Closed loop water treatment protects sealed chilled-water, hot-water and glycol loops with 20 chemicals in 8 functions, led by nitrite and molybdate corrosion inhibitors, because a loop that loses less than 5 % of its volume a year still corrodes, fouls and grows bacteria. Residuals run from 5 mg/L (ppm) of free azole to over 1,000 mg/L of nitrite as NaNO2, so which program fits which loop?
Closed loop water treatment, also called closed system or closed circuit water treatment, draws on 6 of the 16 families of water treatment chemicals.
Every value on this page is checked against primary sources (eCFR, EUR-Lex, NIH technical bulletins, VDI, BSRIA, PubChem, ECHA, EPA pesticide records); see our methodology and fact-checking process.
What Is Closed Loop Water Treatment?#
Closed loop water treatment is the chemical program for a recirculating loop isolated from the atmosphere, whose makeup typically stays below 5 % of system volume per year (NIH Office of Research Facilities, August 2023). The Association of Water Technologies (AWT) guide classes chilled loops below 22 °C (72 °F), hot-water loops below 100 °C (212 °F) and high-temperature hot-water (HTHW) loops at 100 to 204 °C (212 to 400 °F).
Seven system programs are compared under industrial water treatment.
What goes wrong in an untreated closed loop?#
An untreated closed loop fails in 5 ways: oxygen corrosion of steel, copper attack, deposits, microbial growth that consumes nitrite, and glycol degradation to organic acids. The table below compares the 5 failure modes.
| Problem | Cause in a closed loop | Chemical answer | Key number |
|---|---|---|---|
| Oxygen corrosion of steel | makeup oxygen, inhibitor dilution | nitrite or molybdate plus oxygen scavenger | makeup typically < 5 % of volume per year; 25 % per month seen |
| Copper and brass attack | ammonia from nitrite-reducing bacteria, no azole | tolyltriazole, benzotriazole | 5 to 10 mg/L free azole (owner specification) |
| Deposits and fouling | corrosion products, hardness in makeup | dispersant; condensate or softened makeup | 10 to 20 ppm AA/AMPS copolymer |
| Microbial growth (microbiologically influenced corrosion) | loops below 60 °C (140 °F), nitrite as nutrient, glycol below 20 to 25 % | non-oxidizing biocide | shock above 1,000 cells/mL (owner specification) |
| Glycol degradation | heat, aeration, microbes, giving organic acids | inhibited glycol at pH 8.0 to 10.0 | replace glycol below pH 7.0 |
Closed loop failure modes; see corrosion in water systems.
Makeup drives the first failure: losing 25 % of system volume a month is "not uncommon" (NIH, 2023), so the AWT guide calls for a makeup totalizer on every loop.
Closed loop vs open cooling water: what changes for the chemistry?#
A closed loop holds high, long-lasting inhibitor residuals instead of an open tower's low continuous feed, because it loses almost no water: nitrite runs at 600 to 1,200 mg/L as NO2 (Veolia Water Technologies Handbook, chapter 32). Cooling-tower corrosion bands never apply to a loop.
Evaporation, blowdown and cycles are covered under cooling tower water treatment.
The Closed Loop Chemical Program: 8 Chemical Functions#
A closed loop program combines up to 8 chemical functions: steel corrosion inhibitors, azoles for copper, pH buffers, oxygen scavengers, dispersants, non-oxidizing biocides, inhibited glycol and pre-commission cleaners. The table maps each function to its chemicals and key figure.
| # | Function | Chemicals on this page | Key closed-loop figure (basis) | Program node |
|---|---|---|---|---|
| 1 | Corrosion inhibitors | sodium nitrite, sodium molybdate, sodium silicate, HPAA (sodium nitrate as a section) | nitrite 600 to 1,200 mg/L as NO2; molybdate 200 to 300 mg/L as MoO4 (Veolia ch. 32); silicate 50 to 100 mg/L as SiO2 (AWT) | chilled water, heating |
| 2 | Copper and yellow-metal corrosion inhibitors | tolyltriazole, benzotriazole, mercaptobenzothiazole (MBT) | TTA 10 to 50 mg/L as sodium salt (AWT) | none |
| 3 | pH adjustment chemicals | sodium hydroxide (borate and dipotassium phosphate as sections) | pH 9.0 to 10.5 on steel; < 9.0 with aluminum | none |
| 4 | Oxygen scavengers | sodium sulfite, DEHA, sodium erythorbate | DEHA 200 mg/L initial, 125 mg/L maintenance | heating |
| 5 | Scale inhibitors | AA/AMPS copolymer, HEDP | 10 to 20 ppm AA/AMPS (AWT) | none |
| 6 | Non-oxidizing biocides | glutaraldehyde, isothiazolinone, DBNPA, TTPC, polyquat; BCDMH (oxidizing, one-time cleanup only) | no bacterial growth above 60 °C (140 °F) | none |
| 7 | Inhibited glycol | ethylene glycol, propylene glycol (sections) | 25 to 60 % glycol; freeze point at least 3 °C (5 °F) below ambient | glycol |
| 8 | Descaling and chemical cleaning agents | trisodium phosphate | 1 to 2 % TSP for new systems (Dow) | Pre-commission cleaning and flushing of closed systems |
1. Corrosion inhibitors for steel: nitrite, molybdate, silicate and HPAA#
Nitrite and molybdate are the most effective closed loop corrosion inhibitors for steel (NIH, 2025): the Veolia Water Technologies Handbook gives 600 to 1,200 mg/L as NO2 and 200 to 300 mg/L as MoO4 (120 to 180 as Mo). Under-dosed nitrite leaves anodes open to pitting; molybdate needs dissolved oxygen and calcium below 500 ppm. Chromate is historical.
2. Azoles for copper and yellow metals#
Azoles protect copper and brass in almost every closed loop formulation: tolyltriazole is dosed at 10 to 50 mg/L as the sodium salt (AWT) and held at 5 to 10 mg/L free azole in an owner specification at the University of Nevada, Las Vegas (UNLV).
3. pH buffers and alkalinity#
pH buffers hold a closed loop at 9.0 to 10.5 for carbon steel, 8.5 to 10.5 with copper alloys and below 9.0 with aluminum, and borate at pH 9.3 to 9.5 is the most common buffer (AWT).
4. Oxygen scavengers#
Oxygen scavengers, such as sodium sulfite, sodium erythorbate and diethylhydroxylamine (DEHA), remove makeup oxygen in tight loops; DEHA is dosed at 200 mg/L initially and 125 mg/L for maintenance. The same handbook pairs sulfite with polymer in HTHW loops.
5. Dispersants and scale inhibitors#
Dispersants hold iron oxide and hardness in suspension: an acrylic acid/AMPS (AA/AMPS) copolymer runs at 10 to 20 ppm (AWT), and HEDP appears in molybdate blends. Condensate or softened makeup limits the load.
6. Biocides for closed loops#
Closed loops use non-oxidizing biocides, such as glutaraldehyde, isothiazolinone, DBNPA, TTPC and polyquats, because oxidizing biocides convert nitrite to nitrate. Bacteria do not grow above 60 °C (140 °F), so cool loops carry the risk (AWT).
7. Inhibited glycol for freeze protection#
Inhibited glycol protects a closed loop from freezing at 25 to 60 % glycol, with a freeze point at least 3 °C (5 °F) below the lowest ambient; below 20 to 25 % it feeds bacteria (Dow DOWFROST guide). In ASTM D1384 tests, mild steel corrodes at 9.69 mpy in water and 0.04 mpy in inhibited glycol.
The glycol freeze point and dilution calculator sets concentrations.
8. Pre-commission cleaning and flushing chemicals#
New closed loops are cleaned before the first inhibitor charge, typically with 1 to 2 % trisodium phosphate (same guide), because old rust consumes molybdate. BSRIA BG 29/2020 allows at most 10,000 cfu/mL 7 days after cleaning.
Which Program for Which Closed Loop? Chilled, Hot Water, Glycol and Brine#
Temperature, metals and freeze risk select the program: chilled loops should run nitrite or molybdate blends with azole, hot loops nitrite at the high end, glycol loops inhibited glycol and brine loops nitrite. The table compares the 4 loop types.
| Loop | Temperature | Typical program | Key numbers | Watch-outs |
|---|---|---|---|---|
| Chilled water system treatment | < 22 °C (72 °F) | nitrite or molybdate blend + azole + non-oxidizing biocide | 800 to 1,000 ppm as NaNO2; 5 to 10 ppm free azole; or 40 to 60 ppm as Mo with 30 to 40 ppm silica (owner specification) | bacteria, nitrite loss |
| Hot water heating system treatment | < 100 °C (212 °F); HTHW 100 to 204 °C (212 to 400 °F) | nitrite at the high end + silica limit; HTHW sulfite + pH control + polymer | 1,000 to 1,200 ppm as NaNO2, silica < 50 ppm (owner specification); HTHW up to 34 bar (500 psig) | silicate avoided above 82 °C (180 °F) |
| Glycol system treatment | below freezing to above 66 °C (150 °F) | inhibited ethylene or propylene glycol, 25 to 60 % | pH 8.0 to 10.0; dilution water Cl and SO4 < 25 ppm, hardness < 100 ppm as CaCO3 | no automatic makeup; aluminum only below 66 °C (150 °F); no galvanized steel |
| Brine and process closed cooling | process | nitrite at about 2,000 mg/L as NO2 | pH 7.0 to 8.5, caustic only | no molybdate with brine-type antifreeze |
Chilled water loops#
Chilled water loops below 22 °C (72 °F) run the low end of the nitrite range, 800 to 1,000 ppm as NaNO2 with 5 to 10 ppm free azole, in the same owner specification.
Hot water heating and HTHW loops#
Hot water heating loops run 1,000 to 1,200 ppm as NaNO2 in the same specification, while nitrogen-pressurized HTHW loops up to 34 bar (500 psig) take sulfite, pH control and polymer (Veolia Water Technologies Handbook). VDI 2035 Part 1:2021-03 prefers heating water below 100 µS/cm.
Glycol loops#
Glycol loops use dilution water below 25 ppm chloride and sulfate and 100 ppm hardness as CaCO3, run at pH 8.0 to 10.0 and take glycol top-up, not automatic makeup (Dow DOWFROST guide).
Brine and process closed cooling loops#
Brine loops carry about 2,000 mg/L nitrite as NO2 at pH 7.0 to 8.5, adjusted with caustic only, and never molybdate (same handbook).
Closed Loop Control Ranges by Program and Metal#
Closed loop control ranges depend on inhibitor and metals: nitrite programs hold pH above 7.0 (below 9.0 with aluminum), molybdate programs pH 7.0 to 9.0, and every residual states its basis. The table compares the ranges.
| Program or parameter | Range | Basis | pH | Source |
|---|---|---|---|---|
| Sodium nitrite, steel | 600 to 1,200 (about 900 to 1,800 as NaNO2) | as NO2 | > 7.0; < 9.0 with aluminum | Veolia ch. 32 |
| Sodium nitrite, steel (AWT) | 700 to 1,800 (467 to 1,200 as NO2) | as NaNO2 | 8.5 to 10.5 | AWT guide |
| Sodium nitrite, steel-copper couples | 5,000 to 7,000; up to 10,000 with aluminum | as NO2 | as above | Veolia ch. 32 |
| Sodium molybdate | 200 to 300 (120 to 180 as Mo; about 258 to 386 as Na2MoO4) | as MoO4 | 7.0 to 9.0 | Veolia ch. 32 |
| Sodium molybdate (AWT) | 150 to 1,000 (117 to 777 as MoO4; 70 to 466 as Mo) | as Na2MoO4 | 8.5 to 10.5; 7.8 to 8.3 with aluminum | AWT guide |
| Nitrite + molybdate blend | 150 + 150 to 400 + 400; ratio 50:50 to 60:40 | as Na2MoO4 + NaNO2 | 8.5 to 10.5 | AWT guide |
| Sodium silicate | 50 to 100 alone; 10 to 25 as aluminum or copper adjunct | as SiO2 | 8.5 to 10.5; < 9.0 with aluminum | AWT guide |
| Tolyltriazole | 10 to 50 dosed; 5 to 10 free azole (owner specification) | as sodium salt; free azole | 8.5 to 10.5 | AWT; UNLV |
| AA/AMPS dispersant | 10 to 20 | ppm as product | n/a | AWT guide |
| DEHA (tight, low-conductivity or HTHW loops) | 200 initial; 125 maintenance | mg/L as DEHA | 9.5 to 10.0 | supplier figure |
| Brine loop nitrite | about 2,000 | as NO2 | 7.0 to 8.5 | Veolia ch. 32 |
| Glycol | 25 to 60 % (typical 20 to 50 %) | wt % or vol %, stated | 8.0 to 10.0; replace below 7.0 | Dow; AWT |
| Iron / copper | < 1 / < 0.5 | mg/L total | n/a | UNLV owner specification |
| Bacteria | shock with non-oxidizer above 1,000 | cells/mL | n/a | UNLV owner specification |
| Makeup loss | typically < 5 % per year; owner specification < 1 % per year | % of system volume | n/a | NIH; UNLV |
Mo x 1.67 = MoO4 and Mo x 2.15 = Na2MoO4; kits report NaNO2 and Na2MoO4 unless stated, and ppm equals mg/L only near density 1 (Veolia Water Technologies Handbook, chapter 40).
Corrosion rate benchmarks for closed loops#
A closed loop rates excellent at or below 0.2 mpy (0.005 mm/y) on carbon steel and 0.10 mpy (0.0025 mm/y) on copper alloys (AWT). The table lists all 5 bands.
| Rating | Carbon steel, mpy (mm/y) | Copper alloys, mpy (mm/y) |
|---|---|---|
| Excellent | ≤ 0.2 (≤ 0.005) | ≤ 0.10 (≤ 0.0025) |
| Good | 0.2 to 0.5 (0.005 to 0.013) | 0.10 to 0.25 (0.0025 to 0.0064) |
| Moderate | 0.5 to 0.8 (0.013 to 0.020) | 0.25 to 0.35 (0.0064 to 0.0089) |
| Poor | 0.8 to 1.0 (0.020 to 0.025) | 0.35 to 0.50 (0.0089 to 0.013) |
| Very poor | ≥ 1.0 (≥ 0.025) | ≥ 0.5 (≥ 0.013) |
Aluminum rates excellent below 0.1 mpy and poor above 0.5 mpy (Chem-Aqua TB2-009, 2018).
Coupons run 90 days (NIH, 2023) under ASTM D2688-23; racks and probes are explained under corrosion coupons and corrosion monitoring.
Shot feeding and top-up dose: a worked example#
A 10 m³ (about 2,640 US gal) loop dosed to 200 mg/L as MoO4 needs 7.35 kg (16.2 lb) of 35 % sodium molybdate liquid at 27.2 % MoO4 (worked example). The 4 steps are listed below.
- Take the volume: 10 m³ (2,641.7 US gal).
- Multiply by 200 g/m³ as MoO4: 2.0 kg.
- Divide by 0.272: 7.35 kg of product.
- Shot-feed via the pot feeder; retest after circulation.
The chemical dosage calculator converts such doses.
How Is a Closed Loop Monitored and Tested?#
A closed loop is monitored on 6 core parameters (pH, temperature, conductivity, dissolved oxygen, total hardness, bacterial count; NIH, 2023), plus inhibitor residual, iron, copper and 90-day coupons. The table lists each test.
| Test | Why | Frequency or exposure | Source |
|---|---|---|---|
| pH, temperature, conductivity, dissolved oxygen, total hardness | core loop KPIs | each service visit | NIH, 2023 |
| Nitrite as NaNO2 (ceric sulfate or permanganate titration, or kit); molybdate as Na2MoO4 (Hach Method 8036) | dose control | each service visit | Veolia ch. 32; test kit makers |
| Free azole | copper protection | each service visit | AWT; UNLV |
| Iron and copper | corrosion products | each service visit; owner specification Fe < 1, Cu < 0.5 mg/L | UNLV |
| Corrosion coupons | rate in mpy (mm/y) | 90 days | NIH, 2023; ASTM D2688-23 |
| Bacteria (bio-counts) | nitrite consumers | at least monthly with nitrite programs | AWT |
| Nitrate, ammonia, alkalinity | nitrite-loss diagnosis | when nitrite falls | Qualichem (supplier) |
| Glycol %, pH, buffer capacity, inhibitor | glycol condition | annual lab analysis for systems of 950 L (250 US gal) or more | Dow; AWT |
| BS 8552:2012 sampling (UK) | closed-system sampling | quarterly | secondary source |
| Total viable count, BSRIA BG 50/2021 | microbial limit | per sampling plan | limit 10,000 cfu/mL |
Falling nitrite with rising nitrate or ammonia signals nitrite-reducing bacteria (Qualichem, supplier), and triazole in a hot-water loop signals chilled-water intrusion (NIH, 2025).
Kit bases per loop are in closed loop water testing and control ranges.
Which Rules Apply to Closed Loop Chemicals?#
Closed loop chemicals meet 4 rule sets when a loop is drained, spilled or treated: CERCLA reportable quantities, sewer and biosolids limits, biocide registration, and EU chemical rules. Sodium nitrite carries a CERCLA reportable quantity of 100 lb (45.4 kg) and ethylene glycol 5,000 lb (2,270 kg) under 40 CFR 302.4. The table lists each rule.
| Rule | What it sets for closed loops | Value |
|---|---|---|
| CERCLA reportable quantities, 40 CFR 302.4 | release reporting | sodium nitrite 100 lb (45.4 kg); ethylene glycol 5,000 lb (2,270 kg); propylene glycol and sodium nitrate not listed |
| National Pretreatment Program | draining a loop to sewer | 40 CFR 403.5: no discharge below pH 5.0, no closed-cup flashpoint below 60 °C (140 °F), no heat raising treatment-plant influent above 40 °C (104 °F); some sewer authorities restrict molybdate (AWT; UNLV) |
| 40 CFR 503.13 biosolids | molybdenum in land-applied sludge | ceiling 75 mg/kg dry weight |
| FIFRA registration of water treatment biocides | biocide products | glutaraldehyde PC code 043901; CMIT 107103 and MIT 107104; DBNPA 101801; TTPC 128824 |
| EU Biocidal Products Regulation | biocides in EU loops | DBNPA approved for PT11 by Implementing Regulation (EU) 2026/577 from 2027-07-01 to 2032-06-30, only for short-term (up to 14 days) preservation of liquids in closed recirculating and open cooling water systems by industrial or professional users |
| REACH Candidate List | borate buffers | disodium tetraborate SVHC since 18 June 2010 (ECHA decision ED/30/2010), Repr. 1B |
| Drinking-water limits (cross-connection context) | nitrite and molybdenum | nitrite MCL 1 mg/L as N (40 CFR 141.62); molybdenum lifetime health advisory 0.04 mg/L (EPA, 2018), no MCL |
| NSF/ANSI/CAN 60 | certification | no chemical category for nitrite, molybdate or azoles |
Complete List of Closed Loop Water Treatment Chemicals: All 20 in One Table#
The complete list below gives all 20 closed loop water treatment chemicals with CAS number, function and closed-loop figure.
| # | Chemical | CAS | Function | Closed-loop figure (basis, source) |
|---|---|---|---|---|
| 1 | Sodium nitrite | 7632-00-0 | steel inhibitor (anodic, oxidizing) | 600 to 1,200 mg/L as NO2 (Veolia) |
| 2 | Sodium molybdate | 7631-95-0 (dihydrate 10102-40-6) | steel inhibitor (anodic, non-oxidizing) | 200 to 300 mg/L as MoO4 (Veolia) |
| 3 | Sodium silicate | 1344-09-8 (metasilicate 6834-92-0) | multi-metal inhibitor | 50 to 100 mg/L as SiO2 (AWT) |
| 4 | HPAA | 23783-26-8 | steel inhibitor | 50 to 200 mg/L HPA (AWT) |
| 5 | Tolyltriazole (TTA) | 29385-43-1 | copper inhibitor | 10 to 50 mg/L as sodium salt (AWT) |
| 6 | Benzotriazole (BZT) | 95-14-7 | copper inhibitor | copper alloy protection in closed loops |
| 7 | Mercaptobenzothiazole (MBT) | 149-30-4 (sodium salt 2492-26-4) | copper inhibitor | in blends; reduces catalytic glycol oxidation (LANXESS, supplier) |
| 8 | Sodium hydroxide (caustic soda) | 1310-73-2 | pH adjuster | glycol pH adjustment at pH 7.0 to 8.0 (Dow); brine pH, caustic only; caustic embrittlement risk above pH 12 (NIH) |
| 9 | Sodium sulfite | 7757-83-7 | oxygen scavenger | HTHW, electrode and resistance boilers with polymer (Veolia) |
| 10 | DEHA (diethylhydroxylamine) | 3710-84-7 | oxygen scavenger | 200 mg/L initial, 125 mg/L maintenance |
| 11 | Sodium erythorbate | 6381-77-7 (acid 89-65-6) | oxygen scavenger | named closed-loop scavenger (NIH, 2025) |
| 12 | AA/AMPS copolymer | 40623-75-4 (sodium salt 37350-42-8) | dispersant | 10 to 20 ppm (AWT) |
| 13 | HEDP | 2809-21-4 (tetrasodium 3794-83-0) | scale inhibitor | in molybdate and phosphonate blends (supplier) |
| 14 | Glutaraldehyde | 111-30-8 | non-oxidizing biocide | compatible with nitrite and molybdate programs (Dow); activity loss with ammonia at pH 9 |
| 15 | Isothiazolinone (CMIT/MIT) | 55965-84-9 | non-oxidizing biocide | named closed-loop biocide (AWT); incompatible with high pH |
| 16 | DBNPA | 10222-01-2 | non-oxidizing biocide | incompatible with alkaline pH; EU PT11 short-term preservation from 2027-07-01 |
| 17 | TTPC | 81741-28-8 | biocide and biodispersant | "extremely effective" against nitrite bacteria (Qualichem, supplier) |
| 18 | Polyquaternium (WSCP) | 31512-74-0 | biocide | named closed-loop biocide (AWT) |
| 19 | BCDMH | 16079-88-2 | oxidizing biocide | brominator feed, DPD test; one-time cleanup only (supplier); never with running nitrite |
| 20 | Trisodium phosphate (TSP) | 7601-54-9 (dodecahydrate 10101-89-0) | pre-commission cleaner | 1 to 2 % solution, flushed out (Dow) |
Sodium nitrate, borax, dipotassium phosphate, both glycols and chromate are host-page sections; every other chemical is in the water treatment chemical database.
How Is a Closed Loop Water Treatment Program Bought?#
A closed loop program quote rests on 6 inputs: loop type, volume, metallurgy, makeup rate, glycol percentage and the current inhibitor residual.
Program providers are compared under water treatment companies.
A public 2016 commercial HVAC tender scope set closed-loop targets: mild steel below 1.0 mpy, copper below 0.2 mpy, bacteria below 10,000 cfu/mL.
Cost follows loop count, visit frequency and testing scope.
Distributors selling inhibitor drums directly are listed under water treatment chemical suppliers.
Are Closed Loop Chemicals Safe for People, Metals and Drains?#
Closed loop chemicals are safe in a sealed, labelled loop when concentrates are handled to their safety data sheet (SDS), nitrite stays away from acids and ethylene glycol stays out of any loop with possible drinking water or food contact. Drained fluid meets sewer rules.
What is a closed loop in a boiler system?#
In a boiler system, the closed loop is the hot-water heating circuit, which returns water without steam or blowdown and so takes a nitrite or sulfite program.
Steam boilers, feedwater and blowdown are covered under boiler water treatment.
Can automotive antifreeze be used in an HVAC glycol loop?#
No: automotive antifreeze is unsuitable for HVAC glycol loops, because its silicates foul heat exchangers and fail pump seals (Chem-Aqua glycol guideline).
Which glycol is used where food or potable water contact is possible?#
Propylene glycol is the closed loop glycol where incidental food or potable water contact is possible, and ethylene glycol is never used there; neither is listed in 21 CFR 173.310.
Which chemicals must never meet a nitrite loop?#
Acids and oxidizing biocides must never meet a nitrite loop: acids release nitrogen oxides, and chlorine or bromine converts nitrite to nitrate.
Can a nitrite loop be cleaned with sulfamic acid?#
No: sulfamic acid reacts with nitrite and releases nitrogen gas, so a nitrite loop is drained and flushed before any sulfamic acid cleaning.