Glycol systems: concentration, freeze protection and performance

In most of Canada, any hydronic loop that sees outdoor air or outdoor equipment needs freeze protection. Air-cooled chillers, dry coolers, heat recovery coils, makeup air unit preheat coils, snow melting systems and ground loops are routinely filled with a glycol and water mixture instead of plain water.

Glycol solves the freezing problem, but it is not free. It reduces heat transfer, increases pumping power, changes expansion volume, needs its own maintenance and costs money to buy and dispose of. Choosing the right type and concentration is a balance between protection and performance.

Ethylene vs propylene glycol

Propylene glycol is generally selected where a heat exchanger could leak into potable water, in food facilities, and where local requirements or the owner's policy favour it. Plumbing codes and the authority having jurisdiction may also require double-wall heat exchangers between treated loops and potable water, regardless of glycol type. Check provincial plumbing code requirements for the specific application.

Always use an inhibited, HVAC-grade glycol formulated for hydronic systems. Automotive antifreeze contains silicates and other additives that can gel, foul heat exchangers and attack pump seals in HVAC systems.

Freeze protection vs burst protection

Manufacturers publish two different temperatures:

• Freeze protection (freeze point): the temperature at which ice crystals begin to form. Below this, the fluid becomes slushy and cannot be pumped effectively.
• Burst protection: a lower temperature to which the fluid can fall without expanding enough to burst pipes. The slush still occupies the system, but it does not rupture it.

Systems that must operate continuously in cold weather, such as a preheat coil loop or a heat recovery loop, need freeze protection below the lowest expected fluid temperature. Systems that can sit idle in winter, such as a cooling-only loop that is shut down, may only need burst protection, which allows a lower concentration.

Typical freeze points

The values below are approximate freeze points for common concentrations by volume. Exact values vary by manufacturer and inhibitor package, so confirm with the product data sheet.

Concentrations much above 50 to 60 percent are rarely useful: freeze protection stops improving and heat transfer suffers further.

Choosing a design temperature

Start with the outdoor design temperature for the location, from the NBC Appendix C climatic data, and ask what fluid temperature the loop can actually reach. Coils exposed directly to outdoor air during a fan failure or with a stuck damper can approach outdoor temperature. Buried loops and loops inside heated buildings see milder extremes. A common approach is to select a freeze point a few degrees below the lowest expected fluid temperature. See selecting outdoor design conditions for Canadian cities.

Effect on performance

Glycol solutions have lower specific heat, higher viscosity and lower thermal conductivity than water. The effects are greatest at low temperature and high concentration.

• Flow rate: to move the same heat at the same delta-T, more flow is needed because specific heat is lower.
• Pump head and power: higher viscosity increases friction losses, and pumps must be corrected for specific gravity and viscosity. Pump curves published for water must be adjusted.
• Coil and heat exchanger capacity: film coefficients drop, so a coil selected for water will deliver less capacity with glycol. Coils and heat exchangers must be selected using the actual fluid.
• Chiller capacity: chillers operating on glycol have reduced capacity and efficiency; manufacturers publish correction factors.
• Expansion volume: glycol expands more than water over the same temperature range, which affects expansion tank sizing.

For these reasons, designers often isolate the glycol section of a system with a heat exchanger so that only the part exposed to freezing carries glycol. That costs a heat exchanger and an approach temperature, but keeps the rest of the building on plain water. A runaround loop on a heat recovery system and a plate exchanger for a waterside economizer are typical examples.

Filling and makeup

1. Pre-mix glycol and water to the target concentration before filling, rather than adding concentrate to a filled system.
2. Use good quality water for dilution, typically deionized or softened water with low chlorides and sulphates, as recommended by the glycol supplier.
3. Fill through a glycol feed unit, not a direct city water connection. Automatic city water makeup dilutes glycol every time there is a leak.
4. Label the system with glycol type, concentration and date.
5. Record the volume added so total system volume is known.

Testing and maintenance

Test glycol at least annually, and more often in critical systems:

• Concentration by refractometer, using the scale for the correct glycol type.
• pH and inhibitor reserve, since inhibitors deplete over time and glycol can degrade into acidic compounds, especially if overheated or aerated.
• Appearance and odour: cloudiness or a sour smell suggests contamination or biological growth.

Adjust inhibitors or concentration as recommended by the supplier. Degraded glycol should be removed and disposed of according to provincial and municipal regulations; it should not be drained to sewer without approval. The guide on water treatment for closed and open loops covers testing programs more broadly.

Frequently asked questions

Can I mix ethylene and propylene glycol?

It is not recommended. Mixing makes concentration testing unreliable and inhibitor packages may not be compatible.

Why did my glycol loop freeze even though it was filled correctly?

The most common reason is dilution through a city water makeup connection after leaks or repairs. Low concentration may go unnoticed until a cold snap.

Key takeaways

• Use inhibited HVAC-grade glycol; propylene where toxicity matters, ethylene where performance matters and contact risk is controlled.
• Choose freeze protection for loops that must run in cold weather and burst protection for idle loops.
• Select pumps, coils, chillers and expansion tanks using glycol properties, not water data.
• Keep glycol only where it is needed, isolated by a heat exchanger where practical.
• Test concentration and inhibitors at least annually; verifying fluid and flows belongs in building commissioning.