scale. Louisville water runs hard, about 7 grains per gallon, and every gallon through the heat exchanger leaves a little mineral behind. The passages narrow, the exchanger runs hotter than designed, and the unit protects itself by throttling flow or shutting down with a code. A descaling flush runs $500–$600 and clears most of it. The ‘cold water sandwich’ is different: that burst of hot, then cold, then hot again is a quirk of how tankless units fire, not a defect. And if codes keep coming back after a flush, that’s when a real diagnosis earns its keep.
Tankless water heaters are honest machines. When something’s wrong, they usually tell you, right on the little display, in a code you never asked to learn. We service a lot of these around Louisville, and most calls trace back to one root cause, a few to gas or venting, and a couple aren’t problems at all. Here’s how to read what yours is telling you.
The usual suspect: scale in the heat exchanger
A tank heater keeps 50 gallons hot around the clock. A tankless unit heats only while you’re using it: open a tap, a flow sensor wakes the burner, and water picks up heat as it races through a coiled heat exchanger. No tank, no standing water, no sediment bed, which is why tankless units never rumble the way tanks do.
What they get instead is scale. Louisville water carries about 7 grains of dissolved mineral per gallon, and every gallon through the exchanger leaves a trace of it on the hot metal. After a couple of years those traces become a lining, the water path narrows, and the exchanger has to run hotter to move the same heat.
Here’s the part that confuses people. A tankless unit is built to protect itself, so when the exchanger runs hot it throttles flow to hold a safe outlet temperature, which you feel as weak hot-water pressure, or it shuts the burner down and posts a code. The symptoms look electronic. The cause is usually mineral.
Which symptom is yours
- Hot water flow got weak, but cold is fine Classic scale. The unit is pinching flow to keep water from overheating inside a narrowed exchanger. If the cold side of the same faucet runs strong, the restriction lives on the hot side, and the heater is the first suspect. A clogged inlet filter screen can mimic it, and that one’s a five-minute cleaning.
- The temperature swings while you’re in the shower Ongoing swings are usually the unit hunting to hold its set temperature through a crusted exchanger. A single burst of cold near the start of a shower is different: that’s the cold water sandwich, which gets its own section below, and it isn’t a fault.
- It won’t fire at all No hot water anywhere means the unit isn’t lighting: gas off, an ignition problem, or a hard shutdown from a code. Check the display first. If the screen is dark, check the breaker, because a tankless unit needs electricity even though it heats with gas. If you’re in a cold shower right now, we wrote a full walkthrough for no hot water.
- It’s making a banging or rattling noise Not normal for tankless. There’s no tank to rumble, so noise usually means scale debris moving in the exchanger or a burner lighting late. Worth a look rather than a wait.
What the error codes actually mean
Every brand numbers its codes differently. A Rinnai speaks a different dialect than a Navien or a Rheem, so the number matters less than the family it belongs to. Write yours down before you reset anything, because the code is the diagnosis and clearing it throws away the note. Your manual translates it, and almost every code falls into one of five families.
- Ignition failure The unit tried to light and couldn’t. Gas valve off, an empty propane tank, or air in the line after utility work. This is the one code where a single power-cycle and retry is fair. If it comes right back, stop cycling it and get it looked at, because repeatedly relighting a gas appliance that doesn’t want to light isn’t a hobby.
- Flame loss It lit, then lost the flame mid-run. Often gas supply again: a borderline gas line can feed the unit fine in summer and starve it in January, when the furnace pulls from the same line.
- Overheat and scale warnings The exchanger is running hotter than spec. This is the scale family, and it’s the most common thing we find behind a code in Louisville. Some brands even post a dedicated maintenance code when the unit calculates it’s overdue for a flush.
- Venting and combustion air The unit can’t breathe in or out. Walk outside and check the intake and exhaust terminals: leaves, snow, ice, a bird’s nest, a wasp nest. Clear what you can see. If vent codes continue with clear terminals, the problem is inside the vent run, and that’s a service call.
- Flow sensor and water-side faults The unit can’t tell how much water is moving, or too little is moving to fire safely. A scaled exchanger, a fouled inlet screen, or a failing sensor all land here.
The ‘cold water sandwich’, explained
You turn on the shower and get warm water almost right away. Then, a minute in, a slug of cold hits you. Then it’s hot again and stays hot. That’s the cold water sandwich, and it’s not a malfunction.
The first warmth is leftover hot water sitting in the pipes from the last time somebody used a tap. Behind it comes the slug that passed through the heater before the burner reached full temperature, and behind that comes properly heated water. Short stop-and-start draws make it worse, which is why it shows up during morning routines.
If it genuinely bothers you, a recirculation loop keeps hot water staged near your fixtures so there’s no cold slug to feel. That’s the point of the high-efficiency tankless with built-in recirculation, which runs $6,000–$6,200 installed. For most families, though, this is a personality trait to live with rather than a problem to spend money on.
The flush that fixes most of it
- A descaling flush We isolate the unit, circulate descaling solution through the heat exchanger for about 45 minutes, and rinse it clean. It runs $500–$600, and on a scaled-up unit the difference is dramatic: full flow comes back, the temperature steadies, and the overheat codes stop. With our water, once a year is the right rhythm.
- Clean the inlet filter screen Free, and a reasonable DIY. There’s a small screen where cold water enters the unit. Close the isolation valve, pull the screen, rinse out the grit, and put it back. Your manual shows where it lives.
- A real diagnosis when the picture isn’t clean If the code isn’t in the scale family, or a flush didn’t clear it, a tankless water heater diagnostic runs $100–$200. That’s a tech with the manufacturer’s code chart, a gauge on your gas pressure, and eyes inside the unit, instead of parts bought on a hunch.
When it’s bigger than a flush
Most tankless calls end at descaling. A few don’t, and we’d rather you recognize them early:
- Ignition or flame codes that survive a flush Scale doesn’t cause those. The usual culprit is gas supply: a line too small for the unit’s full fire, or low pressure at the meter. Resizing a gas line is real work we price after measuring, not off a chart, but left alone the unit keeps starving right when it needs the most gas.
- It froze during a cold snap Tankless freeze protection is electric, so it only works while the unit has power. A single-digit Louisville night plus a power outage is exactly the recipe. If the exchanger froze and split, you’ll see leaking when it thaws, and a split exchanger on an older unit is usually a replacement conversation, not a repair.
- The unit is past about 15 years and the codes keep coming Heat exchangers on the units we install carry 10 to 15 year warranties, and with annual flushes the equipment often runs close to 20. Past that, repeat repairs on a tired exchanger stop making sense. A mid-efficiency tankless replacement runs $5,200–$5,400 installed plus Kentucky’s $50 state permit, and if you’re weighing a return to a tank, a standard 50-gallon gas tank runs $2,200–$2,400. The honest tradeoff is in our tankless cost guide.
- You smell gas Stop troubleshooting. Leave the house and call LG&E’s emergency line from outside, then call us. No error code is worth standing next to a leak.
Common questions we get
How often should a tankless water heater be flushed in Louisville?
Once a year. Our water runs about 7 grains per gallon, and scale builds on a schedule whether you think about it or not. Annual descaling keeps the exchanger efficient and the warranty honest, and it’s the cheapest thing you can do for the unit’s lifespan. If yours has never been flushed, start now rather than waiting for a code.
Can I descale a tankless water heater myself?
If the unit was installed with isolation valves, a homeowner with a small pump, two hoses, and descaling solution can do it, and plenty do. Two honest cautions: without those valves a DIY flush turns into a plumbing job anyway, and if the unit is throwing codes outside the maintenance family, a flush alone won’t answer why. If yours lacks the valves, ask about adding them.
Why does my shower go cold for a minute and then get hot again?
That’s the cold water sandwich: leftover hot water from the pipes, then the slug that passed through before the burner hit full temperature, then fully heated water. It’s how tankless units fire, not a defect, and it’s worst with short stop-and-start draws. A recirculation system eliminates it if it truly bothers you, but most people learn to count to ten.
How long does a tankless water heater last?
With annual descaling in our hard water, 15 to 20 years is a fair expectation, roughly double a tank. The heat exchangers on units we install are warrantied for 10 to 15 years. The catch is that neglect compounds: a unit never flushed in Louisville water can scale its way into trouble in half that time.
Why won’t my tankless heater make hot water at a trickle?
Tankless units need a minimum flow, roughly half a gallon per minute, before the burner will fire. A barely-open tap never wakes it up, so you get endless cold water and assume something’s broken. Open the tap further and it should light. If it takes more and more flow to trigger it over time, scale or a fouled inlet screen is fooling the flow sensor, and that’s flush territory.
The next questions to ask
Tankless acting up and not sure if it’s a flush or something bigger?
Ask Goldie walks you through it in a couple of minutes and tells you what’s likely going on before anyone comes out. Try it at goldenplumbingky.com/instant-estimate/