Indirect water heater service and repair

How an indirect differs

An indirect water heater is an insulated storage tank with a heat exchanger coil inside it. Your boiler circulates hot water through that coil, and the coil heats the domestic water in the tank. There is no burner, no flue and no combustion in the water heater itself, it is a heating zone that happens to make hot water.

That design has real advantages. Recovery is fast, because a boiler puts far more energy into the coil than a standalone tank's own burner ever could. There is no second appliance to flue or service. And because the tank has no burner, the tank itself tends to last longer than a conventional gas water heater.

The complication is that hot water now depends on the boiler and on the controls that link them. In most installations the indirect is wired as a priority zone, so when the tank calls for heat the boiler serves it first and pauses the heating zones. That is why a fault in the indirect can present as a heating problem, and a boiler fault can present as no hot water, and why diagnosing one without understanding the other wastes time.

No hot water at all

Work outward from the simplest cause.

• Is the boiler running? No boiler means no hot water. Check the service switch, the breaker, fuel supply and any lockout. Our boiler repair page covers that chain.
• Is the tank aquastat calling? The indirect has its own thermostat, usually a strap-on or immersion aquastat. If it has failed, or its setpoint has drifted, the boiler is never told to make hot water.
• Is the indirect's zone valve or circulator working? This is the most common failure by some margin. The boiler fires, but nothing carries that heat to the coil. Feel the pipes running to the tank when it should be calling, cold pipes with a hot boiler is a clear answer.
• Priority control. On some relay panels, a fault in the priority logic leaves the tank permanently deprioritised behind heating zones that are always calling in winter.
• Coil failure. Uncommon but real: a scaled or perforated coil. A perforated coil shows up as boiler pressure changing when domestic water is drawn, or as domestic water appearing in the heating system.

Because the boiler and the tank are so intertwined, describe both symptoms when you call, whether the heating still works, and whether the hot water fails completely or just runs short.

Hot water that runs out too fast

Running out of hot water is different from having none, and it points somewhere different.

Sizing. An indirect sized for a household of two does not cope with a household of five, and adding a soaking tub or a high-flow shower changes the arithmetic substantially. If the tank has always run short and nothing has failed, it is a sizing problem rather than a fault.

Scaled coil. Scale on the heat exchanger coil is the classic cause of a tank that used to be fine and now recovers slowly. The coil is still transferring heat, just far less of it, so the tank refills with lukewarm water and never catches up during a busy morning.

Aquastat setpoint or differential. A setpoint that has been turned down, or a wide differential letting the tank cool a long way before calling, both shorten usable capacity.

Priority not working. If the boiler is trying to heat the house and the tank simultaneously in January, recovery slows dramatically. Priority exists precisely to prevent that, and when it fails, the symptom is seasonal, fine in September, poor in January.

A failing mixing valve. A thermostatic mixing valve that has failed can bleed cold into the hot supply, which feels exactly like a tank that cannot keep up.

Scale, sediment and water quality

Massachusetts water varies enormously, soft and slightly acidic in much of the western and central part of the state, considerably harder in parts of the south and on private wells with high mineral content.

Hard water deposits scale on the coil, which is the main enemy of indirect performance. Soft, acidic water is gentler on the coil but harder on the tank lining and the anode. Either way, sediment accumulates in the bottom of the tank over years and reduces effective capacity.

Annual maintenance is straightforward and rarely done: flushing sediment from the tank, checking the coil for scale, verifying the aquastat and testing the temperature and pressure relief valve. On hard water, descaling the coil at intervals restores recovery to something close to original. Where water is genuinely hard, treatment upstream is worth considering, it protects the indirect, the boiler and every fixture in the house.

The anode rod

Glass-lined steel tanks are protected by a sacrificial anode rod, a magnesium or aluminium rod that corrodes in place of the tank. When the anode is consumed, the tank starts corroding, and once that begins the clock is running.

Anodes typically last three to five years, less on soft or aggressive water, and almost nobody replaces them. A tank that fails at eight years and one that lasts twenty is very often the same tank with a different anode history. Checking and replacing the anode is one of the highest-value maintenance items on any storage water heater, and it costs a fraction of a replacement tank.

A smell of rotten eggs in the hot water, incidentally, is usually a reaction between the anode and bacteria in the water rather than a plumbing fault, and switching anode material generally resolves it.

Temperature and scalding

Storage temperature is a genuine trade-off. Stored hot enough, the tank suppresses bacterial growth; stored too hot and delivered unmixed, it scalds, and scalding at domestic hot water temperatures happens fast, particularly to children and older adults.

The standard resolution is to store at a temperature high enough to be safe microbiologically and fit a thermostatic mixing valve to deliver water to the taps at a safe temperature. That gives you both the storage safety and the delivery safety, and it also increases usable capacity, because hotter stored water mixed down goes further.

If your system stores hot and has no mixing valve, that is worth addressing. If it stores lukewarm to avoid scalding, that is the wrong solution to the right concern and should be changed.

When to replace

Indirect tanks generally outlast conventional water heaters, and a well-maintained one can run fifteen to twenty-five years. Replacement becomes the answer when the tank itself leaks, a leaking tank is not repairable, when the coil is perforated, or when scale is so advanced that descaling no longer restores useful recovery.

A leaking tank is also worth acting on promptly rather than watching: they rarely improve, and a stored tank of water failing in a finished basement is an expensive way to learn that.

At replacement, it is the right moment to reconsider capacity honestly against how the household actually uses hot water, and to compare against the alternatives: see water heater installation and tankless installation.