Glycol vs. Water for Heat Transfer

Choosing the right heat transfer medium is crucial for HVAC systems, and the choice between glycol and water often sparks debate. While some may assume glycol is better due to its presence in automotive systems, its primary purpose there is freeze protection, not enhanced heat transfer.

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Key Differences Between Glycol and Water

1. Heat Transfer Efficiency

• Water is a more effective heat transfer medium than glycol. Water's superior thermal conductivity allows it to absorb and transfer heat more efficiently.

• Glycol is added to prevent freezing and to inhibit rust or corrosion in the system.

2. Corrosion Protection

• Glycol systems often include corrosion inhibitors, which protect metal components from rust.

• Water systems, when filled or refilled, can introduce oxygen into the loop, promoting oxidation and rust formation.

3. Cold Weather Performance

• Closed-loop heat pump systems may require glycol since heat pumps can pull heat from the loop faster than the earth can replenish it, posing a freezing risk.

• Outdoor hydronic coils require glycol since exposure to freezing temperatures could lead to water freezing in the coil.

• Snowmelt systems rely on glycol to ensure consistent operation in freezing conditions.

4. System Design Considerations

• Glycol increases the fluid's viscosity, requiring proper pump sizing to maintain adequate flow rates.

• Tubing must be correctly sized to accommodate the increased resistance in glycol systems.

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Glycol System Guidelines

• Some boiler manufacturers limit glycol content to 30%, while others allow up to 50%. Always consult the equipment manufacturer for proper glycol ratios.

• Propylene glycol is common in HVAC applications due to its low toxicity, while ethylene glycol has better heat transfer properties but is more toxic.

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Maintenance Considerations

• Glycol systems require annual maintenance since continuous heating and cooling cycles degrade the glycol's chemical properties over time.

• Water systems with automatic fill valves can often be ignored for longer periods but require winterization to prevent freezing.

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Final Recommendations

When selecting between glycol and water:

• Use water where freezing is not a concern for better heat transfer efficiency.

• Use glycol in systems exposed to freezing temperatures or where corrosion inhibitors are needed.

 

For superior protection and performance, consider using Fernox products for glycol treatment and corrosion control. Johnstone Supply offers a range of chemical feeders and treatments to meet your system’s needs.