Glycol plays an indispensable role in preventing freeze damage. Glycol is not going to go away. There will be instances where filling the system with a glycol mix is the best solution. Freeze-ups caused by mechanical failures, empty fuel tanks, power outages, poorly installed insulation or garage doors left open can be avoided if glycol is there to save the day. We do not want frozen piping to give hydronic and solar thermal systems a bad reputation. No one wants to deal with burst solar collectors or ruptured tubing in a concrete slab in their home or on their watch.
So why not install propylene glycol (here after referred to as glycol) anti-freeze in every system? When I started bidding hydronic heat jobs with snow melt or in slab garage heat, it occurred to me that it is often cheaper and easier to add glycol to the whole system than it is to separate the part which might freeze. The material and installation costs for an additional heat exchanger, expansion tank, pressure relief valve, circulator and associated piping could make the difference between being awarded the job and going fishing. Still, lots of experienced heating professionals will insist that glycol be used only where it is absolutely necessary and then only in the minimum concentration required. Why?
Glycol does not behave exactly like water. It would be nice to know what the implications of using propylene glycol for freeze protection are. Consider these questions: What is creep capacity? Will hard water or oxygen shorten the life of my heat transfer fluid? Why is one brand of glycol rated for 250°F and another 325°F? How do we match the glycol to the system? What causes glycol to become acidic? When does glycol boil? Do I need to upsize my expansion tank if I add glycol? How do we compensate for the reduced heat transfer qualities of glycol? Will increased viscosity cut down the flow rates? Does propylene glycol biodegrade? Are all prepackaged propylene glycols non-toxic? What do freeze, flow and burst protection mean?
One reason to avoid the use of glycol is called ‘increased creep capacity’ also known as ‘weepage’. You guessed it, glycol will leak out of systems in which water did not. In this respect it acts a little like penetrating oil. Tom Lane recommends the use of Teflon tape instead of pipe dope on threaded joints in systems with glycol to avoid leaks.1 I was taught to use both pipe dope and Teflon tape on every joint and it has served me well. I also tighten things up just a bit more and solder with added diligence when I know glycol will be used. Leaks involving glycol are not only more common, but also more costly to refill and clean up. Upon returning from a warranty call, a master plumber threw his hands up in the air exclaiming simply: “The water wants to get out.” Who needs the added liability of glycol if it is even better at “getting out” than water?
Control dissolved solids, oxygen content, pH value and temperature to protect your system. If ‘increased creep capacity’ is not enough motivation for you to install that extra heat exchanger or make that solar hot water installation a drain back system, then perhaps the acid/corrosion issue will be. When dealing with water or water-glycol mixes, corrosion must be minimized by controlling four different factors: dissolved solids, oxygen content, pH value and temperature of the heat transfer fluid. Let’s take a closer look at each one of these factors.
1) Dissolved Solids can lead to corrosion and scale. The term ‘dissolved solids’ refers to hardness in the water and other impurities which might be left in the system from the manufacturing or installation processes. For instance, flux and chlorinated water can both introduce chloride, a corrosive element (especially for aluminum), into the heat transfer fluid. For this reason, tap water should not be used if it contains more than 100 ppm chloride and new systems should be cleaned to prevent contamination of the heat transfer fluid (HTF)2 . Some people recommend TSP (tri-sodium phosphate) be used as a cleaner (one lb. to 50 gallons and run system for two hours at design temp then flush). Hard water may introduce calcium and magnesium, which results in the formation of scale. This scale is most likely to form inside boilers, heat exchangers and solar collectors where heat causes it to precipitate out of the HTF. These high temperature regions are also the places where scale will most adversely affect system efficiency.
Dissolved Solids can react with inhibited glycol. The problem of corrosion due to dissolved solids is increased when glycol is used. Glycols for use as heat transfer mediums contain inhibitors and buffers as additives. These inhibitors and buffers are often dissolved solids themselves (how ironic is that) but are there to protect the system. Specific inhibitors protect specific materials by coating them with a thin film. For example, if you are installing a boiler with a cast aluminum heat exchanger, make sure to use aluminum safe glycol which contains inhibitors specifically formulated to protect aluminum. Also watch the use of galvanized piping because glycol reacts with zinc, resulting in sludge. Glycols also contain buffers which help them maintain a neutral pH value. Buffers contribute to what is called residual alkalinity by neutralizing acids that occur as the glycol reacts and degrades. Glycol mixes which do not contain inhibitors or buffers, while less expensive, are undesirable. Without inhibitors and buffers, chemical reactions between the glycol, system components and oxygen produce lactic acid particularly in the presence of heat. There are horror stories about uninhibited glycol corroding its way out of expensive equipment. Sometimes the terminology is confusing since some chemicals act as both inhibitors and buffers. Both inhibitors and buffers can be ‘used up’ in reactions with other dissolved solids introduced into the system, leaving the system unprotected from corrosion. These reactions can also produce scale and sludge inside the system. De-ionized or de-mineralized water can be used to mix down concentrated glycol and minimize the introduction of dissolved solids. Some glycol products come premixed with de-ionized water and specify that their product be used undiluted. Controlling the quality of the water used to mix down the glycol is worth more than the additional cost of shipping a mixture which is half water. There is an opportunity here for people ‘in the know’ to save some money. Most glycol manufacturers offer their glycol in various concentrations, so buy the concentrated stuff and mix it down on site with high quality water to save shipping and packaging costs on water. Just make sure not to over-dilute the inhibitors and buffers.
When you dilute the pre-packaged glycol mix you also dilute the inhibitors and buffers. Over-diluted inhibitors may not be able to prevent corrosion. The Noburst web site states: “A minimum NOBURST -100 concentration of 50% is required to maintain adequate corrosion protection for the long term…”3 Some manufacturers sell inhibitor packages which can be added to boost inhibitors weakened through reactions or dilution.
“Glycol concentrations less than approximately 20% volume must be avoided since glycol is a nutrient source for bacteria at these low concentrations.” 4 I have had this bacteria problem myself. It resulted in a modulating boiler making some undesirable whistling noises. On the other hand, there are a lot of systems out there with highly diluted glycol which seem to show no bacteria related problems.
2) Oxygen can also react with inhibited Glycol. It is well known that oxygen must be kept out of closed-loop hydronic systems containing ferrous metals. I have had the pleasure of telling a costumer he had stapled up potable tubing under an entire section of his house and it would have to be redone with tubing containing an oxygen barrier. We pay more for PEX piping with an oxygen barrier so that we can use cast iron boilers or pumps as well as sections of black iron pipe. Similarly, glycol must be protected from atmospheric oxygen. One reason for this is the inhibitors and buffers can react with oxygen. Systems containing glycol should be closed systems with intact oxygen barriers.
3) Low pH values (acid) in the HTF can destroy a heating system, solar or otherwise. In some cases water from a well or municipality is acidic right out of the tap. Green coloring is a sign of dissolved copper in the water, and if the copper is in the water it is not part of the pipes any more. ANSI/NSF standards certify copper only when the pH is 6.5 or higher. For aluminum, high pH is also bad. At pH values greater than 10, the oxide film starts to dissolve, resulting in rapid corrosion unless controlled by inhibitors. The glycol and its additives must be maintained to prevent low pH values.
4) Excessive temperatures can break glycol down. While controlling dissolved solids and oxygen in the HTF helps to reduce corrosion and maintain pH values, excessive temperatures should also be avoided. In general, the more heat, the more chemical reactions to stress the glycol and its inhibitors. Standard glycols may be rated for 250°F. Often glycols for high temperatures and solar applications are rated for temperatures around 325°F. Flat panel solar collectors with selective coatings on the absorbers can reach temperatures above 325°F, vacuum tube collectors can achieve still higher temperatures. In order to prevent the glycol mixture from degrading, temperatures above those for which the fluid is rated should be avoided and time spent at high temperatures minimized. The variation in the temperature ratings of different glycols is due predominantly to differences in the inhibitor package or additives which make up only about 5% of the glycol. Dowfrost (1 year rating at 250°F) fluid is a formulation of 96% propylene glycol, 4% additives. Dowfrost HD (1 year rating at 325°F) is a formulation of 94 percent propylene glycol and 6% additives.5 Matching the additives to the temperatures and materials in the system is the critical factor in selecting the right glycol mix. To see what kind of stagnation temperatures you might be dealing with see the COLLECTOR EFFICIENCY CALCULATOR located on this site.
Collector stagnation temperatures may be too much for glycol. Most of the time, solar collectors are kept relatively cool due to the circulation of the HTF. Excessive, stagnation type temperatures result when, for some reason, the solar generated heat is not transferred away from the collectors. When the circulator quits and it is -20°F glycol can save the day, but when it’s sunny out and the circulator for that solar closed loop glycol system stops working your glycol might be cooked. Tom Lane states: “One hour of stagnation in the full sun and the inhibitors that buffer the glycol start breaking down in the anti-freeze and over time it turns to an acid.”6 Stagnation occurs when circulation through the collectors ceases during full sun conditions. The resulting high collector temperatures act as a catalyst for undesirable chemical reactions and may even vaporize the HTF.
Avoiding stagnation. Malfunctioning sensors or controls, broken pumps, power outages and high limiting of storage can all result in stagnation. Photovoltaic panels used to power DC pumps (a subject for its own article) provide flow through the collectors even during power outages. The high limiting of storage does not have to result in stagnation if the excess heat can be dumped elsewhere. Installing a ‘drain back’ system where the heat transfer fluid drains by gravity out of the collectors and all piping exposed to freezing or stagnation temperatures can eliminate the need for both heat dumps and anti-freeze. Even in non-freezing regions drain back systems are installed so that the collectors can be left empty when the storage high limit is reached. SRCC rated collectors have been tested in full sun conditions when empty for a limited time.
Stagnation temperatures at standard system pressures (30 psi pressure relief valve) will vaporize the HTF. The resulting increase in volume (liquid to gas) can push the HTF through the pressure-relief valve. A service call will most likely be required to refill the system and possibly replace a pump. If vaporization of the HTF is possible it often makes sense to size the expansion tank(s) so that the heat transfer liquid can vaporize out of the collectors and into the expansion tank(s). Tyfocor LS is ‘reversibly evaporizable’ (translated from the German) which means it is formulated to evaporate from solar collectors in the event of stagnation and reabsorb any residue left behind when pumping resumes. This is achieved through the use of a customized liquid inhibitor package. The temperature rating of 338°F for Tyfocor LS is conditional upon the expansion tanks being sized large enough to accommodate the increased volume incurred by vaporization of the HTF in the collectors. When the glycol in the collectors vaporizes the portion of the glycol exposed to temperatures above its boiling point is dramatically reduced, but any residues left behind experience true stagnation temperatures. With glycols that are not ‘reversibly evaporizable’ these residues may be ruined inhibitors. In general, vaporization is to be avoided. At atmospheric pressure, Tyfocor LS has a boiling point between 216 and 221°F, with 55 to 58% water in the mix right out of the bucket. With an additional system pressure of 15 psi the boiling point rises to about 255°F. At 73 psi system pressure, the boiling point rises to around 313°F. Thus system pressure can be used to reduce the chances of vaporization. Make sure system components are compatible with the system pressure. If the system pressure can exceed domestic water pressure, the use of single wall heat exchangers may be prohibited because the HTF could be forced into the potable water supply. Similarly, water at 15 psi system pressure boils at about 250°F, at 73 psi the boiling temp rises to approximately 300°F. Water and water glycol mixes boil at around the same temperatures. Failsafe temperature control is one of the hurdles in designing solar thermal systems. There are even solar systems out there designed to dump domestic hot water down the drain when storage temperatures rise too high rather than allow stagnation.
How to upsize the expansion tank to allow for vaporization. After scratching my head for a while and searching around on the internet to no avail, it occurred to me how to size the expansion tank(s) to accommodate vaporization of the HTF. Upsize the expansion capacity beyond that normally required for the fluid state by the volume of HTF in the collectors and any piping above or within about five feet of the collectors. That way all the volume of all the hot regions can be absorbed into the expansion tank(s). In some small solar applications designers even size the expansion tank to hold the entire fluid volume of the system. From the manufacturer literature I have read most expansion tanks are rated for temperatures in the 210 to 240°F range. The expansion tank will last longer if it is exposed to less heat. The best heat technician I know pipes his expansion tanks low and off to the side to minimize high temperature exposure. 7
Glycol mixes expand more than water. Here is a “rule of thumb” I came across: Upsize expansion tanks by 20% for systems with glycol. Here’s another one: The expansion tank should be sized to allow about 4% greater expansion than for plain water in the same temperature range. I won’t site the sources. Given the often contradictory nature of rules of thumb, maybe we need to be a little more systematic and a little less rule of thumb. The two issues here are temperature range and glycol concentration. In the typical hydronic system the temperature might fluctuate between 50 and 200°F, resulting in expansion of 3.8% with water or 5.9% with 50% glycol. Because of these erroneous rules of thumb I confirmed the above calculation with Amtrol tech support. There is 55% more expansion with glycol. Expansion capacity must be increased beyond the above numbers to account for vaporization of the HTF or larger temperature swings.
The heat transfer qualities of propylene glycol are inferior to those of water. When someone says ‘heat exchanger’ I think about flat plate heat exchangers but really all boilers, baseboards, radiant floors and solar collectors contain heat exchangers. So the effects of glycol on heat exchange are something worth understanding. When glycol is added to an existing system, heat exchange will decrease due to lower thermal conductivity (R-value of HTF), reduced heating capacity (specific heat) and reduced flow (viscosity). Let’s deal with these issues one at a time.
Basically, lower thermal conductivity means glycol has a higher R-value than water. The thermal conductivity of a 50% glycol mix is about 59% that of water.8 This means, heat is transferred less efficiently between the fluid and the piping or heat exchanger wall. For example, the difference between the average fluid temperature and the inner wall of the pipe will be about twice as large with a 50% glycol mix.9 But this difference as you might imagine is small and twice small is still small. The overall heat transfer through a radiant floor or heat exchanger will not be reduced by 50%. Only the heat transfer between the fluid and the inner wall is affected. The addition of glycol does not affect the pipe wall, concrete or flooring. In general, reduced thermal conductivity is compensated for by a slight increase in the difference between the average fluid temperature and the inner wall of the vessel.
Heat transfer between the fluid and the wall is further complicated because viscosity influences whether the flow is turbulent or laminar. Turbulent flow is when the fluid mixes within the pipe. Laminar flow is when the fluid moves in concentric layers, not mixing water near the pipe wall with water in the center of the pipe. Because the glycol is more viscous it will require higher velocities to create turbulent flow. Intuitively, it takes more energy to mix a thick fluid than a thin one. We care because turbulent flow helps to transfer the heat. In half inch PEX at 125°F with .5 GPM, 50% glycol may be laminar while water is turbulent. Laminar flow reduces heat transfer by preserving greater temperature differences between fluid in the center and near the pipe wall. On the upside, laminar flow requires less pumping power than turbulent flow. Laminar flow can partially compensate for the increased pumping power required to circulate the more viscous fluid.
Reduced heating capacity is a result of glycol holding less heat per gallon than water does. From the definition of specific heat we know it takes one BTU to raise one pound of water 1°F. It takes about .88 BTU to raise one pound of 50% glycol mix one degree. This means that when one pound of 50% glycol mix cools off one degree it only gives up 88% as much heat as a pound of water. Now let us consider gallons. At 100°F it takes 8.3 BTU to raise one gallon of water one degree, whereas it takes 7.48 BTU to raise one gallon of 50% glycol mix one degree. For every gallon pumped less heat is delivered. The 50% glycol mix carries 90% as much heat per gallon as water does. If you think of the piping system as a conveyor belt moving heat then using a 50% glycol mix is like reducing the bucket size by 10%. Assume we have designed a system to operate at 10 GPM with a 20°F delta T (20 °F drop between supply and return) using water. Because of the glycol mix’s reduced heating capacity we will either have to increase the GPM to 11 gallons or increase the delta T to 22°F to transfer the same amount of heat. What do you know: 10% more GPM or 10% more delta T or some combination of the two is required to make up for the lower specific heat of a 50% glycol mix. This, in itself, is not a deal breaker for glycol but when we combine the reduced heating capacity of glycol mixes with the next issue of increased viscosity, the plot thickens.
Glycol is more viscous than water. This means glycol mixes are thicker than water. We would not care but thicker fluids are harder to circulate. At -20°F a 50% glycol mix has a viscosity of .156 Pa s (Pascal-seconds) while water at 180°F has a viscosity of .0004 Pa s. That is 390 times more viscous. Of course, water is frozen at -20°F so I will have to cede that battle to glycol. The viscosity of water varies between .0017 around 32°F and .0004 Pa s at 210°F. While a 50% glycol mix ranges between .4744 Pa s at -40°F and .0006 Pa s at 210°F. In short, hot glycol flows pretty much like water, cold glycol flows like something else. The viscosity of olive oil at room temperature is about the same as 50% glycol at -5°F, around .08 Pa s. From this we can see that a potential pumping problem can occur, especially if we are pumping cold glycol.
Viscosity and heating capacity effect how glycol is pumped. There are three issues to consider when pumping glycol mixes instead of water. First, as noted above, the reduced heating capacity means more GPM will be required to deliver the same number of BTU’s. Second, the greater viscosity will increase the head or pressure needed to push the thicker HTF through the piping. Third, the viscosity can affect the performance of the pump itself by reducing available head, flow rates and efficiency. The issues of reduced heating capacity and viscosity compound one another.
Consider a radiant floor containing ten 250’ loops of ½” PEX in parallel flowing at 10 GPM with an average fluid temperature of 100°F and a 20°F delta T. How do we compensate if we replace the water with a 50% glycol mix? The first issue, reduced heating capacity, will necessitate a 10% increase in flow. The second issue, head loss through the piping will increase from 10.2’ to 14.8’, or 45% at the original 10 GPM. Combining issues 1 and 2 we increase the flow 10% to 11 GPM, making up for the reduced heating capacity, the head loss with glycol then jumps to 17.5’ or a 72% increase. The third issue, de-rating the pump for the viscosity of the glycol (about 36 SSU) is negligible for this example but could become an issue at colder temperatures and/or greater concentrations of glycol. So to compensate for the glycol, a pump capable of 11 GPM at 15.1’ of head would be required. 10 For more information about pumping viscous fluids see: the Grundfos Technical Guide (HVAC), www.lightmypump.com and www.mcnallyinstitute.com.
The whole reason we use propylene glycol instead of other freeze resistant fluids like ethylene glycol (found in car radiators) is safety. Many heating and solar systems have connections or potential connections to the potable water supply. Heat exchangers immersed in domestic hot water and system fill valves are very common. We do not want to poison ourselves or any pets if there is a leak, cross connection or someone looking for water opens a boiler drain. Propylene Glycol is both non-toxic and biodegradable. It is even used as a food additive. For the environmentally conscious there are bio-glycols available, made from domestic corn, and possessing superior viscosity characteristics. The only thing we need to watch out for is the additives which usually amount to about 5% of the mix. In particular some aluminum safe glycols are not considered non-toxic. Check the labels and/or MSDS sheets on the products you use.
With glycol there are three levels of freezing. Glycol manufacturers typically provide a freeze protection chart for their products. These charts list values for the freeze, fluid flow and burst protection points of various concentrations of glycol. Glycol products come premixed with varying proportions, generally ranging in concentration from about 45% to 95% of propylene glycol. Even different products from the same manufacturer can vary significantly in glycol concentration. When choosing a glycol concentration, manufacturers often recommend that a 5°F safety margin be incorporated. The chemistry of the water added to dilute the glycol mix may affect the freeze protection values slightly.
So why have three categories of freeze protection? With water, freezing is freezing, but with glycol the freeze point is only the beginning. The freeze point is when the first ice crystals form in the fluid. Down to the flow point ice crystals are present in the fluid but it will still flow. The dreaded burst point is when the fluid becomes a solid, expanding and bursting the vessel. As ice crystals form the concentration of glycol in the remaining fluid increases because the water freezes first. In simple heat systems glycol is added to provide burst protection during unexpected shut downs or mechanical failure. The flow point can be an issue in snow melt and solar installations. Clearly, the HTF will have to flow at start up for a snow melt or solar system to function. For this to occur, the outdoor snowmelt or attic solar piping will have to be warmer than the flow point of the HTF.
As John Siegenthaler says, “The only good thing about anti-freeze is that it doesn’t freeze”. We have seen that the correct use of glycol is not a slam dunk. If you are going to use glycol here are some things to watch out for:
1. Choose a glycol with the right additives for the job at hand. Consider the expected temperature range, materials in the system and possible toxicity.
2. Don’t mix glycol with hard water, chlorine or residual flux left in the pipes.
3.Size the expansion tank(s) for the HTF you are using, glycol expands more than water. For solar systems consider the possibility of vaporization.
4.Keep in mind that the glycol mix will not transfer heat or pump at the same GPM water would.
It may well be that glycol systems have been given a bad rap simply because installers were not aware of the limitations involved. Under the right conditions with the appropriate glycol mix the fluid can go ten years or even longer without problems. Of course, glycol mixes should be checked and maintained regularly. When I asked a manufacturer how long their glycol would last all I was told is that it has a two year shelf life.
If you are a heating/solar professional you most likely know from experience that common hydronic systems work just fine if you mix in glycol to protect against freezing, not that big a deal. Now we know why and what to watch out for. In modern solar thermal systems, HTF’s are often subjected to extreme temperatures requiring designers and installers to be better informed. I hope the above information will help us to install glycol correctly and with confidence.
4 http//: www.infrastructure.alberta.ca
8 M. J. Assael, E. Charitidou, S. Avgoustiniatos, W A. Wakeham, Absolute Measurements of the Thermal Conductivity of Mixtures of Alkene-Glycols with Water, International Journal of Thremophysics, Vol. 10, N0. 6, 1989, p. 1127-1139.