{"id":20060,"date":"2026-11-06T15:26:29","date_gmt":"2026-11-06T15:26:29","guid":{"rendered":"https:\/\/az-broquetas.es\/glycol-in-solar-thermal-systems-how-to-control-its-degradation-and-protect-the-circuit"},"modified":"2026-11-06T15:26:35","modified_gmt":"2026-11-06T15:26:35","slug":"glycol-in-solar-thermal-systems-how-to-control-its-degradation-and-protect-the-circuit","status":"publish","type":"post","link":"https:\/\/az-broquetas.es\/en\/glycol-in-solar-thermal-systems-how-to-control-its-degradation-and-protect-the-circuit","title":{"rendered":"Glycol in Solar Thermal Systems: How to Control Its Degradation and Protect the Circuit"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">In a solar thermal system, <strong>glycol should not be treated as a consumable that you simply fill up and forget about<\/strong>. Its formulation, its exposure to high temperatures, and the maintenance performed throughout the system\u2019s lifespan all determine both its antifreeze capacity and the protection it provides to metals.<\/p>\n\n<p class=\"wp-block-paragraph\">A new solar fluid, when properly treated and used under its specified operating conditions, can be compatible with copper, aluminum, stainless steel, brass, elastomers, and other materials. Problems arise when the product is improperly selected, mixed, or diluted without authorization; when it degrades due to excessive heat; or when it loses the ability to maintain the circuit\u2019s chemical balance.<\/p>\n\n<p class=\"wp-block-paragraph\">This article explains what happens in the fluid, what the installer should check, and how to relate the results of the analysis to a maintenance decision. The piping matters, but <strong>no material can make out-of-specification glycol acceptable<\/strong>.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Quick Reference for Installers<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>There is no universal interval for changing the glycol<\/strong>, nor <strong>is<\/strong> there a single pH value that applies to all products. The evaluation must be based on the data sheet for the installed fluid and its service history.<\/p>\n\n<p class=\"wp-block-paragraph\">At a minimum, it is advisable to identify the product, measure its concentration or freeze protection using the specified method, compare the pH to its specific range, check the alkaline reserve when the manufacturer uses it as a criterion, and check for turbidity, precipitates, or contamination. The history of stagnation, air ingress, leaks, and replenishments should also be reviewed.<\/p>\n\n<p class=\"wp-block-paragraph\">If the fluid is out of specification, it must be corrected or replaced in accordance with the manufacturer\u2019s instructions. <strong>Changing the piping material does not resolve the issue of a degraded fluid<\/strong>.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>What is the function of glycol in a solar system?<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">In many closed-loop solar systems, a formulated fluid based on propylene glycol or ethylene glycol is used to prevent freeze damage. <strong>Unformulated technical glycol should not be used, nor should mixtures be prepared based on guesswork<\/strong>: heat transfer fluids contain corrosion inhibitors, stabilizers, and other additives selected for a specific application and specific materials.<\/p>\n\n<p class=\"wp-block-paragraph\">Concentration affects:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>The crystallization onset temperature.<\/li>\n\n\n\n<li>Protection against freeze-induced breakage.<\/li>\n\n\n\n<li>Viscosity and, therefore, pressure drop.<\/li>\n\n\n\n<li>Density and heat capacity.<\/li>\n\n\n\n<li>The behavior of the mixture during vaporization.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Therefore, a rule such as \u201c30% provides protection up to a certain temperature\u201d cannot be applied to every product. The installer must use the manufacturer\u2019s curve and determine which parameter is being measured: freezing point, ice crystal formation, or protection against bursting.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Propylene glycol and ethylene glycol are not interchangeable<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Propylene glycol-based fluids are frequently used when lower acute oral toxicity is a priority or there is a possibility of accidental contact with drinking water. This does not mean that any formulation is harmless: inhibitors, dyes, and other additives must also be considered, as well as the safety data sheet.<\/p>\n\n<p class=\"wp-block-paragraph\">Ethylene glycol has different heat transfer and viscosity properties and is used when the application and risk assessment allow it. <strong>Two products should not be mixed simply because they share the word \u201cglycol.\u201d<\/strong> The chemical base, inhibitor package, concentration, and authorized application must match the system documentation.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Stagnation: the condition that most severely affects solar fluid<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">Stagnation occurs when the collector receives radiation but <strong>the circuit does not dissipate the heat<\/strong>. The temperature rises, the fluid begins to vaporize, and some of the liquid is displaced out of the collector.<\/p>\n\n<p class=\"wp-block-paragraph\">In a system with good drainage performance, the steam expels most of the liquid toward cooler areas and into the available expansion volume. If small amounts remain trapped in the receiver, the water may evaporate preferentially, the glycol becomes concentrated, and the residue remains subjected to intense thermal stress. Repeated occurrences of this phenomenon accelerate the depletion of additives and can lead to precipitates, flocculation, or solid residues.<\/p>\n\n<p class=\"wp-block-paragraph\">The fact that glycol raises the mixture\u2019s boiling point does not mean that it improves performance during stagnation on its own. <strong>Protection comes from the system\u2019s design<\/strong>: collector drainage, vapor path, heat dissipation in the pipes, pressure, expansion tank, and the fluid\u2019s thermal limits.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Expansion tank and intermediate protection tank<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">The diaphragm expansion tank must be sized to accommodate the expansion of the fluid and the volume displaced during vaporization. In solar thermal systems, it is not sufficient to simply apply the criteria used in a conventional heating system.<\/p>\n\n<p class=\"wp-block-paragraph\">It is also necessary to determine how far the steam or high-temperature fluid can travel. If the calculation and circuit layout indicate that the thermal load could reach the expansion tank, it may be necessary to install an intermediate or protective tank upstream of it. Its function is to keep excessively hot fluid away from the diaphragm and reduce its thermal degradation.<\/p>\n\n<p class=\"wp-block-paragraph\"><strong>It is not an accessory that should be automatically added to all installations<\/strong>. Whether it is needed depends on the steam capacity, the drainage characteristics, the length and heat dissipation of the pipes, the position of the vessel, and the maximum temperature the membrane can withstand.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>How Glycol Degrades Chemically<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>The key process in a solar circuit is thermo-oxidative degradation<\/strong>. Temperature and oxygen promote reactions that progressively transform glycol into products with lower molecular weights, including organic acids.<\/p>\n\n<p class=\"wp-block-paragraph\">Experimental studies using ion chromatography identified differences between the two most common bases:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>In ethylene glycol solutions, the main acid identified was glycolic acid. Depending on the conditions, formic, acetic, glyoxylic, and oxalic acids may also be present.<\/li>\n\n\n\n<li>In propylene glycol solutions, the primary acid identified was lactic acid, accompanied by other acids depending on temperature, oxygen, and the materials present.<\/li>\n\n\n\n<li>The formation of these products depletes the fluid\u2019s buffering capacity and can lower the pH. Inhibitors are designed to delay corrosion and control this process, but their effectiveness is not unlimited.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\"><strong>The factors that accelerate the process are:<\/strong><\/p>\n\n<ul class=\"wp-block-list\">\n<li>High temperatures or repeated stagnation.<\/li>\n\n\n\n<li>Oxygen ingress due to leaks, purges, or refills.<\/li>\n\n\n\n<li>Dilution or mixing of incompatible formulations.<\/li>\n\n\n\n<li>Water that does not meet the manufacturer\u2019s requirements.<\/li>\n\n\n\n<li>Residues of flux, cleaners, oils, or particles.<\/li>\n\n\n\n<li>Areas with trapped fluid and localized overheating.<\/li>\n\n\n\n<li>Progressive loss of the inhibitor package.<\/li>\n<\/ul>\n\n<h2 class=\"wp-block-heading\"><strong>What is the relationship between copper and glycol degradation?<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>New, properly inhibited glycol may be compatible with copper<\/strong>. However, copper is not a chemically inert bystander when the fluid undergoes thermal oxidation.<\/p>\n\n<p class=\"wp-block-paragraph\">Studies published by Rossiter et al. showed that the presence of metallic copper increased the amount of degradation products in aqueous solutions of ethylene glycol and propylene glycol. Other studies from the same program described a catalytic effect of copper on the oxidative degradation of ethylene glycol. The acids produced can subsequently react with copper to form copper salts.<\/p>\n\n<p class=\"wp-block-paragraph\"><strong>This does not mean that glycol will inevitably corrode copper piping or that copper should be ruled out<\/strong>. It means that the fluid intended for use in a copper circuit must be formulated and tested to protect it, and that the loss of the inhibitor system can simultaneously affect both the fluid and the metal surface.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Copper-specific inhibitors<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">Anti-corrosion packages may contain organic inhibitors capable of adsorbing onto copper and limiting its dissolution. Among the compounds studied for this purpose are azoles, such as benzotriazole and tolyltriazole, which can form a protective film on the surface.<\/p>\n\n<p class=\"wp-block-paragraph\">It should not be assumed that all solar fluids contain these compounds, nor should their concentration be inferred from the trade name. Formulations are often proprietary and may use different technologies. The guiding principle is that the manufacturer should declare compatibility with copper and, preferably, provide test results for multi-metal systems.<\/p>\n\n<p class=\"wp-block-paragraph\"><strong>If the documentation does not expressly mention copper, it should not be assumed that the product protects it<\/strong>.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>And what role does stainless steel play?<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>Austenitic stainless steels offer good resistance to many fluids, but they are not immune<\/strong> to corrosion. Chlorides, temperature, deposits, crevices, surface contamination, or a harsh outdoor environment can cause localized corrosion.<\/p>\n\n<p class=\"wp-block-paragraph\">The molybdenum in AISI 316L improves its resistance to pitting corrosion compared to AISI 304, but this does not mean it can be used with degraded glycol, nor does it automatically reduce the frequency of fluid maintenance. Compatibility must be verified for the specific pipe grade, gaskets, fittings, and heat transfer fluid.<\/p>\n\n<p class=\"wp-block-paragraph\">In a mixed-media system, the question is not which metal \u201cwins,\u201d but whether the fluid has been tested against all the materials present and whether the design limits the conditions that compromise their protection.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Aluminum: The Material That Can Narrow the Compatibility Window<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">Aluminum and its alloys are found in certain absorbers, heat exchangers, pumps, and other components. Their performance depends on the alloy, temperature, pH, chloride content, and inhibitor package. Therefore, <strong>compatibility should not be assumed simply because the fluid is suitable for copper or stainless steel<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\">In a system containing aluminum, the coolant manufacturer must expressly state that its formulation protects the aluminum and, when possible, provide a corrosion test that includes the corresponding alloy. There are commercial solar fluids that incorporate cast aluminum in their tests, but that result pertains to the specific formulation tested and cannot be generalized to any glycol mixture.<\/p>\n\n<p class=\"wp-block-paragraph\">The operating range is not determined solely by the fluid\u2019s pH. Both the range specified by the fluid manufacturer and the limits of all wetted components must be met simultaneously. In a multimetal circuit, the valid operating range is determined by the most restrictive material or component. This is particularly relevant for aluminum, because deviations toward excessively acidic or alkaline conditions can compromise its protective film.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Gaskets, elastomers, and other nonmetallic components<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>Some of the issues attributed to \u201cglycol\u201d or the piping actually originate at a joint<\/strong>. The glycol-based solution, concentration, inhibitors, temperature, and exposure time all affect the swelling, hardening, loss of elasticity, and permeability of elastomers.<\/p>\n\n<p class=\"wp-block-paragraph\">It is not enough to simply verify that a gasket is generically EPDM, FKM, NBR, or silicone. Compatibility must be verified for the specific compound, because materials within the same chemical family may use different formulations and fillers. Sealants, adhesives, plastics, membranes, and coatings must also be verified.<\/p>\n\n<p class=\"wp-block-paragraph\">The fluid data sheet and the component data sheet must match in terms of concentration and operating temperature. If a leak occurs after a replacement, a product mix, or a period of stagnation, it is advisable to inspect the seal and confirm its compatibility before concluding that there is a pipe failure.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Alkaline reserve: what it measures and what it doesn&#8217;t measure<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">Alkaline reserve indicates a fluid&#8217;s ability to neutralize an added acid before reaching a reference pH. <strong>It provides information that differs from the instantaneous pH<\/strong>: two samples may have similar pH values but different remaining buffer capacities.<\/p>\n\n<p class=\"wp-block-paragraph\">ASTM D1121 defines a method for coolants and corrosion inhibitors based on titrating a sample with 0.100 N hydrochloric acid to a pH of 5.5. The result is expressed as the number of milliliters of acid required to titrate 10 mL of the sample to that endpoint.<\/p>\n\n<p class=\"wp-block-paragraph\">There are two important precautions:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>ASTM D1121 was developed for engine coolants. It should only be applied to a solar fluid when the manufacturer uses or accepts it as a method.<\/li>\n\n\n\n<li>The standard itself notes that a higher alkaline reserve does not, by itself, indicate greater corrosion protection nor does it allow for a direct calculation of remaining service life. Some effective inhibitors contribute little to the assessment.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Therefore, alkaline reserve is a variable used for trend analysis and quality control. It should be interpreted in conjunction with the product\u2019s pH, concentration, contamination, appearance, and specifications.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Fluid Diagnostic Protocol<\/strong><\/h2>\n\n<h3 class=\"wp-block-heading\"><strong>Identification and Traceability<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\"><strong>Before taking measurements, you must know what product is in the system<\/strong>. Relevant information includes the manufacturer, part number, glycol base, supply concentration, lot number, fill date, and any subsequent refills. When the product is unknown or different formulations have been mixed, the results lose their diagnostic value.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Frost Protection<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Measurements using a refractometer must be taken using a scale or conversion factor appropriate for the product. A reading calibrated for ethylene glycol may be misinterpreted when used with propylene glycol. The result must be compared with the manufacturer\u2019s curve and the minimum design temperature.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>pH<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">The pH is compared to the specified range for the fluid and to the indicated test method. <strong>There is no universal pH 7 limit applicable to all formulations<\/strong>. A decrease from the initial value may indicate a loss of buffering capacity, but the decision should not be based on a single figure.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Alkaline reserve<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">When the manufacturer includes it among its tests, it allows for the evaluation of changes in neutralizing capacity. It is particularly useful as a time series: new sample, initial testing, and subsequent tests conducted using the same method.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Visual Inspection and Solids<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Turbidity, sediment, flocs, precipitates, phase separation, and abnormal changes in appearance must be checked. <strong>Color alone does not prove that the product is acceptable<\/strong>. A sample that appears clean may have chemical properties that are out of specification.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Thermal and Operational History<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">It is advisable to record blockages, overheating, safety valve openings, pressure drops, air ingress, and replacements. A system with repeated incidents may require an early analysis, even if the fluid has been in service for only a short time.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Sampling<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">The sample must be representative of the system and taken while the system is in a safe operating condition. The container must be clean and compatible with the product. The sampling point, temperature, date, and operating status must be recorded, especially when the results are to be compared with previous tests.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Sampling and Replenishment: Errors That Affect the Diagnosis<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">Before filling, the system must be clean and free of incompatible substances. Residues from soldering, fluxes, oils, cleaners, or particles can contaminate the fluid from the moment the system is put into service.<\/p>\n\n<p class=\"wp-block-paragraph\">Two products should not be mixed simply because they are both propylene glycol-based. The inhibitor packages may be different or incompatible. Water should also not be added automatically:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>A concentrate must be prepared using the specified water quality and proportions.<\/li>\n\n\n\n<li>A premixed product may prohibit any dilution.<\/li>\n\n\n\n<li>If the manufacturer requires topping off with the same product, adding water alters the antifreeze protection and the chemistry of the system.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Each intervention must include a record of the product, lot number, quantity, measured concentration, and reason. Without this record, it is difficult to distinguish between normal aging, dilution, contamination, or accidental mixing.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>When Should the Fluid Be Replaced?<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>There is no universal three- or five-year period<\/strong>. Service life depends on the formulation, temperature, oxygen, contamination, replacements, and the design\u2019s resistance to stagnation.<\/p>\n\n<p class=\"wp-block-paragraph\">Replacement should be considered when:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>The manufacturer specifies this in accordance with its maintenance schedule.<\/li>\n\n\n\n<li>Analysis confirms a property that is out of specification.<\/li>\n\n\n\n<li>There is contamination or a mixture of unknown origin.<\/li>\n\n\n\n<li>Precipitates appear or severe thermal degradation occurs.<\/li>\n\n\n\n<li>If freeze protection cannot be restored through an authorized procedure.<\/li>\n\n\n\n<li>The inhibitor package has lost its effectiveness according to the product\u2019s criteria.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">Before refilling, the cause must be identified. If poor circulation, air ingress, inadequate water quality, or an improperly sized tank persist, the new fluid will deteriorate again.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Intervention Checklist<\/strong><\/h2>\n\n<h3 class=\"wp-block-heading\"><strong>Before filling or replacing<\/strong><\/h3>\n\n<ul class=\"wp-block-list\">\n<li>Identify the product approved for solar thermal systems.<\/li>\n\n\n\n<li>Verify its compatibility with all metals\u2014including aluminum, when present\u2014polymers, elastomers, and gaskets.<\/li>\n\n\n\n<li>Confirm whether it is a concentrate or ready-to-use.<\/li>\n\n\n\n<li>Calculate freeze protection using its own curve.<\/li>\n\n\n\n<li>Check the collector\u2019s stagnation temperature.<\/li>\n\n\n\n<li>Evaluate the drainage behavior and the steam path.<\/li>\n\n\n\n<li>Size the expansion tank for liquid and vapor volume.<\/li>\n\n\n\n<li>Determine whether an intermediate protection tank is needed.<\/li>\n\n\n\n<li>Flush the circuit using a compatible procedure.<\/li>\n\n\n\n<li>Record the product, lot, quantity, and initial measurements.<\/li>\n<\/ul>\n\n<h3 class=\"wp-block-heading\"><strong>During maintenance<\/strong><\/h3>\n\n<ul class=\"wp-block-list\">\n<li>Measure frost protection using the correct method.<\/li>\n\n\n\n<li>Compare the pH to the specified range.<\/li>\n\n\n\n<li>Monitor the alkaline reserve when appropriate.<\/li>\n\n\n\n<li>Check for turbidity, solids, precipitates, and contamination.<\/li>\n\n\n\n<li>Check for leaks, air ingress, and top-offs.<\/li>\n\n\n\n<li>Record instances of stagnation.<\/li>\n\n\n\n<li>Do not mix or dilute without documented authorization.<\/li>\n\n\n\n<li>Request an analysis from the manufacturer or a laboratory when the result is questionable.<\/li>\n<\/ul>\n\n<h2 class=\"wp-block-heading\"><strong>Where Does the CATS System Fit into a Solar Installation?<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\">AZ Broquetas offers a CATS configuration specifically designed for solar thermal systems based on corrugated stainless steel tubing. The product is supplied in double-tube coils with insulation and a cable for installing the temperature sensor. According to AZ\u2019s public information, one end is crimped and finished with two female nuts.<\/p>\n\n<p class=\"wp-block-paragraph\">The PES fiber lining incorporates a protective film that shields against ultraviolet radiation and mechanical wear. AZ specifies a continuous operating temperature of up to 200 \u00b0C for this configuration. This value applies to the assembly and insulation as described by the manufacturer; it does not represent the temperature at which any glycol-based fluid can remain stable indefinitely.<\/p>\n\n<p class=\"wp-block-paragraph\">The published solar product line includes:<\/p>\n\n<ul class=\"wp-block-list\">\n<li>Fittings that can be crimped on-site using the CATS 120 hand crimping tool.<\/li>\n\n\n\n<li>C-Klip system based on O-rings, a clamping ring, and a nut.<\/li>\n\n\n\n<li>Transition fittings between corrugated tubing and copper.<\/li>\n\n\n\n<li>Klingersil C4400 gaskets specified by AZ for the temperatures and fluids specific to the application.<\/li>\n\n\n\n<li>Mounting kit with sleeve, screw, washer, nut, cap, and expandable sleeve.<\/li>\n\n\n\n<li>CATS 120 press for diameters DN8, DN12, DN15, DN20, and DN25.<\/li>\n<\/ul>\n\n<p class=\"wp-block-paragraph\">For each project, the current specification, the exact grade of steel, the joints, the pressure and temperature conditions, and compatibility with the selected commercial fluid must be confirmed. <strong>The fact that the piping is made of stainless steel does not eliminate the need for this verification<\/strong>.<\/p>\n\n<p class=\"wp-block-paragraph\"><strong><a href=\"https:\/\/az-broquetas.es\/instalacion-solar-termica\">CATS Solar System Information<\/a><\/strong><\/p>\n\n<p class=\"wp-block-paragraph\"><strong><a href=\"https:\/\/az-broquetas.es\/caracteristicas-tubo-corrugado-inox\">General Characteristics of the Corrugated Pipe<\/a><\/strong><\/p>\n\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/az-broquetas.es\/tuberia-aerotermia-solar-termica-criterios\" type=\"link\" id=\"https:\/\/az-broquetas.es\/tuberia-aerotermia-solar-termica-criterios\">Terms of Service for Piping in Air-Source Heat Pump and Solar Systems<\/a><\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Conclusion<\/strong><\/h2>\n\n<p class=\"wp-block-paragraph\"><strong>The reliability of a solar circuit is not determined solely by the choice between copper and stainless steel<\/strong>. It is determined by selecting a compatible fluid, controlling the thermal load during periods of inactivity, and maintaining traceability for each fill and refill.<\/p>\n\n<p class=\"wp-block-paragraph\">Chemistry explains why. Oxygen and temperature can break down glycol into acidic byproducts, deplete the buffering capacity, and reduce the effectiveness of the inhibitor system. The presence of copper can accelerate these reactions under certain conditions, but fluids formulated for copper incorporate specific protection. Stainless steel offers advantages over certain mechanisms, but it also cannot tolerate a degraded fluid without consequences.<\/p>\n\n<p class=\"wp-block-paragraph\"><strong>The professional approach involves measuring, comparing the results to the product specifications, and correcting the cause of any deviation<\/strong>. This protocol provides better protection for the installation than a fixed replacement schedule or a blanket statement about the material.<\/p>\n\n<h2 class=\"wp-block-heading\"><strong>Frequently Asked Questions<\/strong><\/h2>\n\n<h3 class=\"wp-block-heading\"><strong>Does glycol damage copper pipes?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Not necessarily. A properly formulated solar fluid can be tested for copper and other metals. Studies show, however, that the presence of copper can increase the thermo-oxidative degradation of glycol under certain conditions. Therefore, the product must include an appropriate inhibitor system and remain within specification. If corrosion or copper salts appear, the temperature, oxygen, water, assembly residues, and the condition of the fluid should also be investigated.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Which inhibitors protect copper in a glycol-based system?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">There are various technologies. Among the inhibitors studied for copper are azoles, such as benzotriazole and tolyltriazole, which are capable of forming a protective film on the metal surface. However, not all products use the same compounds, and their formulations may be confidential. The installer does not need to know the complete formulation, but must require that the manufacturer declare compatibility with copper and with the other metals present in the circuit.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>What acids are formed when glycol breaks down?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Thermo-oxidative degradation can produce various organic acids. In ethylene glycol, glycolic acid has been identified as the primary product, along with formic, acetic, glyoxylic, and oxalic acids, depending on the conditions. In propylene glycol, the main product reported is lactic acid, accompanied by other acidic products. Their formation depletes the buffering capacity and can lower the pH. The actual composition depends on temperature, oxygen, time, and the materials present.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>What should be checked if the circuit contains aluminum?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">It must be confirmed that the commercial fluid is specifically formulated and tested for aluminum or for the specific alloy present. Temperature, chloride content, concentration, and pH range must also be checked. In a multimetal system, it is not enough for the fluid to be compatible with copper or stainless steel: the operating window must meet the requirements of the most restrictive component. A generic inhibitor must not be added, nor should the pH be adjusted, without the manufacturer\u2019s authorization.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Can glycol damage gaskets and elastomers?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Compatibility depends on the product, concentration, temperature, and the exact composition of the seal. An inappropriate formulation can cause swelling, hardening, loss of elasticity, or permeability, especially after exposure to excessive temperatures or mixing. The designation EPDM, FKM, NBR, or silicone is not sufficient to validate a specific seal. The fluid and component data sheets must be consulted to verify that they match in terms of concentration and operating temperature.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>What does the fluid&#8217;s alkaline reserve indicate?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">It indicates the formulation\u2019s ability to neutralize an acid load before reaching a defined endpoint. ASTM D1121 uses a titration with hydrochloric acid up to a pH of 5.5, but this method is intended for engine coolants and should only be applied to solar fluid if the manufacturer specifies it. Furthermore, the standard itself notes that this value alone does not measure corrosion protection or remaining service life. It must be interpreted in conjunction with other variables.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>How often should the solar glycol be replaced?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">There is no universal interval. The decision depends on the formulation, the temperatures reached, periods of stagnation, oxygen ingress, top-offs, and analytical results. Monitoring should follow the fluid manufacturer\u2019s plan. Replacement is warranted when a relevant property falls outside specifications, contamination is present, or degradation is confirmed that cannot be remedied through an authorized procedure. The cause must be corrected before refilling.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>When is an intermediate protective vessel needed?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">It must be determined whether the steam or displaced fluid during stagnation could reach the expansion tank at a temperature higher than that permitted by its membrane. The intermediate vessel creates a thermal separation volume and protects the sensitive component. It is not mandatory by definition in every installation: whether it is needed depends on the drainage behavior, the steam load, heat dissipation from the piping, the position of the vessel, and the manufacturer\u2019s specifications.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Can water be added to a solar loop containing glycol?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">Only if the product data sheet permits it and using the specified water quality. Concentrates require controlled preparation; ready-to-use fluids may prohibit any dilution. Adding water alters the freeze protection, the concentration of inhibitors, and the chemistry of the system. When the manufacturer requires replenishment with the same product, that condition must be followed. All replenishments must be recorded to maintain traceability.<\/p>\n\n<h3 class=\"wp-block-heading\"><strong>Does stainless steel help reduce glycol maintenance?<\/strong><\/h3>\n\n<p class=\"wp-block-paragraph\">No. Choosing stainless steel may provide resistance to certain types of corrosion, but the fluid is still subject to degradation due to temperature and oxygen. Chlorides, deposits, gaskets, and fittings must also be checked. The testing protocol depends on the fluid and operating conditions, not on whether the piping is made of copper or stainless steel. Glycol that is out of specification must be corrected or replaced in both cases.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>In a solar thermal system, glycol should not be treated as a consumable that you simply fill up and forget about. Its formulation, its exposure to high temperatures, and the maintenance performed throughout the system\u2019s lifespan all determine both its antifreeze capacity and the protection it provides to metals. A new solar fluid, when properly&hellip;<\/p>\n","protected":false},"author":1,"featured_media":20050,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[251],"tags":[],"class_list":["post-20060","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-stainless-steel-corrugated-tube-cats-system","category-251","description-off"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v26.9 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Solar Thermal Glycol: Degradation, Analysis, and Compatibility - AZ Broquetas<\/title>\n<meta name=\"description\" content=\"How glycol in a solar system degrades, what the installer should test for, and how to verify its compatibility with copper and stainless steel.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link 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