In any hydronic closed-loop solar heat collector system, the heat transfer fluid is the lifeblood. It must be sealed and pressurized in the solar heat piping, much like the Freon fluid in a refrigeration system. To ensure that the solar heating system is reliable over a very long time, the heat transfer fluid in the system must not leak out, freeze or boil, and it must tolerate high temperatures inside the solar collector without “cooking.” 

Propylene glycol (PG) has become the most common heat transfer fluid used in closed-loop solar heating systems that contain antifreeze. It has a long track record over many decades in this application and is widely available from a number of sources. This is not automotive antifreeze, which is a different substance (ethylene glycol), and is much more toxic and should never be used in domestic solar heating equipment. When working with PG, it is good to get to know its properties, capabilities and limitations that have a direct bearing on the pumping, piping components and temperature controls required by these systems.

Non-toxic

Solar home heating systems are most often used to heat potable domestic hot water, and in-tank heat exchanger coils have become very popular for this purpose. When a single wall heat exchanger fails, it is possible for the heat transfer fluid in the coil to leak into the potable water. Because this (and other environmental leakage) is a real possibility, the ideal solar heat transfer fluid would be biodegradable when released into the environment, and non-toxic if consumed by people or animals. 

Pure PG has a very high score in this regard, as evidenced by its use as a food and drug additive. Millions of people consume pure PG as part of their diet every day mixed into their food, cosmetics, medications and, more recently, inhaled when vaping. So, how pure is the PG used in solar heating systems? The answer is typically 95 percent pure before it is mixed with water. Typical PG heat transfer fluid contains additives to prevent corrosion and boost the resistance to high temperature degradation. The additives make up about 5 percent, by weight, of the concentrated PG fluid. The concentrated fluid is mixed with de-mineralized water before final usage, so for example, if mixed half and half with water, the final concentration of additives would be about 2.5 percent.

These small concentrations of additives are apparently nowhere near toxic levels. The makers of the PG heat transfer fluid provide Material Safety Data Sheets (MSDS) for the concentrated and the pre-mixed products. The MSDS language is very reassuring. For example, “first-aid measures” listed on one of these sheets include the following entries:

• Skin contact: Wash skin with plenty of water.   
• Inhalation: Move person to fresh air; if effects occur, consult a physician.   
• Ingestion: No emergency medical treatment necessary.

The MSDS listing under “Ecological Information” seems equally benign:

• Persistence and degradability — For the major component(s): Material is readily biodegradable.
• Ecotoxicity — For the major component(s): Material is practically non-toxic to aquatic organisms on an acute basis.

Heat tolerance for some common brands

Look for PG manufacturers that specifically formulate their glycol products for compatibility with solar heating systems. Those that do will say so very clearly in their  product labels and literature along with a high temperature rating that indicates compatibility with the normal operating temperatures of hot solar collectors. Pure PG will “cook” at high temperatures, and long exposure will cause it to change from a clean, transparent liquid to a brown substance resembling molasses with a burnt chemical smell.  

Figure 100-1 shows some common brands that typically come premixed with water, such as 60/40 or 50/50 (water to glycol ratios). One hundred percent PG is also available, but is very thick and cannot be pumped with a common hydronic circulator until it is mixed with water.

The MSDS listing for DowFrost, for example, acknowledges this in the section under “Thermal Decomposition,” which states: “Decomposition depends upon temperature, air supply and the presence of other materials. Decomposition products can include and are not limited to aldehydes, alcohols and ethers.” 

In other words, the heat transfer fluid will remain thermally stable in a closed system at recommended temperatures and pressures for a long time. If the high-limit temperatures are exceeded and/or oxygen is introduced into the closed system, the fluid will degrade. During decomposition, gases are generated that can cause extra pressure in closed systems as well. 

So, you can see that preventing the glycol from overheating is a design consideration of primary importance. That is why solar heating design discussions (even in this column) so often focus on controlled overheat dissipation (heat dumping) to keep the solar collectors below the high-limit temperature of the glycol in question. When overheat controls are provided, they are often set to keep the collectors below 220 F  to extend the life of the glycol. Here is a short list of some common glycol brands, and their temperature ratings as listed by the manufacturers.

DowFrost and DowFrost HD: DowFrost inhibited glycol-based fluid has an effective operating temperature range of -50 F to 250 F. DowFrost HD inhibited glycol-based fluid is effective from -50 F to 325 F . 

Use any Cryo-Tek antifreeze in hydronic closed loop solar heating systems that require freeze protection. Operating temperature range for closed system: Up to 250 F. 

Tyfocor L and Tyfocor LS Pre-mixed: Premature aging will occur above 338 F, slow thermal decomposition above 392 F. 

Dynalene Solar Glycol-XT: (BioGlycol made from corn.) Recommended temperature range closed system: -17 F to 350 F. 

DowFrost in depth

There is a wealth of information available for PG heat transfer fluids, and one of the most prolific sources is Dow, and can be found at www.dow.com/heattrans. 

There are many useful publications in PDF format on this site available for free. One of the most comprehensive is the DowFrost “Engineering and Operating Guide,” which is a gold mine of technical information about the properties of PG with advice about how to use it properly. If you want to know freezing point, boiling point, conductivity, specific gravity, density, viscosity, temperature limits and lots of other details, this is the reference to get.

Testing methods

As the PG ages and degrades over time, the freeze protection concentration can change, the acidity can change, and the additives can lose their effectiveness. You can quickly determine the condition of your fluid by examining its appearance and odor. Any drastic variation from the initial fluid specifications, such as a black or dark-gray color, presence of an oily layer, burnt odor or any heavy sludge in the fluid may indicate the need for fluid replacement.

Test equipment is also available to measure the quality of the fluid. This can be done with test strips, supplied as a kit by the manufacturer, that resemble litmus paper. Test strips will tell you the pH, the freeze protection level (indicated by the percent concentration) and the state of the inhibitors. We often use a refractometer gauge that resembles a small telescope to quickly check the freeze point/concentration. A hand held digital refractometer gauge (e.g. by MISCO), that reads out concentration and freeze point directly on an LCD display, has also proven to be very useful. Digital pH meters are available as well.

The following advice is taken from the DowFrost “Engineering and Operating Guide:

Control of pH between 8 and 10 is important to minimize corrosion and glycol degradation. Using narrow range pH paper, such as pHydrion Control paper with a 7.2 to 8.8 pH range is an easy and reliable way to read your pH level.  

A pH tester can also measure alkalinity or acidity of the fluid. The desirable pH range should fall between 8 and 10. Adjustments can be made by using a 50 percent solution of sodium hydroxide or potassium hydroxide, if the pH is between 7 and 8. Any fluid with an acidic pH below 7 should be replaced. 

Final notes

This article is targeted toward residential and small commercial buildings smaller than 10,000 square feet. The focus is on closed-loop pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.   

Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed Mechanical Contractor in New Mexico. He is the chief technical officer for AMENERGY-SolarLogic LLC in Santa Fe, New Mexico, where he is involved in solar hydronic installations, development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.