Using Glycol in Hydronic Heating Systems


Glycol, also called antifreeze, is an important compound that HVAC manufacturers use to protect hydronic heating systems in cold climates. When added to water, glycol lowers the freezing temperature of the resulting mixture, ensuring that it remains liquid even during harsh winter weather.

Without glycol, simply leaving the heat source off for more than a day or two could result in the entire system freezing up; causing ice which will likely burst pipes and create extensive water damage to a dwelling.  Adding glycol to the water prevents such catastrophes from happening, but often compromises system performance.  However, high-quality glycol can mitigate that risk.

The HVAC industry uses two types of glycol in hydronic heating systems:

  • Ethylene glycol. A historically popular and very effective anti-freezing agent, modern HVAC experts try to avoid using ethylene glycol because of its toxicity. Nonetheless, it is commonly implemented in industrial settings where strict regulation prevents any chance of it seeping into drinking water.
  • Propylene Glycol. A more modern compound, propylene glycol is an effective anti-freeze agent generally recognized as safe by the FDA. HVAC experts use this anti-freeze compound in residential and commercial settings. Propylene glycol is also the preferred anti-freeze for industrial food processing applications.

Glycol Characteristics

Glycol has a higher viscosity and lower heat transfer characteristics than water. For this reason, HVAC engineers typically dilute glycol with water inside hydronic heating systems. The maximum recommended ratio for glycol dilution with water is 1:2. A glycol concentration lower than 50% avoids overtaxing the performance of the system’s boilers and pumps while eliminating the risk of damage occurring in low-temperature climates.

For this reason, HVAC system designers must pay close attention to the minimum temperature they expect any particular system to encounter. The amount of glycol incorporated into the system should ideally prevent freezing at that temperature without exceeding concentration parameters – this ensures the best possible performance.

Choosing Glycol for Hydronic Heating Systems

Sentinel X500 Glycol is a propylene-based glycol compound that contains a unique blend of corrosion-inhibiting chemicals. These additives prevent system degradation, significantly extending the life expectancy of hydronic heating appliances and the glycol used in the system.

While most non-inhibited glycol products have a three to five-year lifespan, pre-mixed Sentinel X500 glycol can provide up to 20 years of service before needing replacement.  Pre-mixed Sentinel glycol uses pure, neutral-pH water which is an important life-extending factor, since “heavy/hard water” contains trace metals and other mineral impurities which shorten glycol life spans and corrode hydronic pumps, boilers, valves, and fittings.

HVAC installers with access to neutral pH water can also purchase Sentinel X500 in 100% concentrations.  This is ideal when specific mixture percentages are needed for exact job requirements that need to be fulfilled.

Is Water Quality That Important?

Some HVAC specialists take water quality for granted. However, it can and does play an important role in ensuring problem-free operation of hydronic heating systems. Trace deposits of metals and other contaminants can produce sludge that blocks components of the hydronic system and reduces its efficiency.  Over time, sediment deposits damage the system and deplete a system’s corrosion inhibitors creating the perfect conditions for internal corrosion and eventually shortening the life of the system.

To ensure the best results, don’t use tap water unless you are absolutely sure your local municipality’s water has an acceptable pH (not typical).  If mixing the glycol is the preferred method, use demineralized water or water that has been distilled, deionized, or filtered through a reverse osmosis system.

EP Sales maintains a large inventory of Thermolec and Greentek products and parts to service the needs of our customers.