Antifreeze for solar systems – how to choose the right composition and concentration
Antifreeze for solar systems – how to choose the right composition and concentration
Antifreeze is one of the most important, yet most frequently underestimated components of the entire solar system. While collectors, the storage tank, pump groups and controls are things that both the installer and the customer address intensively, antifreeze stays somewhere in the background – "it's just some liquid, you can buy it anywhere, right?" This underestimation has led to hundreds of premature failures, corroded heat exchangers, clogged pipes and broken solar collectors. From experience, I know that a mistake in choosing or concentrating antifreeze is one of the top three causes of unnecessary service calls in the first two years of operation.
In this article, we will look at the topic comprehensively – from basic chemistry, through choosing the right type and concentration, to specific real-life situations that show why one seemingly small mistake can destroy several thousand euros invested in the system.
Why does the solar circuit need a special antifreeze at all
The first and most natural question most customers ask: "Can I use regular automotive antifreeze?" The answer is clear – no. And there are several reasons, each of which is serious enough on its own.
The solar circuit operates under extremely different conditions than a car's cooling circuit. While in a car the liquid temperature rarely exceeds 110 °C, in a solar collector the stagnation temperature can reach 180–220 °C for flat-plate collectors and even higher values for vacuum tubes. Standard monoethylene glycol with automotive additives starts to thermally degrade at such temperatures – inhibitors break down, acids are formed (mainly glycolic and oxalic acid), which further corrode copper, brass and aluminium.
Besides temperature resistance, the material environment is also a key difference. The solar circuit is typically a combination of copper (collectors, piping), brass (fittings, ball valves), aluminium (some collectors and mounting frames) and stainless steel (storage tanks, heat exchangers). Automotive fluids are formulated for different metal pairings, and their inhibitor package simply does not provide sufficient protection across all these combinations at once.
Types of antifreeze – glycol bases and their differences
There are basically two main types of glycol bases used in solar systems on the market: monoethylene glycol (MEG) and monopropylene glycol (MPG). Each has different properties and its own justification.
Monoethylene glycol (MEG)
Monoethylene glycol is a historically older and technically better-mapped base. It is characterised by lower viscosity at low temperatures, which makes it a more advantageous choice in terms of hydraulic resistance in the circuit. Lower viscosity at freezing temperatures means the pump does not have to overcome as much resistance during a nighttime system start in winter, which saves energy and reduces wear.
The disadvantage of MEG is its toxicity – it is not suitable for applications where contact with drinking water is possible. For closed solar circuits with a double heat exchanger (which in practice is the vast majority of correctly designed systems), this is not a problem. Nevertheless, some local regulations or insurance conditions may require the use of a less toxic medium, so it is worth checking this in advance.
Monopropylene glycol (MPG)
Monopropylene glycol is significantly less toxic – in the event of a leak and contamination of drinking water, it does not pose an acute health risk. It is therefore mandatory in systems with a single heat exchanger directly in the drinking water tank (a less common solution, but it does occur). In some countries (e.g. Germany), MPG is directly prescribed by standard for such applications.
The disadvantage of MPG is higher viscosity at low temperatures – at –15 to –20 °C the hydraulic resistance can be significantly higher than with the same concentration of MEG. This sometimes requires a more powerful pump or redesign of the entire hydraulic system.
Inhibitors – the heart of every solar fluid
Glycol alone, without inhibitors, would be a disaster for the solar circuit. Inhibitors are chemical additives that prevent metal corrosion, foaming of the liquid, biological growth (bacteria, algae) and degradation of the glycol due to heat. The quality and composition of the inhibitor package determines whether the solar fluid lasts in the system for 3 years or 8–10 years.
The following classes of inhibitors are commonly used in solar applications:
- Silicates – traditional, relatively cheap, form a protective layer on the metal. Disadvantage: they deplete faster and can cause deposits at higher temperatures.
- Carboxylate-based (OAT – Organic Acid Technology) – a modern approach, protection works by adsorption on the metal surface, longer lifespan, fewer deposits. Standard choice for premium solar fluids.
- Combined (HOAT – Hybrid OAT) – a combination of silicates and organic acids, a good compromise between quick initial protection and long service life.
- Special silicates and azoles – protect copper and brass against corrosion; an essential part of any well-formulated solar fluid.
Practical consequence: never buy antifreeze just because it is "propylene-glycol based and eco-friendly". That alone is not enough. Always ask about certification for solar applications and the declared temperature resistance, including resistance during stagnation.
Correct concentration – the key parameter most often estimated incorrectly
The concentration of the mixture (ratio of glycol to distilled water) directly determines the freezing point of the liquid. This is a parameter that most installers are aware of, but in practice it is very easily underestimated or overestimated – and both mistakes are costly.
Too low a concentration – risk of freezing
If the glycol concentration is too low, the liquid will freeze during extreme frosts. In mountainous areas of Slovakia (Orava, Tatras, Kysuce, Liptov), extreme minimums can reach as low as –32 to –38 °C. Under such conditions, 30% MEG, which only protects down to –15 °C, is not sufficient. Frozen liquid expands, breaks the piping, solder joints in the collector crack, and the entire collector ends up scrapped. This situation is not hypothetical – every winter with an extreme cold spell, cases of totally destroyed systems appear precisely because the minimum temperature was underestimated.
Too high a concentration – unnecessary loss of performance and aggressiveness
The opposite mistake – too high a glycol concentration (e.g. 60–70%) – is also incorrect. A high concentration means higher viscosity, lower specific heat capacity and reduced heat transfer. The system simply transfers energy from the collector to the storage tank less efficiently. In addition, some inhibitors become less stable at excessively high glycol proportions and salts may precipitate. The optimum is therefore to find the correct concentration according to the actual climatic conditions of the location.
Practical table of MEG concentrations for common Slovak climate zones
| Region / location type | Min. temperature (°C) | Recommended MEG conc. | Freezing point |
|---|---|---|---|
| Lowlands (Danube region, Záhorská lowland) | –18 to –22 °C | 30–35 % | –18 to –22 °C |
| Hilly areas, foothills (central Slovakia) | –22 to –28 °C | 35–40 % | –22 to –28 °C |
| Mountain areas (Orava, Tatras, Kysuce) | –28 to –38 °C | 42–48 % | –30 to –38 °C |
| High-altitude locations, north-facing slopes | below –35 °C | 50 % | around –40 °C |
Note: The freezing point should always be at least 5–10 °C lower than the historical minimum for the given location. A safety margin is not unnecessary – climate extremes are becoming increasingly unpredictable.
How to check the concentration and quality of the fluid in an existing system
If you are taking over an older system, or one for which there is no documentation of the last fluid change, there are several ways to determine the current condition.
Refractometer – quick field check
The most accessible tool for any installer. An optical refractometer measures the refractive index of the liquid and, based on that, displays the freezing point. Simply place a drop of liquid on the prism, close the cover glass and look through the eyepiece. An accuracy of ±1 to 2 °C is sufficient for field work. Digital refractometers are more accurate and less dependent on lighting, but optical models priced at €20–40 are more common in practice. Important: refractometers must be calibrated for the correct type of glycol – MEG and MPG use different scales.
pH test strip and degradation
In addition to concentration, also monitor the pH of the liquid. Fresh solar fluid typically has a pH of 7.5–9.0 (slightly alkaline). If the pH drops below 7.0, the liquid becomes acidic – the inhibitors are depleted and the glycol has degraded into organic acids. Such a liquid actively corrodes copper and brass. A pH test strip can be bought at any pool supply shop or online for a small amount, and it should not be missing from any annual system inspection.
Laboratory analysis
In case of doubt, when taking over a system without history, or after several years of operation without a fluid change, a complete laboratory analysis is recommended. Some solar fluid manufacturers offer it for a symbolic fee, or even free of charge when purchasing replacement fluid. The analysis will reveal not only the concentration and pH, but also the presence of corrosion products (iron, copper, aluminium) and the residual inhibitor content.
Preparing the mixture – distilled water is a requirement, not an option
One of the most common mistakes when filling the solar circuit: people dilute the concentrate with ordinary tap water. Even relatively soft tap water (hardness 1–3 mmol/l) contains calcium and magnesium, which precipitate as carbonates at temperatures above 60–70 °C and form limescale. In a solar collector, with its high temperatures, this process is drastically faster. Limescale on the walls of the absorber meander impairs heat transfer, narrows the cross-section, and can lead to local overheating and even destruction of the absorber.
Distilled or demineralised water is not a luxury – it is a basic requirement. It can be bought at any car service station, supermarket or chemical supplier for €0.50–1 per litre. For a 15–20 litre solar circuit, this is a negligible cost compared to the risks. If in doubt, measure the water's conductivity – for solar use it should be below 10 μS/cm, ideally below 5 μS/cm.
Circuit volume – how to calculate how much fluid you need
Before buying the fluid, you need to know the volume of the entire primary circuit. It consists of:
- Absorber volume in the collector(s) (stated by the manufacturer in the technical data sheet, typically 1.2–2.5 litres per collector)
- Volume of the flow and return piping (depends on length and diameter – for DN 18/1 mm copper pipe, this is approx. 0.2 litres per running metre)
- Heat exchanger volume in the storage tank (stated by the tank manufacturer in litres, typically 3–8 litres)
- Volume of the pump group and other components (approx. 0.5–1 litre)
- Volume of the expansion vessel (this is not included in the thermal volume, but you must not forget the fluid it contains)
A practical example: a two-collector system on a pitched roof for a family house, a 200 l storage tank, piping 2× 8 m DN 18. Collector volume 2× 1.8 l = 3.6 l, piping 16 running metres × 0.22 dl/m × 2 (flow+return) = approx. 0.7 l, heat exchanger 5 l, pump group 0.8 l. Total approx. 10.1 l for the primary circuit. For a 40% MEG concentration, you need 4 l of pure concentrate and 6 l of distilled water, or a suitable pre-mixed product.
Systems with a larger number of collectors, for example expanded using a mounting frame for installing an additional collector, have a correspondingly higher total volume, and it must be recalculated with every expansion of the system. It is not uncommon for a customer to expand the system with an additional collector and forget to add the appropriate amount of antifreeze – the result is a lower concentration and a risk of freezing.
Systems with a larger number of collectors and a manifold
In systems with four or more collectors, where a hydraulic manifold and header are used (for example, the industrial stainless steel manifold/header assembly with ball valves), the volume of the primary circuit is considerably larger. The manifold adds additional hydraulic volume and, at the same time, creates several branches in which the liquid must be evenly distributed. Here, an exact calculation of the total volume is more important, and it is advisable to perform a hydraulic balance calculation, including the antifreeze, before filling. You can find more about manifold sizing in the topic Manifold and header in a solar system – what it is for and when you need one.
Filling and venting the solar circuit
Even a perfectly chosen fluid can cause problems if the circuit is poorly filled and vented. Air pockets in the collector or piping cause:
- Pump cavitation and premature wear
- System noise (bubbling, knocking)
- Uneven temperature distribution in the collector, local overheating and increased fluid degradation
- False temperature readings and incorrect switching of the controller
The correct procedure for filling the solar circuit assumes filling from the bottom (from the lowest point), releasing air from each vent valve, and finally setting the system pressure (typically 1.5–2.5 bar in a cold state). Never leave the circuit filled on the "it will flow in by itself" principle – air in the vertical branches of the collectors does not flow out on its own.
Antifreeze lifespan and when to replace it
Antifreeze is not a "set and forget" product. It has its own lifespan, which depends on:
- Product quality – premium fluids with OAT inhibitors last 6–10 years, standard products 3–5 years
- Frequency and duration of stagnation – every stagnation episode (collectors without flow at maximum temperature) accelerates degradation
- Circuit volume vs. collector area – an oversized system with a small fluid volume and large collectors stagnates more and ages faster
- Quality of the water used for dilution – hard water accelerates the depletion of inhibitors
The recommended inspection interval is once a year (refractometer + pH strip), with replacement based on condition, at most every 5 years. Specific steps for replacing and flushing the circuit are described in the topic Maintenance and replacement of antifreeze in the solar circuit – when and how in our Knowledge Centre.
Environmentally sound disposal of used fluid
Used antifreeze – even the propylene-glycol-based type – must not go down the drain. It contains glycol degradation products, metal corrosion products and inhibitor residues. Proper disposal is done by handing it over to a hazardous waste collection point or through an authorised company for the collection and recycling of chemical waste. Some service companies offer this service directly when replacing the fluid.
Frequently Asked Questions (FAQ)
Can I use regular automotive antifreeze in a solar circuit?
No, definitely not. Automotive antifreeze is formulated for temperatures up to a maximum of 120–130 °C and for different metal materials (aluminium-steel-cast iron). In a solar circuit at temperatures of 150–220 °C during stagnation, this fluid thermally degrades, forms acids, clogs the heat exchanger and corrodes the copper system. Always use fluid with specific certification for solar systems.
How do I find out the concentration in my circuit if I have no documentation?
The simplest way is to use an optical or digital refractometer. Take a sample of the liquid from the drain valve (ideally after the system has been running for a while, not from a "dead" corner), drip it onto the refractometer prism and read the freezing point. At the same time, measure the pH with a test strip – if the liquid is acidic (pH below 7), plan a replacement regardless of concentration.
Can I just top up with water if the fluid level drops?
No, never top up with plain water only. Every top-up with plain water reduces the glycol concentration and lowers frost protection. If the circuit regularly loses fluid, it is either a leak or fluid loss through the safety valve (a sign of an overfilled or faulty expansion vessel). The leak must be fixed first, then top up with a mixture of the same concentration as in the circuit. Always check the result with a refractometer after topping up.
How long does antifreeze last in a solar circuit?
It depends on product quality, operating conditions and stagnation frequency. Premium certified OAT solar fluids have a lifespan of 6–10 years with annual checks. Standard products should be replaced every 3–5 years. However, never rely solely on the declared lifespan – an annual check of pH and concentration is essential. Don't call us surprised after 12 years without a check.
Do I need to replace the entire antifreeze when expanding the system with another collector?
Not necessarily, but you must recalculate the new total circuit volume and top up with the correct fluid (at the same concentration) accordingly, so that the resulting concentration stays within the required range. Always verify with a refractometer after topping up. If the original fluid is older than 4–5 years, we recommend replacing the entire volume while the system is being expanded, since the circuit is open for work anyway.
Is propylene glycol really safe if it leaks into drinking water?
MPG (monopropylene glycol) is a food-approved substance (E1520) and is not acutely dangerous in small quantities. Nevertheless, contamination of drinking water with any technical fluid is not permissible and must be reported to the water supplier immediately. For solar systems with a double heat exchanger (the standard solution), this question is practically irrelevant, since the primary and secondary circuits are not connected. MPG is only mandatory in systems with a single heat exchanger for direct heating of drinking water.
Conclusion – antifreeze as a long-term investment
Antifreeze for a solar circuit costs €30–80 for a typical family system, depending on the volume and quality of the product. This is a negligible amount compared to the cost of the entire solar system, which includes collectors, storage tank, controls, installation and accessories, including collector mounting frames and hydraulic components. Nevertheless, this "cheap liquid" is precisely the most common cause of premature failures that cost significantly more.
A correctly chosen fluid with an adequate concentration, properly prepared with distilled water, and regularly checked and replaced in time, is the foundation of a reliable and long-lasting solar system. An investment in a quality fluid and a €30 refractometer pays for itself many times over in saved service costs. And if you are not sure about the choice for your specific location and system configuration, you won't go wrong by consulting the situation before filling – it is much cheaper to ask the right question in advance than to repair damage afterwards.
Do you have a question on this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us - we will be happy to advise.