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Glycol Concentration for Hydronic Systems: Freeze Protection, the Efficiency Penalty, and Ethylene versus Propylene

Why Glycol Concentration Is a Balance, Not Just a Freeze Setting

Adding glycol to a hydronic loop protects it from freezing, but every percent of glycol also thickens the fluid, raises the pumping energy, and cuts the heat it can carry, so the right concentration is a balance between freeze protection and system efficiency, not simply the highest number that guarantees no ice.

The physics are direct. Glycol lowers the freezing point of water, and more glycol lowers it further, which is why the instinct is to add plenty for safety. But glycol is more viscous than water, has a lower specific heat, and transfers heat less well. A loop charged with more glycol than its climate needs pays for that excess every hour it runs: higher pump power, larger pressure drop, and reduced coil or heat-exchanger capacity. The engineering goal is the minimum concentration that protects the system down to its design minimum temperature, keeping the penalties as small as possible.

The calculator computes the glycol concentration as the glycol volume divided by the total solution volume, or in reverse, the glycol volume needed to reach a target concentration, then classifies the result against a practical operating band. It opens the hydronic sub-cluster and continues directly from the Liquid Cooling Flow Rate article, which noted that glycol's lower specific heat forces a higher flow rate. This article is the dedicated glycol treatment: how much protection a concentration buys, and what it costs. The freeze curves differ between ethylene and propylene glycol, so the type matters as much as the percentage.

Calculator Inputs: Fluid Type, Volumes, and the Reverse Mode

The calculator uses four inputs and operates in two modes.

Fluid Type. Ethylene Glycol or Propylene Glycol. The type is used for interpretation and classification; the concentration formula is identical for both, but the freeze behavior at a given percent differs significantly (see Section 6).

Total Solution Volume [gal or L]. The total mixed fluid volume: the full system charge or the design batch. This is the denominator in the concentration formula.

Glycol Volume [gal or L]. The glycol portion of the mixture. Used with total volume to compute concentration in Mode 1 (concentration from volumes). Leave empty when using target concentration in Mode 2.

Target Glycol Concentration [%]. A desired concentration. Fill this instead of glycol volume to compute the required glycol volume (Mode 2, reverse). The modes use the same relationship, rearranged.

Mode 1 (concentration): total volume + glycol volume → concentration %
Mode 2 (required volume): total volume + target % → required glycol volume

The calculator outputs glycol concentration (%), required glycol volume in Mode 2 (gal or L), and a status badge from TOO LOW through TOO HIGH. Concentration is unit-independent: the ratio is the same in gallons or litres, so long as both volume inputs use the same unit within one calculation.

The calculator does not compute: actual freeze-point or burst-point temperature (needs the manufacturer curve for the fluid type), viscosity or pumping penalty values, percent-by-mass conversion (uses volume fraction only), inhibitor package depletion, dilution or contamination over time, mixing volume contraction, non-glycol coolants, or temperature-dependent fluid properties. It is a concentration screen, not a freeze-point chart.

The Concentration Ratio and the Required-Volume Reverse Calculation

The concentration is a volume ratio. The reverse mode simply inverts it to find how much glycol a target concentration requires.

Mode 1 (concentration from volumes):

C = (V_glycol / V_total) × 100   [%]

where:
  C        = glycol concentration [% by volume]
  V_glycol = glycol volume [gal or L]
  V_total  = total solution volume [gal or L]

Mode 2 (required volume from target):

V_required = (C_target / 100) × V_total   [gal or L]

where:
  V_required = glycol volume needed [gal or L]
  C_target   = target concentration [%]
  V_total    = total solution volume [gal or L]

The two modes are inverses: Mode 1 solves for C given the volumes; Mode 2 solves for V_glycol given the target C. Same relationship, rearranged.

Worked (Mode 1, Example 1):

V_total 100 gal (378.5 L), V_glycol 30 gal (113.6 L):
C = (30/100) × 100 = 30% by volume
Water volume = 100 − 30 = 70 gal (265.0 L)

Worked (Mode 2, Example 2):

V_total 400 L (105.7 gal), C_target 35%:
V_required = (35/100) × 400 = 140 L (37.0 gal) of glycol
Water = 400 − 140 = 260 L

This is percent by volume (glycol volume / total volume), not percent by mass (weight fraction). Glycol is denser than water, so the two differ. Manufacturer charts may use either; match the basis before comparing (Section 13).

Per fluid-mixing basics: concentration equals glycol volume divided by total volume times 100 (percent by volume); reverse mode gives required glycol as target percent times total volume. Water is the remainder. Volume contraction on mixing is a second-order effect for planning purposes.

Freeze Protection versus Burst Protection: Two Different Temperatures

A glycol concentration protects a system at two different temperatures, and treating them as the same leads to over-concentration or under-protection: the freeze point, where ice first forms, is considerably higher than the burst point, where frozen fluid expands enough to rupture the pipe.

Freeze point: temperature where ice crystals first form (fluid becomes slush)
Burst point:  temperature where frozen fluid expands destructively and ruptures the pipe
Burst point is LOWER (colder) than freeze point for the same concentration

Between the freeze and burst points, a glycol-water mixture forms a slush that is not fully solid and does not expand destructively. The pipe survives in slush conditions, even when flow has stopped. That gap between freeze and burst temperatures is the structural safety margin.

The design choice depends on whether the system flows continuously or sits dormant at its minimum temperature:

Freeze protection (flowing systems):
  No ice at all required; slush blocks flow and damages coils.
  Design to the freeze point: the system must stay above it.
  Applications: chilled-water loops, heat exchangers, active hydronic systems.

Burst protection (dormant/idle systems):
  Pipe must survive; some slush is acceptable since there is no flow to block.
  Design to the burst point: a lower (colder) temperature.
  Applications: snow-melt off-season, solar drain-back, idle circuits.
  Needs LESS glycol than freeze protection for the same minimum temperature.

Example (propylene glycol, illustrative; verify against the fluid-type curve):

30% PG: freeze point ~−13°C (9°F), burst point ~−33°C (−27°F)
Same 30% protects against freezing to −13°C, against bursting to −33°C.
Gap: ~20°C between the two protection levels.

The calculator gives concentration, not the protection temperature. Verify the actual freeze and burst temperatures against the manufacturer's curve for the fluid type selected (Section 13).

Per glycol manufacturer data (Dow DOWFROST/DOWTHERM SR-1): freeze point (ice forms) is above burst point (pipe ruptures) for a given concentration. Flowing systems design to freeze protection; dormant systems can use burst protection, needing less glycol. Read both temperatures from the fluid-type curve.

Ethylene versus Propylene: Same Percent, Different Behavior

The two common glycols, ethylene and propylene, are not interchangeable at the same concentration: they have different freeze curves, different efficiency penalties, and different toxicity. The type is as important as the percentage.

Ethylene glycol (EG): better thermal performance, lower viscosity, TOXIC
Propylene glycol (PG): food-safe (FDA GRAS), higher viscosity, slightly less thermal performance

Freeze curves differ at the same concentration:

30% EG: freeze point ~−15°C (5°F); protects more deeply per percent than PG
30% PG: freeze point ~−13°C (9°F)
Same 30% does NOT give the same freeze protection. Check the correct curve.

Toxicity is often the deciding factor:

EG: toxic; cannot be used where leakage could reach potable water or food processing.
PG: non-toxic, FDA generally-recognized-as-safe (GRAS); used in food plants,
    breweries, and potable-adjacent circuits.

Efficiency comparison:

EG: lower viscosity, better heat transfer, lower pumping penalty (thermally preferred)
PG: higher viscosity, more pumping energy required, but safe for potable-adjacent use

Application split:

EG: industrial chillers, closed non-potable loops, data-center coolant distribution
PG: food and beverage, breweries, potable-adjacent HVAC, snow-melt near occupied areas,
    solar-thermal systems

The concentration formula C = V_glycol/V_total is identical for both glycols, but the freeze protection temperature at a given percent differs. Never assume equivalence: substituting PG for EG at the same percent under-protects (PG needs a higher percent for the same freeze point). Re-check the concentration against the correct fluid-type curve whenever glycol type changes.

Per Dow Chemical (DOWFROST propylene / DOWTHERM SR-1 ethylene) and ASHRAE Fundamentals: EG and PG have different freeze curves at the same concentration. EG protects more deeply per percent; PG is food-safe (FDA GRAS). The type determines both the protection temperature and the penalty level.

The Recommended Band and Its Interpretation Tiers

The calculator maps concentration to five tiers, with a recommended band of 25 to 40 percent: the range that protects most HVAC climates without imposing excessive efficiency penalty.

Concentration Status
< 20% TOO LOW
20% to < 25% LOW / MARGINAL
25% to 40% RECOMMENDED
> 40% to 50% HIGH
> 50% TOO HIGH

Tier by tier: below 20% provides minimal freeze protection and is inadequate for most cold climates; the inhibitor package in inhibited glycols is also too dilute at this level. The 20 to 25% band gives light protection adequate only for mild climates or burst-only applications. The recommended 25 to 40% covers most HVAC freeze protection needs (roughly −10 to −25°C, or 14 to −13°F, depending on glycol type) with manageable penalties. The 40 to 50% range gives deep protection for severe climates at a notable viscosity and pumping cost. Above 50%, freeze protection returns diminish (and for some glycols plateau or reverse above about 60%), while the efficiency penalty becomes heavy.

The inhibitor minimum reinforces the lower bound:

Inhibited glycols need roughly 25-30% minimum concentration
for the corrosion inhibitor package to function.
Below that threshold, the inhibitor is too dilute even if freeze protection appears adequate.
This is a second reason not to go below the recommended band.

The band screens typical HVAC conditions. Severe climates justify HIGH; mild or burst-only applications may accept MARGINAL. The threshold is practical, not absolute.

Per ASHRAE HVAC Systems and Equipment and Dow Chemical data: the 25 to 40% band balances freeze protection with manageable penalty for most HVAC climates. Below 20% is inadequate for freeze protection and inhibitor concentration; above 50% carries diminishing freeze return and heavy penalty.

The Efficiency Penalty: Viscosity, Pumping, and Heat Transfer

Every percent of glycol above what the climate requires costs efficiency on three fronts: higher viscosity raises pumping energy, lower specific heat requires more flow, and lower thermal conductivity reduces heat transfer.

The three penalties stack:

1. Viscosity ↑ → pressure drop ↑ → pump power ↑
2. Specific heat ↓ → more flow required for the same load and ΔT (Section 9)
3. Thermal conductivity ↓ → heat-exchanger and coil capacity ↓

Viscosity and pumping: glycol is more viscous than water, more so at lower temperatures and higher concentrations. Higher viscosity raises pressure drop, so the pump works harder. A 50% glycol loop requires noticeably more pump power than a 30% loop, and both require more than a water-only loop.

Heat-transfer penalty: lower specific heat carries less heat per unit mass, forcing higher flow for the same duty. Lower thermal conductivity transfers heat less well across coil or heat-exchanger surfaces. A glycol loop may need a larger heat-exchanger surface or derating compared to a water loop of the same nominal size.

Cold-weather viscosity rises sharply with decreasing temperature. A loop that circulates fine at operating temperature can struggle at startup or in outdoor sections during cold weather. Pump selection should use the cold-viscosity condition, not the warm operating point.

Illustrative property shifts versus water (verify against manufacturer tables at the design temperature):

30% PG at 0°C (32°F): viscosity ~3-4× water, cp ~10% lower, conductivity ~10-15% lower
50% PG: penalties roughly double compared to 30%

The lesson is simple: do not over-concentrate. The minimum effective concentration (Section 10) minimizes all three penalties simultaneously.

Per ASHRAE Fundamentals (Secondary Coolants chapter) and Dow DOWFROST/DOWTHERM data tables: glycol adds viscosity (pumping), lowers specific heat (more flow), and lowers conductivity (heat-exchanger derate). All three penalties worsen with concentration and with decreasing temperature. Over-concentration is costly with no protection benefit once the design minimum temperature is already covered.

Why Higher Concentration Lowers Specific Heat and Raises Flow

The link to the Liquid Cooling Flow Rate article is direct: glycol's lower specific heat means a glycol loop must circulate more fluid than a water loop to carry the same heat, and the flow penalty grows with concentration.

Specific-heat values (approximate; per ASHRAE Fundamentals and manufacturer data):

Water:   cp = 4.186 kJ/kg·K
30% EG:  cp ≈ 3.7 kJ/kg·K  (~12% below water)
50% EG:  cp ≈ 3.3 kJ/kg·K  (~21% below water)
cp drops as glycol concentration rises

From the sensible-heat equation (Q = ṁ·cp·ΔT, solved for flow):

flow ∝ 1/cp
Lower cp → more flow for the same load and ΔT
30% EG: ~12% more flow than water
50% EG: ~20%+ more flow than water

Worked link (continuing from the Liquid Cooling Flow Rate example):

Water loop, 10 tons at 10°F: 24 GPM
Same loop, 30% EG:          ~27 GPM (cp ~12% lower → ~12% more flow)
Same loop, 50% EG:          ~29 GPM (cp lower still)

The compounding: higher concentration both lowers cp (needing more flow) and raises viscosity (adding more pressure drop per unit flow). Pump power rises on both counts: more flow through a more resistant fluid.

Sizing implication: pump and pipe selection must use the glycol flow rate, not the water-side calculation. Using the water result for a glycol loop under-sizes the pump. The Liquid Cooling Flow Rate article gives the water-side basis; the glycol concentration gives the correction factor.

Per ASHRAE Fundamentals (Secondary Coolants chapter): glycol mixture cp drops with concentration, raising required flow inversely. At 30% ethylene glycol, approximately 12% more flow than water; at 50%, approximately 20% more. Combined with higher viscosity, pump power rises on both fronts. Size for the glycol flow.

The Minimum Effective Concentration Principle

The governing design principle for glycol is to use the minimum concentration that protects the system to its design minimum temperature, because every excess percent adds penalty without adding protection value.

The principle:

Choose the LOWEST concentration that:
1. Protects to the design minimum temperature (freeze or burst, per system type)
2. Meets the inhibitor minimum (~25-30% for corrosion protection)
Then stop. More glycol adds penalty, not protection value.

Design steps:

1. Determine the design minimum temperature (local climate, outdoor sections, dormant conditions)
2. Decide freeze protection (flowing) or burst protection (dormant)
3. Read the concentration needed from the fluid-type freeze curve
4. Verify it meets the inhibitor minimum
5. Use that concentration, not more

Unlike many design margins, excess glycol is not neutral safety. It actively harms performance: higher pumping energy, more flow, reduced heat-exchanger capacity. The apparent "safety" of extra glycol beyond the protection the climate needs is a penalty on every operating hour.

A small margin for dilution and measurement uncertainty is reasonable. Concentration drifts over time (water top-ups dilute; slight evaporation concentrates), so design at the minimum effective plus a moderate margin and measure field concentration periodically (Section 13).

Worked logic:

Design min temperature: −15°C (5°F), flowing system (freeze protection needed)
30% PG protects to −13°C (9°F) freeze: not enough
35% PG protects to ~−18°C (−0.4°F) freeze: meets −15°C with margin
Use 35%, not 50%. Meets the inhibitor minimum, manageable penalty.

Per ASHRAE HVAC Applications and Dow glycol data: use the minimum concentration that protects to the design minimum temperature and meets the inhibitor minimum (~25-30%). Excess concentration is a penalty. A small dilution margin is reasonable; large excess is not.

Worked Example: 30 Gallons of Glycol in 100 Gallons of Solution

A closed hydronic loop is charged with glycol, total solution volume 100 gal (378.5 L), glycol volume 30 gal (113.6 L).

Step 1. Concentration:

C = (V_glycol / V_total) × 100 = (30/100) × 100 = 30% by volume

Step 2. Water balance:

Water = 100 − 30 = 70 gal (265.0 L)
30 gal glycol + 70 gal water = 100 gal solution

Step 3. Classification:

30% falls in 25-40%: RECOMMENDED

Step 4. Freeze protection (fluid-type dependent; verify against the curve):

30% PG: freeze point ~−13°C (9°F), burst point ~−33°C (−27°F)
30% EG: freeze point ~−15°C (5°F), burst point lower (EG protects more deeply per %)

Step 5. Inhibitor check:

30% is above the ~25% inhibitor minimum: corrosion protection adequate

Step 6. Efficiency penalty at 30%:

Viscosity ~3-4× water cold; cp ~10-12% lower; conductivity ~10% lower
Flow correction: ~12% more flow than water for the same load and ΔT
Penalty is manageable within the recommended band

Step 7. Flow correction (cross-reference to Liquid Cooling Flow Rate):

If this loop carries 10 tons at 10°F: water flow 24 GPM → ~27 GPM at 30% glycol
Size pump for 27 GPM (glycol flow), not 24 GPM (water-side result)

Step 8. Fluid type decision:

Non-potable industrial loop: EG (better thermal; 30% protects slightly deeper)
Food/potable-adjacent system: PG (food-safe; 30% protects to ~−13°C; verify sufficiency)

Step 9. Result:

30% concentration: RECOMMENDED
Protects to ~−13 to −15°C (9 to 5°F) depending on glycol type
Above inhibitor minimum; manageable penalty; ~12% flow correction
Conventional HVAC glycol charge for moderate climates
Verify freeze temperature against the fluid-type curve before finalizing

30% is RECOMMENDED and suits moderate climates. Cross-reference Liquid Cooling Flow Rate for the ~12% flow correction; Hydronic Balancing for distributing the glycol flow across the loop's circuits.

Reverse-Mode and High-Concentration Worked Examples

Reverse Mode (Example 2): Required glycol volume for a 400 L system at 35%

Step 1. Known: total volume 400 L (105.7 gal), target concentration 35%.

Step 2. Required glycol volume:

V_required = (C_target / 100) × V_total = (35/100) × 400 = 140 L (37.0 gal) of glycol

Step 3. Water balance:

Water = 400 − 140 = 260 L
140 L glycol + 260 L water = 400 L at 35%

Step 4. Classification:

35% falls in 25-40%: RECOMMENDED

Step 5. Practical application: reverse mode answers "how much glycol to buy or add to reach 35% in a 400 L system." For a new system charge, order 140 L of glycol and fill to 400 L with water. For a diluted system, the reverse calculation gives the glycol top-up volume needed to bring concentration back to target.

High-Concentration Example (Example 3): 60 gallons of glycol in 120 gallons

Step 6. Concentration:

V_total 120 gal (454.2 L), V_glycol 60 gal (227.1 L)
C = (60/120) × 100 = 50%

Step 7. Classification:

50% falls in 40-50%: HIGH

Step 8. Interpretation:

50% gives deep freeze protection for severe-climate applications.
Viscosity roughly double the 30% case; cp lower; conductivity lower; pump power up significantly.

Step 9. When 50% is justified: only for design minimum temperatures roughly −30 to −35°C (−22 to −31°F) where freeze protection requires that concentration. For climates with design minimums warmer than −25°C (−13°F), 50% over-concentrates and imposes unnecessary penalty.

Results:

Reverse: 35% target in 400 L requires 140 L of glycol (RECOMMENDED)
High:    50% is HIGH, justified only for severe-climate deep protection
         For milder climates, reduce to minimum effective concentration

Per ASHRAE HVAC Applications and Dow Chemical: reverse mode gives required glycol volume directly from target percent and total volume. 50% is HIGH and appropriate only for very cold design minimums; for moderate climates it over-concentrates with heavy penalty for no additional benefit.

Application Boundaries: Freeze Charts, Inhibitors, Percent by Mass, Dilution

The calculator scope is volume-fraction concentration, the required-volume reverse calculation, and the interpretation band. Several analyses fall outside that scope.

Actual Freeze and Burst Temperature. The calculator gives concentration, not the protection temperature. The freeze point and burst point come from the manufacturer's curve for the specific glycol type. Verify the actual protection temperature against the chart (Section 5). Concentration without temperature verification is incomplete.

Fluid-Type Freeze Curves. EG and PG differ at the same percent (Section 6). The fluid-type field identifies which curve applies; read the protection temperature from that curve. Do not apply one curve to the other glycol type.

Inhibitor Package. Inhibited glycols carry corrosion inhibitors that require a minimum concentration to function and deplete over time. The calculator does not track inhibitor level. Monitor and maintain per the glycol manufacturer's recommendations; a properly inhibited charge protects metal components; a depleted one does not, even at the correct concentration.

Percent by Mass versus Volume. The calculator uses percent by volume. Field refractometers and some data sheets use percent by mass or specific gravity. Glycol is denser than water, so the two differ. Convert to a consistent basis before comparing to this calculator or to a manufacturer chart. The glycol type matters for the conversion because EG and PG have different densities (per ASTM E1177 glycol testing standards).

Viscosity, Pressure Drop, and Heat-Transfer Numbers. The calculator flags that the efficiency penalty exists but does not quantify viscosity, pressure drop, pump-power increase, or heat-exchanger derate. Use fluid-property tables from ASHRAE Fundamentals (Secondary Coolants chapter) or Dow property sheets for those values.

Mixing Volume Contraction. Glycol and water mix with slight volume contraction; the total is marginally less than the sum of the components. This is a second-order effect for planning purposes; account for it in precise laboratory batching.

Dilution and Degradation over Time. Concentration drifts: water top-ups dilute it; slight evaporation concentrates it; inhibitors deplete. Measure field concentration periodically with a refractometer or hydrometer, matching its basis (mass or volume, and glycol type) to this calculator. Correct the charge as needed.

Non-Glycol Secondary Coolants. Other coolants (methanol, calcium chloride brine, dielectric fluids) have different properties and different concentration-to-freeze relationships. This calculator is glycol-specific.

Per ASHRAE Fundamentals (Secondary Coolants), ASHRAE HVAC Systems and Equipment, ASHRAE HVAC Applications (snow-melt, radiant, freeze protection), Dow Chemical (DOWFROST / DOWTHERM SR-1), and ASTM E1177: volume-fraction concentration screening is the calculator's scope. Actual freeze/burst temperature, fluid-type curves, inhibitor maintenance, percent-by-mass conversion, penalty quantification, mixing contraction, and dilution drift require separate analysis or manufacturer data.

Glycol Concentration Calculator

Open Glycol Concentration Calculator

Computes glycol concentration by volume fraction (glycol volume divided by total solution volume) or, in reverse, the glycol volume required to reach a target concentration in a given system. Classifies the result against the 25 to 40 percent recommended band. Ethylene and propylene glycol differ at the same concentration, so verify the actual freeze and burst temperatures against the fluid-type curve.

Open Glycol Concentration Calculator

FAQ

How do you calculate glycol concentration?

Per fluid-mixing basics: divide the glycol volume by the total solution volume and multiply by 100, giving C = (V_glycol / V_total) × 100 in percent by volume. In reverse, the glycol volume needed to reach a target concentration is (target% / 100) × total volume. Both modes use the same volume-ratio relationship, rearranged.

What glycol concentration is recommended for HVAC?

Per ASHRAE HVAC Systems and Equipment and Dow Chemical data: 25 to 40 percent suits most HVAC climates with manageable efficiency penalty. Below 20 percent is too dilute for reliable freeze protection and for the inhibitor package to function. Above 50 percent carries heavy viscosity and heat-transfer penalty with diminishing freeze-point return. Use the minimum concentration that protects to the design minimum temperature.

What is the difference between freeze protection and burst protection?

Per glycol manufacturer data (Dow DOWFROST / DOWTHERM SR-1): the freeze point, where ice crystals first form, is a higher (warmer) temperature than the burst point, where frozen fluid expands destructively and ruptures the pipe. Flowing systems design to freeze protection; dormant or idle systems can design to burst protection, requiring less glycol for the same minimum temperature. Both temperatures come from the fluid-type freeze curve.

Are ethylene and propylene glycol interchangeable at the same concentration?

Per Dow Chemical and ASHRAE Fundamentals: no. They have different freeze curves at the same concentration, with ethylene glycol providing more freeze depression per percent. They also differ fundamentally in toxicity: ethylene glycol is toxic; propylene glycol is food-safe (FDA GRAS). Substituting one for the other at the same percent changes both the protection temperature and the safety profile; verify the concentration against the correct curve whenever the glycol type changes.

Why not add extra glycol as a safety margin?

Per ASHRAE Fundamentals and HVAC Systems: because excess glycol raises viscosity (pump power), lowers specific heat (more flow required), and lowers thermal conductivity (reduced heat-exchanger capacity). Beyond the concentration the climate needs, more glycol is a compounding penalty on every operating hour, not a safety improvement. Use the minimum effective concentration; a small margin for dilution drift is reasonable, but significant over-concentration is not.

How much does glycol increase the required flow rate?

Per ASHRAE Fundamentals (Secondary Coolants chapter): glycol's lower specific heat raises required flow inversely. At 30% ethylene glycol, cp is roughly 12 percent below water, requiring approximately 12 percent more flow for the same load and temperature rise. At 50%, the flow increase reaches 20 percent or more. Combined with higher viscosity, pump power rises on both fronts. Size the pump and pipe for the glycol flow rate, not the water-side result.

Is percent by volume the same as percent by mass for glycol?

Per fluid-property basics and ASTM E1177: no. Glycol is denser than water, so the same mixture has a different value in percent by mass than by volume. Field refractometers may read either scale depending on the instrument. Match the basis (volume fraction or mass fraction) and the glycol type to the chart or calculator before comparing readings, as using the wrong basis can leave the system under-protected.

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