Filling a solar system with antifreeze - what you need to know

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Filling a solar system with antifreeze – a complete technical guide

Filling the solar circuit with antifreeze is one of those steps that may seem simple at first glance, but in practice can ruin the entire installation if overlooked. Over the years of practice, I have seen systems where the mixture was poured from a bucket through a funnel, systems filled from the tap that resulted in frozen collectors in January, and systems with a perfect mixture, yet with such a concentration that the solar pump struggled with the viscosity like with syrup. This article covers the entire topic from the basics – what is antifreeze, which one to choose, how to calculate the required amount, how to properly fill the system, and how to avoid common mistakes that ultimately cost the owner unnecessary money and nerves.

Why a solar system needs antifreeze at all

A solar thermal collector is a device that is exposed to outdoor conditions throughout the year. Unlike a boiler circuit, where the water temperature rarely drops below zero, collectors can reach temperatures well below the freezing point in winter – even when they are not in operation. A cold clear night is enough to cool an unheated absorber down to minus fifteen or even minus twenty degrees Celsius, especially in mountainous areas.

If the circuit were filled with pure water, the expanded ice mixture would simply burst the copper pipe in the collector or deform the soldered joints. Repairing such damage is expensive and time-consuming, and in the worst case, it can completely destroy the collectors. Therefore, antifreeze in the primary circuit of a solar system is a standard, not a luxury.

Antifreeze in a solar system also performs other functions – it inhibits corrosion of internal components (collectors, piping, heat exchanger, pump), prevents the formation of limescale and biological growth (algae, bacteria), and due to its higher boiling point, it reduces the risk of sudden medium discharge in the event of system overheating during the summer holidays.

Composition and types of antifreeze for solar systems

For solar thermal systems, only a mixture based on propylene glycol (propane-1,2-diol) is used, not ethylene glycol. The reason is simple – ethylene glycol is toxic and in the case of a leak in the heat exchanger, it could contaminate the potable water in the tank. Propylene glycol is labeled as a safe substance for food (E1520) and even in the case of a minor leak into the domestic water supply, it does not pose a health risk.

Commercially available solar mixtures are usually pre-mixed concentrates, where the manufacturer specifies the freezing point at various dilutions. Some products are also sold as ready-made mixtures with a fixed concentration. It is important that no automotive coolant should ever be added to the solar circuit – although it is cheaper, it contains inhibitors based on silicates, amines or phosphates, which are incompatible with soft solder and seals of solar components, and at higher temperatures produce aggressive decomposition products.

Common parameters of solar mixtures

Freezing point: typically –28 °C to –35 °C at standard dilution
Boiling point: at atmospheric pressure around 105–115 °C, at system operating pressure (1.5–3 bar) effectively higher
Maximum long-term operating temperature: 160–180 °C (stagnation temperatures can temporarily exceed 200 °C – that is why an expansion vessel and proper system design are important)
Density: approx. 1.04–1.07 g/cm³ at 20 °C (depends on concentration)
Kinematic viscosity: at 20 °C around 3–8 mm²/s, at –10 °C can be as high as 20–50 mm²/s – this directly affects the performance of the circulation pump
pH: 7–9 (mildly alkaline, inhibitors slow down corrosion)
Lifespan: usually 5–10 years, after which it should be changed or the inhibitors should be replenished

How much antifreeze you need – calculating the volume of the circuit

Before you start buying liquid or preparing the pump, you need to know the total volume of your primary solar circuit. This volume consists of the sum of the volume of the collectors, piping, and the heat exchanger in the tank.

Most flat solar collectors have an internal volume of 1.0 to 2.0 liters per collector (depending on the absorber design). Vacuum tube collectors can have a smaller volume – around 0.5–1.5 l. Copper pipe DN12 (12/10 mm) has a volume of approx. 0.07 l/m, pipe DN15 (15/12 mm) approx. 0.11 l/m, and DN18 approx. 0.15 l/m. The heat exchanger in the tank typically has a volume of 1–4 liters depending on the power.

For a simple family system with two collectors and 20 meters of piping, I am talking about a total volume of around 6–10 liters. A system with four collectors and a longer distribution can have 15–20 liters. Manufacturers of collectors always specify the volume in the technical documentation – do not neglect this, it is a key input for dimensioning the expansion vessel and for purchasing the correct amount of liquid. More about the dimensioning of the expansion vessel can be read in the article What expansion vessel volume do I need for my solar circuit.

Practical calculation – example from practice

Real customer case: a family house, 2 flat collectors with a volume of 1.3 l/pc, copper pipe 15/12 mm total length 18 m (including return), heat exchanger in the tank 2.5 l, check valve + fittings ≈ 0.3 l:

Collectors: 2 × 1.3 = 2.6 l
Piping: 18 m × 0.11 l/m = 2.0 l
Heat exchanger: 2.5 l
Fittings: 0.3 l
Total volume: 7.4 l

For this system, I will purchase 10 liters of prepared mixture (a small reserve for air venting and topping up), or alternatively 4 liters of concentrate and dilute it with distilled water to the desired concentration.

Preparation before filling – what must be ready

Filling the solar circuit is not the first thing you do. Before that, the entire primary circuit must be fully assembled – collectors, piping, tank, valves, solar pump, expansion vessel, and all seals. Check that the filling and draining valves are freely accessible (they are usually located on the solar station or near the pump group). Without these two valves, the system cannot be filled or properly vented.

The expansion vessel must have the correct pre-pressure set before filling – more about this in the article Setting the pre-pressure in the solar system expansion vessel. If the pre-pressure is incorrect, filling the system will not damage it, but the expansion vessel will not function properly and during the first heating, the safety valve may discharge or, conversely, a vacuum may occur. The pre-pressure of the expansion vessel is set dry, before filling – remember this.

Also check:

Safety valve (usually set to 6 bar) – must be installed and functional
Pressure gauges – at least one should be visible from the filling location
Air vent valves at the highest points of the circuit (usually at the collectors)
Closing valves – all open except the filling/draining port
Room temperature – ideally, do not transfer cold mixture into an overly hot system (in summer, fill in the morning or evening)

Step-by-step procedure for filling the solar system

There are two basic methods for filling the solar circuit – manual pump filling (most common method for smaller systems) and electric pump filling (used for larger commercial installations). For family homes, a manual pump is practically standard, as the pressure, speed, and dosing are precisely under the installer's control.

For this purpose, you can use, for example, a manual pump for filling solar systems – a simple but reliable tool consisting of a container (usually 10–15 liters), a manual plunger, and hoses with adapters for connecting to the filling valves. The advantage over an electric pump is that the installer can precisely feel when the resistance increases (the system is full), and premature over-pressurization does not occur.

Procedure for filling with a manual pump

The entire procedure is described in more detail in the article Manual pump for filling solar systems – how to properly fill and vent the circuit. Here, I summarize the basic steps:

Prepare the ready-to-use antifreeze mixture in the pump container. Check the concentration with a refractometer.
Connect the pump's output hose to the filling (lower) valve of the solar station.
Connect the return hose (or the free end) to the draining valve, leading it back to the pump container – you are creating a closed loop for recirculation.
Open both valves (filling and draining). Start pumping – the liquid enters the circuit, air is pushed out.
Monitor the pressure gauge – the target pressure in a cold system is typically 1.5 to 2.0 bar.
Gradually open the air vent valves on the collectors (at the highest points), until liquid flows out without bubbles. Then close the valves.
When the pressure reaches the target value and no air is coming out of the system, close the filling and draining valves.
Start the solar pump (circulation pump) for a few minutes – check the pressure again and top up if necessary.
After stabilization, vent again – after the first flow of liquid, additional air usually comes out from corners and dead ends.
Record the final pressure and the filling date in the service record.

Correct antifreeze concentration – how to set and verify

This is the point where mistakes are made most often. The installer buys the concentrate, pours it directly into the pump without dilution – and the result is a mixture that is indeed freeze-resistant down to –50 °C (which is not needed), but has such high viscosity that the circulation pump has problems with circulation, pressure losses are enormous and the solar system operates with significantly lower efficiency.

On the other hand – a too diluted mixture does not provide sufficient protection. Example: in a village at an altitude of 700 m a.m.s.l., –22 °C was measured in January. The owner diluted the concentrate 1:1 (50/50), which gives protection down to about –28 °C. His neighbor’s system was diluted to 30/70 – and had a frozen return pipe behind the collector when the temperature dropped to –25 °C during the night. A few hundred euros for a damaged collector is worth the correct concentration.

Recommended propylene glycol concentrations for individual climatic zones of Slovakia:

Lowlands and the Danubian Lowland (minimum around –15 °C): 35–38 % propylene glycol
Hilly central Slovakia (minimum around –20 to –25 °C): 40–45 %
Mountainous areas, High Tatra Mountains, Orava (minimum below –25 °C): 45–50 %

Always calculate with a reserve of at least 5 °C below the historical minimum in your area – a collector on the roof cools down faster than the air temperature in the garden (it can be 3–5 °C colder than the surrounding air due to radiation into the sky).

Measuring concentration with a refractometer

A refractometer is a small optical device into which you drop a few drops of liquid and read the refractive index – which is directly converted to the freezing point. For solar mixtures based on propylene glycol, use a refractometer with the appropriate scale (not the automotive one, which is calibrated for ethylene glycol – the values would be incorrect). The measurement takes a second and is accurate to ±1–2 °C of the freezing point. Every serious installer should have a refractometer with them for every solar job.

Water quality for diluting the concentrate

If you are not diluting a pre-prepared concentrate, but preparing the mixture yourself, the water for dilution must be soft, demineralized or distilled. Hard tap water has a high content of calcium and magnesium – these ions react with inhibitors in the glycol and can form deposits that clog the heat exchanger or damage the pump seals.

Approximate limit: total hardness should not exceed 2 mmol/l (about 200 mg/l CaCO3). In practice, the simplest approach is to buy distilled water (sold in bottles in pharmacies and auto service centers) and mix it with the concentrate in the correct ratio.

Deaeration – the most critical step after filling

Air in the solar circuit is the number one enemy. It causes noise (bubbling), cavitation on the pump, uneven flow distribution in the collectors and in the worst case can completely stop circulation (air block). Solar circuits are more prone to air entrapment than standard heating systems, because during stagnation (overheating in summer, when there is no heat withdrawal) part of the liquid can evaporate and condense back as liquid – and air remains trapped.

The first deaeration takes place directly during filling (by opening the vent valves at the highest points). After filling and starting the pump, the system should run for at least 15–30 minutes and deaeration should be repeated. An air trap and an automatic vent valve (usually on the solar station) should catch the remaining air during the first few days of operation. Some systems require repeated deaeration after the first solar day.

A more detailed deaeration procedure, typical problems and their solutions can also be found in the article Manual pump for filling solar systems – how to correctly fill and deaerate the circuit.

Working pressure in the solar circuit – setting and checking

After filling and deaeration, the cold system (ambient temperature 10–20 °C) should be under pressure of 1.5 to 2.0 bar. This pressure must be higher than the pre-charge pressure of the gas membrane in the expansion vessel (typically 1.0–1.5 bar), otherwise the expansion vessel would not function correctly. Exact values depend on the height of the collectors above the expansion vessel – a height of 10 m of water column corresponds to a pressure of 1 bar, which must be added to the membrane pre-charge pressure.

When the system is heated (in summer under full sun), the pressure increases – during normal operation (60–90 °C) it is usually 2.5–3.5 bar, which is normal. The safety valve (6 bar) should not open in a properly filled system. If it does open, there is a problem either with the overdimensioning of the circuit, an undersized expansion vessel, or the system was filled at too high a pressure.

Lifespan of the mixture and replacement – when is it time

The antifreeze mixture based on propylene glycol degrades over time. Corrosion inhibitors are gradually consumed, the pH of the mixture decreases (it becomes acidic) and the mixture becomes aggressive towards the components of the circuit – corrosion affects the soft solder joints in the collector, seals and aluminum parts of the pump. Degradation is accelerated by high temperatures during stagnation periods.

Recommended replacement frequency of the mixture depends on the product quality and operating conditions, but as a general rule applies:

Every 3–5 years in systems where frequent stagnation occurs (systems with excess capacity in summer)
Every 5–10 years in systems with good regulation, where the temperature rarely exceeds 130 °C
Always after any repair involving fluid replacement (leak, component replacement)

Changes in the color of the mixture (from light yellow to brown or dark) and a strong sour smell should be taken seriously – these are signs of advanced degradation. Check with pH indicator strips: a value below 6.5 means the mixture needs to be replaced immediately.

Work safety with antifreeze mixture

Propylene glycol is non-toxic to humans, but when working with solar circuits, you should keep a few things in mind. Hot mixture (during service work on a hot system) can have a temperature of 70–100 °C and may spray out quickly when a valve is opened. Always let the system cool down before any repair or draining.

When filling in spring or autumn, the mixture temperature is usually safe, but protect your eyes (goggles) and avoid unnecessary contact with the mixture in case of spills – a slippery floor can cause a fall. Used or degraded mixture should be disposed of at a collection center, not poured into the garden or into the sewer system.

Most common filling errors – real cases from practice

Error 1: Filling without checking the concentration. A customer bought "solar fluid" in a 5-liter can and poured it in directly. The liquid was a pre-prepared product protected down to –15 °C. In the Liptov valley, this was not enough – the minimum temperatures in January reached –22 °C, and the collector pipes froze. Always check the freezing point with a refractometer.

Error 2: Filling without venting the collectors. The system was filled via a pump, the pressure was fine, the pump was running – but the solar yield was catastrophically low. Upon inspection, it turned out that the entire supply pipe to one collector was full of air. The air was not pushed out because the vent valve was forgotten to be opened.

Error 3: Using hard water for dilution. They bought a concentrate and diluted it with tap water (hardness 480 mg/l CaCO3 – a common value in many Slovak towns). After two years, the heat exchanger was covered with a layer of CaCO3 and the flow through the exchanger dropped by 60 %. Result: expensive cleaning of the heat exchanger and replacement of the mixture.

Error 4: Ignoring the expansion vessel pre-charge. The system was filled with a pre-charge of 0.5 bar in the expansion vessel (originally set for a different system). As a result, the membranes did not have enough space to expand – after the first hot summer day, the safety valve sprayed the mixture onto the roof. The expansion vessel must have the correct pre-charge even before filling.

For more information about problems with expansion vessels and pumps, read the article Common faults of expansion vessels and solar pumps – causes and solutions.

Additional equipment needed for filling

Along with the manual pump for filling solar systems, you will need the following during the work:

Refractometer – calibrated for propylene glycol (not ethylene glycol)
pH indicator strips – for checking the condition of the mixture during service inspections
Densimeter or hydrometer – an alternative to a refractometer
Pressure hose with adapters – for connecting to the filling valve (usually ¾" or ½" thread)
Distilled water – for diluting the concentrate
Pressure gauge – for measuring pressure at the expansion vessel location
Safety goggles and gloves

Service check after filling and during operation

After filling the system, it is advisable to perform a test run and record key parameters: cold system pressure, mixture concentration (freezing point), mixture pH, and filling date. These data are valuable for each future service inspection.

During operation, we recommend checking the system pressure at least once a year (ideally in spring before the season). If the pressure repeatedly drops, a leak should be looked for – even a small glycol leak will become noticeable over time. A significant pressure loss can also be caused by a worn membrane in the expansion vessel, which allows air to pass through. For more information on how to check the expansion vessel, read the article How to check and maintain an expansion vessel in a solar system.

Most frequently asked questions (FAQ)

Can I use regular automotive antifreeze in a solar circuit?

No. Automotive mixtures are based on ethylene glycol (toxic) or contain inhibitors incompatible with soft solders and seals in solar components. At higher stagnation temperatures (above 150 °C), these inhibitors can break down and form aggressive acidic products. Always use a mixture certified specifically for solar thermal systems.

How long does it take to fill and vent a typical residential solar system?

An experienced installer can fill a system with two collectors and a standard layout in 45 minutes to 1.5 hours, including venting. For first-time experience, expect 2–3 hours. A system with more collectors or a more complex layout may require repeated venting after the first few hours of operation.

Why did the pressure in the system drop a few days after filling?

The reason may be additional air that was released from the circuit after the first heating – and the expansion vessel partially compressed the membrane, which is reflected in a pressure drop. It is sufficient to add the mixture to the correct level. If the pressure drops repeatedly and over a long period, a leak should be looked for or the expansion vessel checked (perforated membrane).

How many liters of fluid should I prepare for filling?

Always buy 20–30 % more than the calculated circuit volume. Some of the fluid is used for flushing, some remains in the pump hoses, and a small amount is lost at each valve during venting. For a system with a volume of 8 liters, therefore, buy 10–12 liters of mixture. Keep the remainder for possible topping up during service checks.

Can I add pure water to the system if the pressure is low?

Yes, in exceptional cases and in small quantities (e.g., 0.3–0.5 liters) – if you have no other means. But every addition of water reduces the concentration and shifts the freezing point upward (toward zero). With repeated topping up with water, you can reach a state after a few seasons where the frost protection is no longer sufficient. Always check the concentration with a refractometer after topping up and, if necessary, add concentrate.

Where can I buy a manual filling pump if I don't have one?

For one-time or occasional use (service of your own system), the best price/performance ratio is a simple manual pump for filling solar systems with a 10–15 liter tank. Installers who handle multiple jobs will appreciate a version with a pressure gauge directly on the pump, which allows monitoring of pressure without looking at the solar station.

Conclusion – Filling a solar system deserves attention

Filling a solar circuit with antifreeze is a one-time job, but its consequences accompany the system for its entire lifetime. The right mixture, the right concentration, clean water for dilution, thorough venting, and correct working pressure – these are the factors that determine whether a solar system reliably works for 15 years or starts causing problems after the first winter. Do not save time on this installation phase and do not neglect annual or biannual service checks – maintaining the system in good condition is much cheaper than dealing with failures caused by worn or degraded fluid.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.