Boiler water treatment before heating season: what the chemistry needs to look like before the first firing.

Why pre-season is the right time to test

Water chemistry in a closed hydronic system changes during the idle period. A system that was correctly treated at the end of last heating season may have drifted during the summer months. Inhibitor concentrations deplete over time as the chemistry reacts with metal surfaces and with oxygen that enters through system joints and air vents. pH can shift as dissolved gases come out of solution or as biological activity changes the carbonate balance in the water. Glycol degrades slowly over time and with exposure to heat, and a system that had adequate glycol concentration at shutdown may have marginally acceptable or inadequate concentration at startup.

The pre-season water sample gives a snapshot of the current chemistry before the system is stressed by heating load. If the chemistry is out of range, there is time to correct it before the first cold snap creates urgency around getting heat into the building. If the chemistry is corrected under time pressure during a heating emergency, the correction is more expensive, less thorough, and made in a system that is already operating at conditions that accelerate whatever damage the out-of-range chemistry was causing.

pH and alkalinity

pH is the most fundamental water chemistry parameter in a commercial hydronic system. The target pH range for most closed hydronic systems is 8.0 to 10.0, with the specific target depending on the metals in the system and the treatment program. At pH below 7.0 the water is acidic and corrosive to all metals in the system. At pH above 10.5 the water can become aggressive toward copper and aluminum components and can cause seal degradation in pumps and control valves.

A system that has drifted below the target pH range has been operating in conditions that accelerate corrosion on all wetted surfaces. The longer it has operated out of range, the more corrosion product is circulating in the system. Correcting pH before startup stops the active corrosion and allows the inhibitor program to establish a protective film on the metal surfaces. Correcting it after months of operation at low pH stops the corrosion but does not reverse the damage that has already occurred.

Alkalinity, measured as M-alkalinity or total alkalinity, supports pH stability and indicates the buffering capacity of the system water. Low alkalinity means the system has limited ability to resist pH swings when acidic contaminants enter the system. High alkalinity can contribute to scale formation in high-temperature applications. Confirm that both pH and alkalinity are within the target range for the specific treatment program before startup.

Dissolved oxygen

Dissolved oxygen is the primary driver of corrosion in closed hydronic systems. Oxygen reacts with iron to form iron oxide, which is the rust and scale that accumulates on boiler heat transfer surfaces, in strainers, and on pump impellers. The corrosion product itself becomes a problem beyond the surface damage it represents: iron oxide circulating in the system deposits on heat transfer surfaces, clogs strainer elements, and in severe cases causes impeller erosion in centrifugal pumps.

A properly maintained closed hydronic system should have very low dissolved oxygen levels because the system water has already consumed the oxygen present at fill and the system is sealed against ongoing oxygen ingress. A system with elevated dissolved oxygen levels at pre-season testing has an oxygen ingress problem that needs to be identified and corrected, not just treated chemically.

Common sources of oxygen ingress include air vents that are not sealing correctly, expansion tank bladder failures that allow air contact with system water, makeup water additions that bring dissolved oxygen with them, and system joints that admit air under negative pressure conditions. Correcting the source of oxygen ingress is more important than adjusting the chemical treatment, because chemical treatment alone cannot maintain low dissolved oxygen levels in a system with active oxygen ingress.

Inhibitor concentration

Corrosion inhibitors form a protective film on metal surfaces that physically separates the metal from the water and reduces the corrosion rate. The inhibitor concentration needs to be within the target range specified by the treatment program for this film to be effective. Below the minimum inhibitor concentration, the protective film is incomplete and corrosion proceeds at an accelerated rate. Above the maximum concentration, some inhibitor chemistries become aggressive toward specific metals or cause foaming in the system.

Pull the pre-season water sample before adding any makeup water or treatment chemicals, so the test results reflect the actual current concentration rather than a diluted or freshly treated sample. If the inhibitor concentration is below target, calculate the volume of treatment chemical required to bring the system to the target concentration based on the system volume, add the calculated amount, and test again to confirm the result.

For systems where the treatment program has not been reviewed recently, a pre-season is also a good time to confirm that the inhibitor chemistry is compatible with the current system materials. Systems that have had components replaced or added since the original treatment program was specified may include materials that are not compatible with the original inhibitor chemistry.

Glycol concentration and inhibitor package condition

For systems that use glycol for freeze protection, two separate parameters need to be checked at pre-season. The glycol concentration determines the freeze protection level. The inhibitor package condition determines whether the glycol is still providing corrosion protection in addition to freeze protection.

Glycol concentration is checked with a refractometer. The reading should be compared against the target concentration for the lowest expected ambient temperature at the site. A system where the glycol concentration has dropped below the freeze protection target needs additional glycol before startup. The concentration can drop through makeup water additions that dilute the glycol, through leaks that are repaired with water rather than glycol, or through glycol degradation over time.

Inhibitor package condition in glycol systems is checked by pH and by the reserve alkalinity or inhibitor concentration test specific to the glycol formulation. Fresh glycol is typically buffered to a pH of 8.5 to 10.0 and contains inhibitors that protect the metals in the system. As the glycol ages and is exposed to heat and oxygen, the inhibitors deplete and the pH drops. Degraded glycol with depleted inhibitors and low pH can be more corrosive than plain water in some conditions. A system with degraded glycol needs the glycol replaced or refreshed with an inhibitor package rather than simply topped up with fresh glycol.

Using the pre-season test as a diagnostic tool

The pre-season water test is most useful when it is compared against previous test results rather than evaluated in isolation. A system where the pH has dropped two tenths of a point since the last test is showing a trend that needs investigation. A system where the inhibitor concentration has dropped to half of the previous result has experienced something that caused inhibitor depletion faster than expected. A system where the iron content has increased significantly since last season has active corrosion somewhere that the test cannot locate by itself but that warrants investigation.

Request iron, copper, and pH along with the standard inhibitor concentration and glycol checks. Elevated iron indicates active corrosion of ferrous components. Elevated copper indicates active corrosion of copper components. These metals in solution are evidence of a problem occurring somewhere in the system, not just a chemistry adjustment that needs to be made.

For the pump systems serving the boiler plant, pre-season pump checks including strainer inspection and variable speed drive diagnostics run on the same schedule as the water treatment assessment. The Merion Pump Company team handles pump pre-season checks for commercial hydronic systems across Pennsylvania, New Jersey, Delaware, and Maryland. Visit merionpump.com for more.

GP Energy Products provides commercial boiler service and pre-season inspections across Pennsylvania, New Jersey, Delaware, and Maryland. If you want a water chemistry assessment included in the pre-season boiler inspection, reach out and we will coordinate the sampling and testing as part of the inspection visit.

References
1. ASHRAE. HVAC Systems and Equipment Handbook, Chapter on Water Treatment. Covers pH, inhibitor concentration, dissolved oxygen, and glycol treatment for closed hydronic systems. ashrae.org
2. Association of Water Technologies. Closed Loop Water Treatment Guidelines. Covers treatment program parameters, inhibitor chemistry, and pre-season assessment procedures for commercial hydronic systems. awt.org
3. ASTM International. Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. Covers corrosion assessment methodology applicable to hydronic system metal analysis. astm.org
4. American Boiler Manufacturers Association. Boiler Water Treatment Guidelines. Covers feedwater and system water treatment requirements for commercial boiler applications. abma.memberclicks.net