Gas water heater noise that pointed to limescale in the coil

The heater noise that gave away limescale in the coil

Shut off the main valve, remove the shower tap, clean the filter and let the water run for a few seconds so the pipe would spit out the muck. Textbook maintenance, the kind done in any home on a Sunday. And from then on, the gas water heater started roaring. The case is described by the owner of an apartment with a Cointra Godesia located just over a metre from the nearest tap, on the same floor, who started the thread looking for someone who had been through the same thing.

The symptoms, clearly defined: the noise appears with hot water at any point in the house, whether the burner is on or off, always above a certain flow rate. It is not gradual — either silence or a racket — it takes three or four seconds to kick in, and it does not change when the flow regulator wheel is turned. That last detail rules out half the armchair diagnoses: if it were the burner or the combustion, changing the flow would alter it.

What lies behind a noise that only appears with hot water

The first hunch going around pointed to air in the circuit: a trapped bubble that vibrates as the water passes and that can be purged by opening a small valve at the top of the unit, after removing the casing. It fits the story — the system was handled, air got in — but it falls short when the flow rate drops steadily.

The second, more prosaic theory pointed to a heat exchanger clogged with limescale. A part that, according to one user, costs around 200 euros and that in a five-litre open-flue water heater accumulates years of scale. The outcome proved this line right: when the coil inlet was disconnected, it turned out the water was arriving with far more flow than was leaving the unit. The plug was inside.

Muriatic acid, an accepted risk and an ending without a technician

The fix was not signed off by an official service. The owner, with bad past experiences — a hundred euros for a diagnosis and an invitation to buy another unit — dismantled the heater and poured a diluted muriatic acid solution into the coil to dissolve the limescale. It worked: the unit performed like on day one. It should be said clearly: tampering with a gas appliance and handling acid without training is reckless, no matter how well it turned out in this case.

The story has an epilogue that borders on the absurd. Another affected user, with no hot water and strange noises after a bathroom renovation, discovered that the workman had connected a cold-water hose to the hot water line. A short circuit of circuits. The heater had nothing wrong with it; the installation had everything.

Servicing, warranty and the underlying debate

Underneath it all remains the uncomfortable question: how much of these troubles is maintenance, and how much is a stock of old heaters that nobody checks? The periodic gas inspection is, in practice, limited to checking that combustion does not produce carbon monoxide; it does not diagnose a clogged coil. And users only call when the tap stops delivering water.

The full calculation of what a technician costs versus a new unit, fine print included, is another matter entirely. What was proven here is that the ear, sometimes, detects what the bill does not yet show.