The water inside a building's HVAC systems is a working fluid, and like any other component it needs care. Untreated or poorly treated water corrodes piping from the inside, deposits scale on heat transfer surfaces, and supports bacterial growth. The damage is usually invisible until a pinhole leak appears, a chiller loses capacity or a pump fails early.
Closed loops (heating water, chilled water, glycol) and open loops (cooling tower condenser water) have very different treatment needs. This guide explains the principles of each, the tests that matter and how to tell whether a treatment program is working.
The three problems water treatment addresses
| Problem | What happens | Consequences |
|---|---|---|
| Corrosion | Metal reacts with oxygen and other agents in the water | Pipe thinning, leaks, magnetite and rust sludge, failed valves and pumps |
| Scale | Dissolved minerals such as calcium carbonate precipitate on hot surfaces | Reduced heat transfer, higher chiller and boiler energy use, blocked tubes |
| Biological growth | Bacteria, algae and fungi form biofilm | Fouling, microbiologically influenced corrosion, legionella risk in open systems |
A fourth concern, suspended solids, is handled mainly with filtration and separation rather than chemistry. See air and dirt separation in hydronic systems.
Closed loops
A tight closed loop, once filled and treated, should need very little new water. That is the key to closed-loop treatment: if makeup is low, oxygen and minerals are added only once and the chemistry stays stable.
Common treatment approaches
- Nitrite-based programs: long used for steel systems. Nitrite is an effective corrosion inhibitor but can be consumed by certain bacteria.
- Molybdate-based programs: effective inhibitors, often blended with nitrite; molybdate is easy to test.
- Organic or blended programs: various proprietary blends with azoles for copper protection.
- Azoles (such as tolyltriazole): protect copper and brass components in mixed-metal systems.
- Oxygen scavengers: used in some heating systems, especially those with steel boilers.
The choice depends on system metallurgy, temperature, the presence of aluminium heat exchangers (some condensing boilers are sensitive to pH), and local discharge regulations. Condensing boiler manufacturers often specify allowable pH and chemistry ranges; following them is usually a warranty condition.
What to test in closed loops
- Inhibitor level (nitrite, molybdate, or the product-specific marker)
- pH, commonly maintained in a mildly alkaline range set by the treatment program and equipment manufacturers
- Conductivity or total dissolved solids
- Iron and copper as indicators of active corrosion
- Bacteria counts, particularly in nitrite systems or those with low temperature
- Glycol concentration and reserve alkalinity, where applicable
Closed-loop testing is commonly done monthly to quarterly, with a full laboratory analysis at least annually. Corrosion coupons in a coupon rack provide a direct measurement of corrosion rates over time.
The makeup water meter
A meter on the makeup line is the most useful single instrument in a closed system. Rising makeup means a leak, and every litre of makeup brings in dissolved oxygen and hardness. If inhibitor levels keep falling, check the makeup meter before adding more chemicals.
Open loops: cooling towers
Open condenser water systems concentrate minerals as water evaporates, absorb dust and organic material from the air, and stay warm. They need continuous treatment.
Program elements
- Scale and corrosion inhibitors fed in proportion to makeup or blowdown.
- Blowdown control using conductivity to keep cycles of concentration at the target.
- Biocides, typically an oxidizing biocide (such as chlorine or bromine) supplemented by a non-oxidizing biocide on a rotating schedule.
- Filtration, often side-stream, to remove suspended solids that shelter bacteria.
- Physical cleaning of basins, fill and distribution systems at regular intervals.
- Monitoring and sampling, including legionella sampling where required.
Target cycles of concentration depend on makeup water chemistry. Soft water in much of British Columbia allows different strategies than harder water in parts of the Prairies and Ontario. The condenser water systems and cooling towers guide explains how cycles affect water use.
Legionella management
ASHRAE Standard 188 sets out the framework for a building water management program, and ASHRAE Guideline 12 provides practical guidance. Some Canadian jurisdictions mandate specific cooling tower registration, maintenance and sampling, Quebec being the most prominent example. Requirements vary, so owners should confirm what applies to their location. More detail is in cooling tower maintenance and legionella control.
Pre-commissioning cleaning
New systems should be flushed and chemically cleaned before treatment chemicals are added, to remove oil, mill scale and construction debris. A typical sequence:
- Flush with terminal equipment bypassed.
- Circulate a pre-commissioning cleaner at the concentration and temperature the treatment specialist specifies.
- Drain and flush until clean and within specified conductivity of the makeup water.
- Fill, add inhibitors and record baseline readings.
- Clean strainers and separators.
Skipping cleaning leaves oils that interfere with inhibitors and debris that plugs valves and coils.
What owners should expect from a treatment provider
- A written program stating chemicals, targets and test frequencies for each system.
- Service reports after every visit, with test results compared to targets.
- Trends of key results over time, not just single readings.
- Corrosion coupon results for critical systems.
- Recommendations when results drift, with reasons.
- Safety data sheets and secondary containment for chemicals stored on site.
Owners should keep these records with other maintenance documentation. A program that reports "within range" on every visit while makeup water rises and iron climbs is not working.
Frequently asked questions
How often should closed loop water be tested?
Many programs test monthly or quarterly on site, with a laboratory analysis annually. Critical systems and systems with known problems warrant more frequent testing.
Can I discharge treated water to the sewer?
Municipal sewer use bylaws govern what can be discharged, including limits on certain chemicals, glycol and temperature. Check local bylaws and obtain approval where required.
Key takeaways
- Water treatment controls corrosion, scale and biological growth; filtration handles suspended solids.
- Closed loops need stable inhibitor levels and low makeup; a makeup meter reveals leaks.
- Open tower loops need continuous inhibitor feed, blowdown control, biocides and cleaning.
- Legionella management is essential for cooling towers and is regulated in some provinces and cities.
- Insist on trended test results and coupons, and include water quality in retro-commissioning reviews of existing plants.
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