Glycol Explained: The Top 5 Questions HVAC Pros Are Asking

There are a lot of questions surrounding glycol, these are 5 of the most common ones we field regularly. 

1a. What percentage of glycol do I actually need?

The percentage of propylene glycol you need depends entirely on the lowest temperature the system will be exposed to and whether you need freeze protection or just burst protection. Using too much glycol hurts system efficiency and increases costs, so getting it right is key. The system manufacturer will also have recommendations that can be specified to your needs. 

Freeze Protection vs. Burst Protection

This is the most important distinction.

• Freeze Protection 🥶: This means the fluid will remain liquid and pumpable down to the rated temperature. You need this for systems that must operate during freezing conditions, like snow melt systems, outdoor coils, or process cooling that runs year-round.
• Burst Protection 💥: This means the fluid will turn into an ice slush but will not expand enough to break pipes. This is sufficient for systems that are shut down and idle during the winter, like a hydronic AC system or a seasonal chiller. It can save you money by using only just enough glycol to get through without breaking pipes.

1b. Why More Isn’t Better

It’s tempting “C.Y.A.” and add extra glycol, but over-concentrating glycol has serious downsides:

• Reduced Efficiency: Propylene glycol transfers heat less effectively than water. A 50% solution can reduce heat transfer efficiency by 15-20% compared to pure water, forcing pumps and chillers to work harder and use more energy. 
• Increased Pumping Costs: Glycol is more viscous (thicker) than water, which increases the pressure drop in the system and requires more energy to pump, this can also lead to premature failure and costly replacements.
• Improper Inhibitor Levels: Corrosion inhibitors are designed to work within a specific concentration range, typically 25% to 60%. Below 25%, there isn’t enough inhibitor to protect the metals in your system.

The bottom line: ✅ Use the manufacturer’s data to select the precise percentage needed for your job’s specific temperature requirement—and no more. This is where a custom pre-blended fluid is ideal, as it guarantees you get the exact concentration without waste or mixing errors.

2. Why can’t I just use tap water to mix with concentrate?

Using tap water to mix glycol is one of the most common and damaging mistakes a contractor can make. Tap water is full of minerals and chemicals that can actively harm a hydronic system.

The two main culprits are:

1. Minerals (Calcium & Magnesium): When heated, these minerals precipitate out of the water and form scale on heat transfer surfaces, like the inside of a boiler or heat exchanger. This scale acts like an insulator, forcing the system to work harder, reducing efficiency, and eventually causing overheating and component failure. Think of it like plaque clogging an artery.
2. Chlorides & Contaminants: Tap water contains chlorides and other dissolved solids that are highly corrosive to metals like steel, copper, and aluminum. They attack system components and accelerate the breakdown of your corrosion inhibitors.

Using purified Deionized (DI) or Distilled water is critical. It’s a clean slate, free of these damaging minerals and contaminants. This ensures your corrosion inhibitor package can focus on protecting the system’s metals instead of fighting with the water itself, leading to a much longer and more reliable system life.

An aside: we’ve been hearing more and more reports of new systems being left with residue/fouling inside them. Consider using a quality hydronic system cleaner and/or flushing with lots of water before kicking things on for the first time. 

3. Propylene vs. Ethylene Glycol: When should I use one over the other?

The primary difference between propylene and ethylene glycol is toxicity. This single factor should guide your choice for almost every HVAC application.

• Propylene Glycol (PG): This is the one you should use for most jobs. It has very low toxicity and is often referred to as “GRAS,” generally recognized as safe.
  • Use it for: Any system in a residential home, school, hospital, or food processing facility, there are NSF versions available. It’s the only choice for applications where a leak could potentially come into contact with potable water, people, pets, or food products. 👨‍👩‍👧‍👦
• Ethylene Glycol (EG): This is the traditional automotive antifreeze. It is toxic if ingested. While it offers slightly better heat transfer performance and is less viscous (easier to pump), its toxicity makes it a significant liability in most settings.
  • Use it for: Strictly controlled industrial applications where human contact is extremely unlikely and peak thermal performance is the absolute priority. The system must be sealed, clearly labeled, and regularly monitored. 🏭

The bottom line: For HVAC applications, the significant safety advantage of propylene glycol almost always outweighs the minor performance benefits of ethylene glycol. When in doubt, choose propylene.

Generally speaking, GRAS or NSF glycol, from a quality source, is high enough quality for the vast majority of glycol systems you’ll encounter. Occasionally you might hear of someone asking for “food safe glycol,” NSF glycol is what you’re looking for. Food safe or pharmaceutical grade glycols are used in cosmetics or food products that come into contact with or are ingested by humans…100% overkill for your residential chiller.  

 4. How often should I be testing the glycol, and what am I looking for?

You should test the system fluid annually as part of routine maintenance, ideally just before the start of the heating or cooling season. Annual testing is cheap insurance against expensive system failures.

You’re checking for three key things:

1. Freeze Point (Concentration): Make sure the fluid still provides protection to the required temperature. The concentration can become diluted over time from leaks being topped off with water.
   • Tool to use: A refractometer. Do not use a floating-ball hydrometer, as they are notoriously inaccurate for glycol.
2. pH Level: This tells you the health of your corrosion inhibitors. The fluid should be slightly alkaline (typically a pH between 8.0 and 10.0). If the pH drops below 8.0 and becomes acidic, the inhibitors are depleted, and the fluid is now actively corroding the system from the inside out.
   • Tool to use: High-quality pH test strips or a digital pH meter. Note: make sure you regularly calibrate your digital meter to make sure your testing is accurate.
3. Visual Appearance: Simply pull a sample. The fluid should be clear (even if colored). If it’s cloudy, rusty, full of sediment, or has a burnt smell, it’s a clear sign of corrosion, contamination, or glycol degradation that needs to be addressed immediately. 

Glycol is usually dyed to help with leak detection (except when clear is required), the industry standard for propylene was blue, ethylene typically pink for HVAC applications. These days, each manufacturer uses a different color to signify different blends, orange, yellow, green and blue are all common colors you’ll encounter. 

 5. Is buying pre-mixed glycol better than mixing concentrate on the job site?

For the vast majority of jobs, buying a quality, custom pre-mixed glycol is far superior to mixing concentrate on-site. It’s about ensuring quality, saving time, and eliminating risk.

Here’s why pre-mixed is the professional choice:

• ✅ Guaranteed Accuracy: Pre-blends are mixed to the exact percentage you need. This eliminates mixing errors, ensures correct freeze/burst protection, and maximizes system efficiency.
• ✅ Superior Quality: A reputable supplier uses pure, DI water. This prevents the scale and corrosion caused by using poor-quality job-site tap water, protecting the client’s equipment from day one.
• ✅ Time & Labor Savings: It eliminates the time your technicians spend measuring, mixing, and handling pails/drums/totes on site. You can get more done in a day, and the process is cleaner and simpler.
• ✅ Reduced Waste & Spills: You order the exact amount you need, reducing leftover concentrate and the risk of messy, costly spills.

While mixing concentrate on-site might seem cheaper upfront, the risks of inaccurate ratios, water contamination, and added labor time often make it the more expensive option in the long run due to callbacks and potential system damage.

As always, if you have any specific questions, reach out to [email protected] and we’re happy to help. If you need help determining what percentage of glycol you need, or how much to add or remove from a system to reach a desired system, checkout our glycol calculator here: link.