High conductivity in boiler water is caused by a buildup of dissolved solids, primarily minerals and salts that accumulate as water evaporates into steam and leaves those solids behind. Every ion dissolved in the water, whether it arrived with the makeup supply, the chemical treatment program, or a leak in the condensate system, increases the water’s ability to conduct electricity. Conductivity is essentially a proxy measurement for how many dissolved solids are present, with a standard conversion of conductivity (in µS/cm) multiplied by 0.7 to estimate total dissolved solids in parts per million at 25°C.
How Dissolved Solids Drive Conductivity
Pure water is a poor electrical conductor. What makes boiler water conductive are the charged particles (ions) dissolved in it: calcium, magnesium, sodium, chloride, silica, sulfate, carbonate, and others. The more of these ions present, the more easily current flows through the water, and the higher the conductivity reading climbs. Both the type and the amount of dissolved solids matter. Alkalinity and acidity have an especially large influence on readings, which is why boiler water samples are typically neutralized before measurement to get a reliable number.
The Concentration Effect of Evaporation
The single biggest reason conductivity rises over time in any boiler is the basic physics of steam generation. When water boils, it leaves the drum as relatively pure vapor. The dissolved solids stay behind in the liquid. With every cycle of evaporation, those solids become more concentrated in the remaining water, similar to reducing a sauce on a stove.
This accumulation is described by a ratio called “cycles of concentration,” which compares the dissolved solids in the boiler water to those in the incoming makeup water. If your makeup water enters at 200 µS/cm and your boiler water reads 2,000 µS/cm, you’re running at roughly 10 cycles. Without regular removal of concentrated water through blowdown, conductivity will keep climbing until it causes problems.
Makeup Water Quality
The raw water feeding your boiler is the original source of most dissolved solids. Well water, municipal water, and surface water all carry varying loads of calcium, magnesium, sodium, chloride, and bicarbonate. Hard water areas can deliver significantly higher mineral loads than soft water regions. If the water treatment system upstream of the boiler (softeners, reverse osmosis, demineralizers) isn’t performing properly or is undersized for demand, more minerals pass through into the boiler and accumulate with each evaporation cycle.
A sudden jump in conductivity often points to a softener that has exhausted its resin, a failed RO membrane, or a change in the source water supply. Checking makeup water conductivity first is the fastest way to rule this in or out.
Chemical Treatment Additives
The chemicals deliberately added to protect the boiler also contribute to conductivity. Oxygen scavengers, alkalinity builders, phosphate-based scale inhibitors, and pH adjusters all introduce ions into the water. Sodium hydroxide (caustic soda) and sodium carbonate, commonly used to maintain alkalinity, are particularly strong contributors because sodium ions are highly mobile in solution.
This means a certain level of conductivity is expected and even necessary for a properly treated boiler. The goal isn’t zero conductivity; it’s keeping conductivity within the recommended range for your operating pressure while maintaining the protective chemistry the boiler needs. Overfeed of treatment chemicals, however, can push conductivity above safe limits just as effectively as poor makeup water quality.
Contaminated Condensate Return
Condensate returning from process heat exchangers, coils, and steam traps should be nearly pure water with very low conductivity, typically between 1 and 10 µS/cm. When a heat exchanger develops even a small leak, process fluids (cooling water, product liquids, oils, or other chemicals carrying dissolved salts, acids, or alkalis) contaminate the condensate. Because clean condensate conductivity is so low, even the slightest leak produces a sharp, unmistakable spike in conductivity readings.
This contaminated condensate then returns to the boiler and adds dissolved solids that wouldn’t normally be present. Common culprits include glycol from heating loops, sugars in food processing, and cooling tower water that leaks through tube failures. If your boiler conductivity rises and makeup water quality checks out fine, the condensate return system is the next place to investigate.
Insufficient Blowdown
Blowdown is the controlled release of concentrated boiler water, replaced by fresher makeup water, and it’s the primary tool for keeping conductivity in check. Two types serve different purposes: surface blowdown runs continuously to skim off dissolved solids concentrated near the water surface, while bottom blowdown is performed periodically to flush sludge and sediment from the lowest point of the boiler.
If blowdown rates are set too low, or if automatic blowdown controllers malfunction, dissolved solids have no exit path and simply keep concentrating. A stuck blowdown valve, a clogged blowdown line, or an operator who skips scheduled bottom blows can all let conductivity creep upward over days or weeks. Conversely, excessive blowdown wastes energy and water, so the aim is to blowdown just enough to hold conductivity within the target range.
Why High Conductivity Is a Problem
Elevated conductivity isn’t just an out-of-spec number on a log sheet. When boiler water exceeds recommended limits, several things can go wrong. High dissolved solids promote foaming at the water surface inside the steam drum, which causes moisture and dissolved minerals to carry over into the steam lines. This carryover deposits scale on superheater tubes, turbine blades, and downstream equipment. Foaming also destabilizes drum water levels, triggering nuisance alarms and, in serious cases, automatic boiler trips that shut down production.
Concentrated salts also accelerate corrosion. Sodium hydroxide concentrating under deposits can cause caustic gouging of boiler tubes. Acid-forming salts like magnesium chloride or magnesium sulfate can create localized low-pH zones that eat through metal. Scale formation from concentrated calcium and magnesium insulates heat transfer surfaces, reducing efficiency and creating hot spots that can lead to tube failure.
Conductivity Limits by Operating Pressure
Recommended conductivity limits vary significantly depending on boiler pressure and type. Higher-pressure boilers require much tighter water chemistry because the energy density is greater and the consequences of carryover are more severe. ASME guidelines for industrial watertube drum-type boilers with superheaters illustrate the range:
- 0 to 300 psig: 1,100 to 5,400 µS/cm
- 301 to 450 psig: 900 to 4,600 µS/cm
- 451 to 600 psig: 800 to 3,800 µS/cm
- 601 to 750 psig: 300 to 1,500 µS/cm
- 901 to 1,000 psig: 200 to 1,000 µS/cm
- 1,001 to 1,500 psig: 150 µS/cm or less
- 1,501 to 2,000 psig: 80 µS/cm or less
Firetube boilers and watertube boilers without superheaters are more forgiving, with limits up to 7,000 µS/cm at lower pressures. These are guidelines, not absolute rules. Your boiler manufacturer’s specifications and your water treatment provider’s recommendations for your specific system should take priority.
Getting Accurate Conductivity Readings
Temperature has a direct effect on conductivity measurements. As water temperature rises, ions move more freely, producing higher readings. A sample at 80°C will read significantly higher than the same water cooled to 25°C. All standard conductivity limits are referenced to 25°C, so using a meter with automatic temperature compensation is essential for meaningful readings. Without it, you’ll need to cool the sample to exactly 25°C or manually apply a correction factor.
Because alkalinity and pH strongly influence conductivity, many testing protocols call for neutralizing the boiler water sample before measurement. This strips out the conductivity contribution from hydroxide alkalinity and gives a reading that more accurately reflects the dissolved mineral load. If your readings seem inconsistent or unexpectedly high, checking whether samples are being properly neutralized and temperature-compensated is a good first step before chasing mechanical causes.