Solar system in winter: how it works at low temperatures and how to prevent the circuit from freezing

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Solar system in winter: myths, reality and practical solutions

Every autumn the same question comes up: "Is a solar system worth using in winter too, or should I simply switch it off for the cold months?" Experience from dozens of installations shows clearly that most solar system owners underestimate the winter potential of the system, because no one has properly explained what happens in the circuit at low temperatures, what risks there are, and how to prevent them. This article will fix that. We'll go through the physics of a solar collector in freezing weather, look in detail at winter performance, focus on the real risk of freezing, and show specific preventive measures - from the composition of the antifreeze mixture to control settings.

How a solar collector works in winter conditions

The basic principle of a solar collector doesn't change in winter: the absorber captures solar radiation and converts it into heat, which the heat transfer fluid carries to the storage tank. What changes is the amount of available energy and the physical conditions of the surroundings.

It's key to understand that a solar collector doesn't work with air temperature, but with solar radiation - i.e. irradiance measured in W/m². Even in January at -10 °C, a clear low winter sun can deliver 400-600 W/m² of global radiation. That's less than the 900-1,000 W/m² of summer, but still enough for a noticeable heating effect. An additional factor is the low angle of the sun - in December in Slovakia it's around 18-25° above the horizon. This is a disadvantage for flat collectors installed on the ground, but at a roof pitch of 35-45°, the radiation hitting the collector can actually be even more efficient than in summer, when the sun shines almost vertically and collectors on a sloped roof receive less energy.

Real measured values from practice: a solar system with two flat-plate collectors (total absorber area approx. 4.4 m²) on a southwest-facing façade at a 45° tilt can heat a 250 l tank by 20-35 °C on a sunny January day. On a cloudy day, the contribution is minimal, but the system still doesn't operate "at a loss" - it simply doesn't switch on, because the control won't let heat flow into the tank if cooling would occur.

The chart clearly shows that the winter months (November-February) are the weakest period, but that's far from meaning zero yield. January typically brings 10-20 kWh of solar gain, which won't cover the whole family's hot water needs, but it does reduce the load on the boiler. More importantly, however, the system must remain functional and protected all year round.

Heat transfer fluid: the heart of winter protection

The most important element of a solar system's winter resilience is the heat transfer fluid in the primary circuit. Under no circumstances should this be plain water - it freezes at 0 °C, and expanding ice under pressure can burst the collector, piping and pump unit. Damage typically runs into hundreds of euros, and in the worst case the entire collector has to be replaced.

The standard solution is a mixture of propylene glycol antifreeze solution with distilled water, sold under various brand names (Solaren, Tyfocor, Solar Fluid and others). Most manufacturers recommend a concentration of 40-45% by volume, which provides freeze protection down to -25 to -28 °C. In extreme conditions at higher elevations in Slovakia (Beskydy, Tatras), a 50% concentration is advisable, protecting down to -35 °C.

Important: unlike ethylene glycol, propylene glycol is non-toxic, which is practically significant in case of a leak near drinking water or a garden. Most professional systems work specifically with propylene glycol.

How the fluid's behavior changes at low temperatures

Here we come to one of the most important, and at the same time most often underestimated, physical facts of winter operation: as the concentration of the glycol solution increases, so does the fluid's viscosity, especially at low temperatures. A fluid with a 40% propylene glycol concentration has a viscosity at -10 °C about 3-4 times higher than at 60 °C. This has a direct impact on pump performance - the control unit must be able to account for this change.

Practical consequence: at very low temperatures (-15 °C and below), morning circulation may be slower until the fluid in the collector warms up to operating temperature. Some cheaper systems with simple pumps can have problems in such situations - the pump lacks the power to overcome the resistance of cold, viscous fluid. That's why manufacturers such as Vaillant or Protherm dimension the pumps in their solar sets with a safety margin.

Aging and replacement of the heat transfer fluid

Glycol doesn't last forever. Repeated heating (in summer the collector can reach 150-180 °C during stagnation) causes the corrosion inhibitors and the glycol itself to degrade. Acidic glycol (pH below 7) is corrosive and damages copper piping, seals and the tank. That's why it's necessary to have the fluid analyzed every 4-5 years and replace it if needed. You can read how to do this in the article Maintenance and servicing of a solar system: what to check every year and when to call a technician.

How solar control works in winter - and where it can fail

A modern solar controller (differential thermostat) continuously compares the temperature at the collector absorber (sensor T1) with the temperature at the bottom of the storage tank (sensor T2). The pump switches on when the difference is sufficient - typically ΔT = +6-8 °C. It switches off when the difference drops to ΔT = +3-4 °C or when the tank reaches the maximum set temperature (usually 60-70 °C).

In winter, a specific scenario occurs: in the morning after a frosty night, the collector temperature may be negative (for example -8 °C), while the tank is at 40 °C. The controller correctly won't start the pump - there would be no heat gain; on the contrary, the collector would cool the tank. At the same time, the cold glycol mixture circulating through the collector would be needlessly stressed. Advanced controllers today automatically check for this scenario too and block the pump at a negative ΔT.

Nighttime collector cooling and reverse circulation

One of the lesser-known winter problems is so-called reverse or nighttime circulation. If the controller doesn't have the correct protective threshold values set, it can appear to be working fine while actually cooling the tank over a long period. This happens with an incorrectly configured differential or a mechanically stuck pump.

Experience shows that owners notice this because in the morning the tank is warmer than it was the evening before going to bed, but a closer look at the controller reveals that the pump was needlessly running for several hours during the night. The solution is to check the ΔT switch-off settings and activate the "night cooling inhibit" function offered by most mid-range controllers.

Anti-freeze control function

Many modern controllers also include an active anti-freeze function: if the collector temperature drops below a set value (usually 4-6 °C), the controller briefly starts the pump to bring warmer fluid from the tank into the collector. This is a safeguard in case the antifreeze mixture is too dilute or has partially degraded.

Note: this function is not meant to replace a properly mixed heat transfer fluid. It's only a last safety net. If the controller has to trigger the anti-freeze function regularly, it's time to check the concentration of the glycol solution and top up or replace the fluid.

Sizing the expansion vessel in winter

The expansion vessel in the solar circuit performs two functions: it absorbs the thermal expansion of the fluid when heated and provides a pressure cushion during summer stagnation. In winter, the opposite happens - the fluid contracts as it cools. The pressure in the circuit at -15 °C can drop by 0.3-0.5 bar compared to the operating value. If the expansion vessel is undersized or has an incorrect pre-charge pressure, negative pressure can occur in the circuit, air can be drawn in through seals, or sensor readings can be distorted.

Correct procedure: the expansion vessel must be sized for the total volume of fluid in the primary circuit and for temperature differences from -20 °C (cold collector on a frosty night) up to +150 °C (summer stagnation). The vessel's pre-charge pressure is always set with the circuit cold (drained) - typically 1.0-1.5 bar for a typical single-story installation, 1.5-2.0 bar for a greater height difference.

When choosing a specific product, for example the Vaillant auroSTEP VSL S 250/2 T for a pitched roof or the Vaillant auroSTEP VSL S 250/2 F for a flat roof, you have the advantage that both systems come with pre-prepared hydraulics, including a correctly sized expansion vessel and solar station. Vaillant also accounted for European winter conditions here, so the expansion vessel is certified for a temperature range from -28 °C to +200 °C.

Freezing of the circuit: when and how it happens

Freezing of the primary circuit isn't a typical scenario in a properly maintained system - it's always the result of neglect. From practical experience, freezing occurs in the following typical situations:

Diluted heat transfer fluid - the owner topped up the circuit with plain water after a leak, or the fluid wasn't replaced for many years and degraded. The glycol lost its inhibitors, the pH dropped, and the fluid partially broke down. The freezing point shifted from -25 °C to -5 °C.
Pump or control failure - the pump got stuck, the contactor failed, or there was a power outage. The fluid sits in the collector during a frosty night and freezes. This is the most dangerous scenario, because even the areas where the fluid is stationary - i.e. the entire collector surface - will freeze.
Too long a pipe run in an unprotected zone - for example, an uninsulated section of piping between the collector and the roof in a ventilated attic. Even if the fluid is strong, a small volume of cold fluid in an exposed section can freeze before the control's protective function has a chance to catch it.
System switched off during frost - the owner went on vacation, switched off the boiler, and forgot that the solar system needs at least a minimal power supply for the controller and pump. Without anti-freeze circulation and with a diluted mixture, a breakdown is just a matter of the first frosty night.

What physically happens when a collector freezes

Water expands by about 9% in volume when it freezes. In a closed pressurized system, there's nowhere for it to expand - the pressure rises to values far exceeding the system's rated working pressure (typically 3-6 bar). The result: a cracked absorber, deformed collector coils, a shattered glass cover, or damaged seals at the connection fittings. Damage to the collector typically amounts to €300-800 in spare parts alone, plus installation.

Interestingly, this damage may not be immediately visible. The collector may look undamaged after thawing, but the absorber's internal coils are deformed and the system slowly loses pressure. The owner only notices after a few weeks, when the solar system's performance visibly begins to drop.

Preventive measures: complete winter checklist

A properly set-up solar system doesn't need a "winter mode" in the sense of being switched off. Instead, it needs preventive preparation and ongoing checks. Here is a list of everything an experienced technician does before winter and throughout the cold season:

Check and measure the glycol solution concentration - using a refractometer (a measuring device costing around €15-20). It reads the freezing point directly on site. If the freezing point is higher than -25 °C, the fluid needs to be topped up or replaced.
Measure the pH of the heat transfer fluid - using litmus paper or a digital pH meter. The correct value is 7.5-9.0. Acidic fluid (pH below 7) must be replaced entirely.
Check the pressure in the primary circuit - with a cold system (collector + fluid at ambient temperature), the pressure should be 1.5-2.5 bar. If it's lower, top up via the filling/drain valves. If it's significantly lower than at the last check, there's a leak in the circuit.
Check the expansion vessel - the pre-charge pressure is checked with the circuit completely cold and drained. If it's lower than the specified value, top up the air using a standard car-type valve.
Test the pump and controller - manually start the pump via the controller's service menu and check the flow (if the controller has a flow display). Pump lockup after the summer shutdown is a common occurrence.
Check the controller settings - verify the correct ΔT switch-on values (typically 6-8 K), ΔT switch-off (3-4 K), maximum tank temperature, and activate the anti-freeze function.
Visual inspection of pipe insulation - especially along the route between the collector and where it passes through the roof, where pipe sections run through ventilated attics or along an outer wall.
Removing snow from the collector after heavy snowfall - this is a special topic, see the next section.

Snow on the collector: leave it or remove it?

Snow on the collector is a topic where opinions typically differ. The facts are as follows:

Snow layer under 5 cm: on a sunny day the collector quickly warms up from the absorbed radiation passing through the glass, and the snow layer slides off on its own. In this case, no action is needed. Modern glass covers of solar collectors have a surface tilt of at least 15-20° (30-45° on pitched roofs) and snow usually slides off by itself.

Wet, heavy snow over 10 cm: here a problem arises, not only in terms of performance but also mechanically. Solar collectors are typically designed for a snow load of 1.5-2.5 kN/m² (corresponding to a snow layer of about 50-80 cm). Wet snow has a higher density, so 20 cm of wet snow can be heavier than 50 cm of dry snow. If extreme loading is a risk, it makes sense to carefully remove the snow - with a soft broom from the bottom up, without pressing mechanically on the glass.

Frozen ice on the glass: never use a shovel or a hard tool. Let the collector thaw on its own - only the surface layers of the glass freeze, the absorber is still warm from the previous day. Pouring hot water risks thermal shock and cracking of the cover glass (safety tempered glass does resist it, but repeated thermal shocks wear it out over time).

Protherm HelioSet and Vaillant auroSTEP in winter practice

If you're choosing a system, winter conditions should be one of the criteria. From experience, both the Protherm HelioSet FES2 250 BM and the Protherm HelioSet 2.250C HT are systems where the heat transfer fluid is included in the delivery - which guarantees that the installation starts with the correct concentration. The controllers of both series include an anti-freeze function and differential control with adjustable ΔT. The difference between the FES2 and 2.250C HT models lies mainly in the type of tank and the hydraulic scheme - you can read more about this in the article Comparison of Vaillant auroSTEP vs. Protherm HelioSet solar systems: differences and suitability of use.

For less demanding applications and smaller family homes, the Solar System No. I S is also an interesting choice - a compact solution for 1-3 people, where winter performance covers a substantial part of hot water preparation in the transitional months (September-November and February-April). In January and December, the contribution will be modest, but the system will operate safely provided it's properly prepared.

Why it's not worth "switching off" a solar system in winter

This topic comes up regularly, and the answer is clear: don't switch off your solar system in winter unless you have a serious technical reason (servicing, fluid replacement). There are several reasons for this:

1. Loss of performance during the transitional period too - the biggest solar gains come in October, November, February and March, not December and January. If you switch the system off "for winter" in October and turn it back on in March, you lose two months of solar gain of 30-50 kWh each, which makes no economic sense.

2. Risk of a stationary circuit freezing - paradoxically, a stationary system without circulation is more at risk of freezing than a system where the controller actively manages fluid flow, including anti-freeze circulation.

3. Summer stagnation damage after shutdown - if you restart the system in March after a long winter shutdown, there's a greater likelihood of finding a stuck pump, acidic fluid, or loosened seals.

The only exception is leaving for a longer stay in an extremely cold environment without the ability to ensure power for the controller and pump. In that case, it's safest to drain and blow out the primary circuit and refill it with a fresh mixture in spring. A detailed procedure is part of the article Installing a solar system step by step: what you can do yourself and what needs a professional.

Combining with a backup heat source in winter

In winter, a solar system works best as a support - not as the primary heat source. That's why correctly setting up the backup is key. Most solar tanks have an electric heating element (backup) or a connection to a gas boiler via a second heat exchanger (or directly into the tank). The backup control should be set so that the boiler (or heating element) only finishes heating the tank when the solar circuit isn't sufficient - i.e. after several hours without adequate solar gain.

A typical mistake: the boiler is set to a fixed heating time each day (for example, every day at 6:00), regardless of how much the solar system has already heated. Result: the boiler "gets ahead of" the solar system, the tank is hot in the morning, and the collector has nowhere to deliver its energy. This reduces annual solar gain by 15-25%. Correct settings are described in the article Combining a solar system with a boiler or heat pump: how to correctly connect the systems.

Most common winter faults and their diagnosis

From a servicing perspective, the following scenarios recur in winter:

Pump won't start after frost - the pump rotor has seized due to long inactivity or slight corrosion. Solution: manual release with a screwdriver (most pumps have an access screw) or pump replacement.
Air in the primary circuit - air can remain after summer stagnation or after a service intervention. In winter, air collects at the highest point (in the collector) and blocks circulation. Solution: bleed via the valve on the collector, top up the fluid to the specified pressure.
The controller doesn't start the pump despite a sunny day - the collector sensor (T1) or tank sensor (T2) is damaged or disconnected. Measure sensor resistance and compare with the manufacturer's tables. This fault is described in more detail in the article Common solar system faults: overheating, air in the circuit, insufficient water heating.
Pressure fluctuating with every temperature change - a faulty expansion vessel (ruptured membrane, lost pre-charge pressure). As the fluid temperature rises, pressure quickly climbs above 4 bar and the safety valve opens. Solution: replace or re-pressurize the expansion vessel.

Frequently Asked Questions (FAQ)

Is it normal for a solar system to operate only a few hours a day in December?

Yes, this is completely normal. In December, the sun shines at Slovak locations on average 2-3 hours a day at levels sufficient to activate the system (global irradiance above 150-200 W/m²). The controller only starts the pump when the collector temperature is sufficiently higher than the tank's - in practice this is 1-3 hours around midday in clear weather, 0 hours on an overcast day. This isn't a fault, but the physical reality of winter solar radiation in Slovakia.

What's the correct antifreeze concentration for Slovak conditions?

For most locations in Slovakia up to an altitude of 600 m above sea level, a 40% propylene glycol solution with a freezing point of -25 °C is sufficient. For mountain regions (Orava, Tatras, Kysuce), I recommend a 45-50% concentration with a freezing point of -29 to -35 °C. Always measure the concentration with a refractometer, don't estimate it by color or density - these methods aren't reliable.

Can I top up the primary circuit with plain water in an emergency?

In an extreme emergency (for example, if the system is losing pressure during frost and there's a risk of freezing), you can add a small amount of distilled or softened water, but this is only a temporary solution. Every addition of water lowers the concentration of the glycol solution and raises the freezing point. After such an emergency intervention, you must measure the current concentration with a refractometer as soon as possible and, depending on the result, top up with concentrated glycol or replace the entire charge. Never top up with hard (non-distilled) tap water - minerals accelerate corrosion processes.

What happens if the power goes out during frost and the pump stops?

A short-term outage (a few hours) with a correctly concentrated glycol solution (-25 °C or lower) doesn't pose an immediate risk. The fluid in the circuit simply sits still, and if it has a sufficiently low freezing point, it won't freeze. The problem is a long-term outage (several days) at very low temperatures - here it depends on the collector temperature and the fluid's freezing point. That's exactly why the correct glycol concentration is the most important safeguard, not the control unit or the pump.

Is it worth investing in a solar system when it barely works in winter?

Don't judge the economics of a solar system by winter performance alone - annual savings are what matter. A typical two-collector system for a family home covers on average 50-70% of the annual hot water needs. The winter months (November-February) contribute about 5-15% of this gain, while the summer months (May-August) contribute 50-60%. You'll find a detailed calculation based on number of occupants and consumption in the article What solar system output do I need: a calculator based on number of occupants and hot water consumption. Investment payback and subsidy options are summarized in the article Frequently asked questions about solar systems: return on investment, subsidies, permits and connection.

Do I need to shut down and drain the solar system for winter, or can it run all year round?

With a properly prepared system with a quality heat transfer fluid and a functional controller, year-round operation isn't just possible - it's recommended. Draining and refilling the primary circuit is a maintenance burden and contributes to seal degradation and surface oxidation. Winter operation is safe as long as you have the correct glycol concentration, a functional pump, and a controller with an anti-freeze function. Shutting down only makes sense when leaving the property for a long time without access to electricity.

Conclusion: winter isn't a reason to switch off the collectors, but a reason to prepare properly

A solar system is a year-round investment - and winter is when the quality of the installation, fluid and control really shows. Three things are key: the right heat transfer fluid at the right concentration, a functional controller with anti-freeze protection, and a regular autumn check of the system's condition. If you have this under control, your solar circuit can run reliably for decades - no matter what the thermometer on the roof shows.

If you're considering a new solar system or upgrading an existing one, take a look at the specific products we offer - whether it's the Vaillant auroSTEP VSL S 250/2 T for a pitched roof, the Vaillant auroSTEP VSL S 250/2 F for a flat roof, or Protherm HelioSet systems with various tank configurations. Each of these systems is designed for year-round operation in European climate conditions - including cold Slovak winters.

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