Winter Operation of Solar Collectors – What You Need to Know

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Winter Operation of Solar Collectors – What You Need to Know

Solar collectors have a reputation as a summer-only solution. Many homeowners assume that in winter these devices are practically useless – covered with snow, snowed in, and shut down. The reality, however, is considerably more complex, and understanding winter operation properly requires clarifying a few physical and technical facts. An experienced installer who has completed hundreds of jobs will tell you that winter operation is exactly what determines whether a system truly justifies its investment – or remains just a seasonal add-on.

This article looks in detail at what happens to a collector during the freezing months, how to avoid damaging it, how to properly maintain the entire circulation system, what a solar system realistically brings you in winter, and when it makes sense to shut the system down. If you're looking for basic information on choosing a collector, we recommend first reading the article How to Choose a Solar Collector – What to Watch Out for Before Buying. Here we will focus exclusively on winter.

Basic physics: why a collector works even at sub-zero temperatures

The first and most important misconception to dispel: a collector doesn't need heat outside. It needs sunlight. These are two different things. Solar radiation hits the collector's absorber even at -15 °C, as long as the sky is clear. The absorber converts this radiation into heat. If the absorber is properly thermally insulated from the surrounding air (and that is exactly the point of collector construction), it can reach a temperature of several tens of degrees Celsius even in severe frost.

The stagnation temperature of a quality flat-plate collector can reach 80–120 °C on a cold but sunny winter day. Under operating flow, the working temperature at the outlet drops to a realistic 30–55 °C, which is still enough to preheat domestic hot water. The critical parameter is not the outdoor temperature, but the amount of solar radiation – measured as global solar irradiance G [W/m²].

The graph clearly shows that December and January have significantly lower daily radiation doses than the June maximum (0.8–1.1 vs. 5.2 kWh/m²/day), but they are not zero. A properly sized system can cover 10–25% of the heat demand for hot water preparation even in January. February and March are considerably more favorable, which is why collectors are sometimes referred to as an "early-spring" energy source.

Frost and solar collectors – where the real danger lies

Frost itself does not harm the collector. The problem arises when a liquid that freezes at low temperatures flows through or sits stagnant in the collector. Pure water freezes at 0 °C, and upon freezing it expands by about 9% in volume – this pressure destroys the absorber, collectors, fittings, and pipes. That is why pure water is never used as the heat transfer medium in solar systems.

Antifreeze mixture – the foundation of winter safety

The standard in solar systems in Slovakia is a mixture of propylene glycol and water in a ratio that provides protection down to -28 to -35 °C. Unlike ethylene glycol, propylene glycol is non-toxic, which is important in case of possible contamination of drinking water through the heat exchanger. A concentration of 40% propylene glycol protects down to -24 °C, which is sufficient for most Slovak locations. In mountainous areas (the Tatras, Orava, Kysuce), we recommend a 45–50% concentration, which shifts the freezing point to -30 to -35 °C.

An important note from practice: antifreeze mixtures age. Their pH decreases over time (acidity increases), corrosion inhibitors get depleted, and the mixture starts to aggressively attack the metal parts of the system. Replacement or replenishment of inhibitors should take place every 4–5 years. Always check the current concentration with a refractometer – don't just trust the fact that you filled the mixture five years ago. Evaporation of water changes the concentration, and topping up with pure water after a leak weakens it.

What happens with an incorrectly mixed solution

Freezing of the absorber – if the concentration is lower than the protective value, the liquid may freeze directly inside the collector during extreme frost. The thin meander or harp tubes of the absorber crack, and the repair usually means replacing the entire collector.
Corrosion – an over-acidified mixture corrodes aluminum absorbers, copper pipes, and steel tanks. It shows up as sludge, clogging, reduced flow, and eventually leaks.
Pump cavitation – with reduced mixture viscosity (incorrect temperature or degraded mixture), noise and premature wear of the circulation pump can occur.

Snow on the collector – myth versus reality

Many people believe that snow on a collector is a disaster. In practice, it's different. A quality flat-plate collector has a glass front surface with a tilt of at least 35–45°. Snow slides off such a surface by itself on the first sunny day – even faster than off a roof tile, because glass has a lower friction coefficient. South orientation and sufficient tilt are therefore important not only for maximum yield, but also for the self-cleaning ability against snow and dirt. You can find more about optimal setup in the article Collector Tilt and Orientation – How to Maximize Energy Yield.

When is snow a real problem? With flat installations (tilt below 20°), during prolonged cloudy periods without sun, when an ice crust forms on the snow, and with horizontal installations on flat roofs. In such cases, a layer of snow with ice can remain for a week or longer. The solution is not to climb onto the roof and shovel the snow off (dangerous!), but to design a system with a sufficient backup heat source to cover these outages.

Stagnation – a winter and summer problem alike

Stagnation is a state in which the liquid in the collector stops moving, but the collector continues absorbing solar energy. The temperature in the absorber then rises to the stagnation temperature (80–200 °C depending on the collector type). Stagnation occurs:

During a power outage (the pump stops)
When the tank overheats – the controller disconnects the pump
In case of controller or pump failure
In winter: if the tank is full, the sun is shining, but the tank temperature has reached its maximum

In winter, stagnation is less dangerous than in summer (lower radiation, shorter days), but it can still cause problems. During stagnation, lighter fractions of the antifreeze mixture evaporate from the liquid, the concentration increases, and the composition changes. Repeated stagnation accelerates the degradation of the glycol mixture. It is therefore important that the expansion system is properly designed (diaphragm expansion tank) and that the controller has both a tank overheating protection function and a night cooling function.

Night cooling as winter protection

Some solar system controllers offer a so-called night cooling function – at night, when the outdoor temperature is low, the controller starts the pump and moves heat from the tank to the collector, where it dissipates into the night air. This function is primarily a protection against tank overheating in summer, but during transitional periods (spring/autumn) it also helps maintain the system's capacity. In winter it is generally not used, because the tank does not overheat on its own.

Control and management of the solar system in winter

The controller is the brain of the entire system. In winter it must handle several specific situations that either don't occur in summer or occur only rarely.

Differential control – the basis of winter management

The controller measures the collector temperature (sensor at the absorber outlet) and the tank temperature (sensor at the bottom). The pump starts when the difference reaches a set value – typically 5–8 °C (switch-on differential). The pump switches off when the difference drops below 2–4 °C (switch-off differential). In winter, the collector temperature is very low in the morning but rises quickly in good sunshine. A problem arises if the switch-on differential is set too low – the pump may run even when the collector is not actually contributing to heating, but instead cooling the tank. Correct differential settings are crucial for winter operation.

Frost protection in the controller

Modern controllers have a protective pump-start function at low collector temperature – for example, when the collector temperature drops below +4 °C, the controller briefly starts the pump and pumps warm liquid from the tank into the collector. This prevents freezing in extreme cases (for example, if the glycol mixture concentration dropped and wasn't checked in time). It is a safety function, not a substitute for the correct antifreeze concentration.

Real yield in winter months – what you can expect

Let's be specific. A solar system with 2 flat-plate collectors (total area approx. 4.4 m²) and a 200-liter tank for a family house in central Slovak locations (e.g. around Zvolen, Nitra, Trenčín) shows in practice the following approximate monthly yields:

December: 15–30 kWh/month (solar fraction of DHW heating: 8–15%)
January: 18–35 kWh/month (solar fraction: 10–18%)
February: 35–60 kWh/month (solar fraction: 18–30%)
March: 70–100 kWh/month (solar fraction: 35–50%)
April: 100–140 kWh/month (solar fraction: 50–70%)

These figures depend on the location, tilt and orientation of the collector, quality of pipe and tank insulation, daily hot water consumption, and of course the actual weather. In cloudy winters, yields in December and January may be close to zero during long dark periods. In cold but sunny winters (occurring in the continental climate of central Slovakia), they can be significantly higher.

If you want more precise calculations for your house, take a look at the article What Solar Collector Output Do I Need for My House, where you'll find calculation methodology and irradiance tables for various Slovak regions.

Differences between collector types in winter operation

Not all collectors respond to winter in the same way. You will find a more detailed comparison in the article Flat-Plate vs. Tube Collector – Which Type Is More Worthwhile. Here we focus on the practical consequences for winter.

Flat-plate collectors in winter

Flat-plate collectors have a slight disadvantage on days with diffuse radiation (cloudy, foggy days) – their yield is lower. On the other hand, they have a natural resistance to stagnation due to their greater thermal capacity and lower stagnation temperature. In frost and snow, their flat glass self-cleans well at a tilt above 35°. A quality flat-plate collector with well-insulated construction and a selective absorber operates reliably even at -20 °C outdoor temperature.

For example, the flat-plate AlCu solar collector with structural glass is equipped with an aluminum-copper absorber with a selective coating and quality mineral wool thermal insulation – these are exactly the properties that determine performance in cold weather, when the temperature difference between the absorber and outside air is greatest. For more demanding conditions and maximizing yield even in weaker winter light, the flat-plate AlCu solar collector with structural anti-reflective glass is available – the anti-reflective glass treatment reduces reflection losses and lets more light through to the absorber at low angles of incidence, which is a typical situation in winter, when the sun sits low above the horizon.

You can find more about the differences between glass types and their effect on yield in the separate article Structured vs. Anti-reflective Collector Glass – What's the Difference.

Tube (vacuum) collectors in winter

Vacuum tube collectors have better thermal insulation properties at low outdoor temperatures (the vacuum in the tubes eliminates convective losses). In deep frost with sunshine, they can operate more efficiently than flat-plate collectors. The disadvantage is that snow hangs on the tubes and doesn't fall off as easily, which can lead to longer coverage of the active surface. Also, if one tube fails, the unit doesn't lose functionality entirely, but output drops.

Winter maintenance of a solar system – a concrete procedure

A solar system is not a maintenance-free device. Winter is the season that tests it from all sides. Every solar system owner should be able to perform, or have a service technician perform, the following tasks:

Before winter (September – October)

Check the glycol mixture concentration with a refractometer – if the value is lower than the protective threshold, top up or replace it
Visual inspection of the collector – cracks in the glass, damaged seals, signs of moisture inside the frame
Check the safety valve (system pressure, condition of the diaphragm in the expansion tank)
Verify the controller and pump function – trial run, checking the differential settings
Check the insulation of the outdoor piping – damaged insulation causes high heat losses even over short outdoor sections

During winter (November – February)

Visual inspection – snow on the collector, ice around the collector or manifold, leaks
Monitor controller data – daily yields, operating temperature. If the controller shows zero even on sunny days, something is wrong
Do not try to mechanically remove snow from the collector – risk of slipping and damaging the collector and glass

After winter (March – April)

Spring inspection of the frame and glass after frosts and possible temperature shocks
Check the pH and concentration of the mixture again
Check the anode in the tank (if it's a steel tank)
Clean the filter cartridge (if the system has one installed)

Does it make sense to shut down the solar system in winter?

This question comes up fairly often, mainly from owners whose system doesn't work reliably or who don't want to risk failures during frost. Shutting down the system makes sense only in very specific cases:

A system with water without antifreeze mixture (some older installations with drainable systems) – here, winter draining is necessary
Long-term absence – when the house is unoccupied for the entire winter and the tank would be full of hot water with no consumption
A detected fault – cracked pipe, serious leak, pump failure – in such a case, shut down the system and repair it

In the normal operation of a family house, it does not make sense to shut down the system. The benefits of winter operation are not dramatic, but February and March bring decent preheating of hot water. A system that is shut down for the entire winter does not have the right temperature in March, and the yield at the start of the season is lost. In addition, shutting down and restarting the system can introduce air into the circulation loop, complicating further operation.

Installation details that determine winter reliability

An experienced installer knows that differences in winter operation are very often due to installation details rather than the collector type itself. What we pay attention to in practice:

Insulation of outdoor piping

All outdoor sections of solar piping (from the collector to the building entry point) must be insulated with closed-cell thermal pipe insulation (PE or EPDM), at least 25 mm thick, ideally 32–40 mm. The insulation must be protected against UV radiation and moisture – either with self-adhesive ALU tape, ALU cladding, or spiral PVC tape. Uninsulated or poorly insulated outdoor piping causes heat losses in winter that can turn the whole system balance negative (the system loses more than it gains).

Deaerating the system

Air pockets in the system are a year-round problem, but they show up more prominently in winter. A pump with air in the circuits works noisier, has lower efficiency, and if the problem goes unnoticed for a long time, the pump can overheat. The automatic air vent at the highest point of the system (usually at the collector or manifold) must be functional and checked regularly. In winter, make sure the valve is not frozen.

Correct flow direction

Cold liquid must enter the collector from the bottom and warm liquid must exit at the top (principle of natural convection + gravity-assisted deaeration). Reversed connections cause lower performance and air problems. This is a detail worth checking when inspecting an older installation.

Comparison of monthly gains for different collector tilts in winter

Collector tilt has a greater impact in winter than in summer. The reason: the sun sits low above the horizon (elevation angle 10–25° above the horizon), so a collector tilted at a steeper angle catches sunlight more perpendicularly and has a higher yield. A collector tilted below 30° is significantly less efficient in winter than the same collector at 55–65°.

Approximate monthly yield (kWh) – 2 flat-plate collectors, 4.4 m², south orientation, central Slovakia
Month	Tilt 30°	Tilt 45°	Tilt 60°
December	12 kWh	22 kWh	28 kWh
January	15 kWh	28 kWh	34 kWh
February	30 kWh	50 kWh	58 kWh
March	65 kWh	88 kWh	90 kWh
June (ref.)	165 kWh	155 kWh	130 kWh

The table clearly shows that a 60° tilt is more advantageous for winter months than the optimal 45° for year-round yield. If you want to maximize winter performance (e.g. a recreational property with a summer break, or a house with excess heat supply in summer), a steeper tilt is beneficial. For year-round systems focused on total annual yield, the sweet spot is 40–50°. You can find more on this topic in the article Collector Tilt and Orientation – How to Maximize Energy Yield.

Practical examples: what we actually see on winter jobs

From practical experience during service visits and inspections of solar systems, we repeatedly encounter several typical situations:

Case 1 – older house, system from 2009: The owner complained that the system "does nothing in winter." Upon arrival, we found that the glycol mixture had a concentration of only 25% (freezing point -10 °C) instead of the required 40%. The controller activated the protective function and refused to start the pump below +6 °C collector temperature, preventing a real risk of freezing on frosty nights. After replacing the mixture and calibrating the controller, the system worked reliably in February and March.

Case 2 – new building, incorrect pipe insulation: Installation was carried out in September; during the first frosts in winter, it turned out that a 20 cm section of pipe between the roof and the attic entrance was insulated with standard thermal tubing without UV and moisture protection. Moisture from condensation during temperature swings destroyed the insulation by the end of January. Heat losses on this section absorbed practically the entire winter yield of the system. Fixing it required removal and new insulation with ALU cladding.

Case 3 – mountain cottage, 35° tilt: The owner wanted "winter yield," but the collector wasn't tilted enough. After recalculation, we proposed adjusting the mounting structure to a 60° tilt, which increased the December–February winter yield by about 60%. Summer yield decreased slightly, which in this case was acceptable, because the tank overflowed through the safety valve in summer anyway due to low occupancy of the cottage.

Frequently Asked Questions (FAQ)

Can a solar collector work when it's -20 °C outside?

Yes, if it's filled with the correct antifreeze mixture and the sun is shining. The absorber can reach a temperature of +50–70 °C even in -20 °C outdoor frost – the collector's thermal insulation separates it from the outside air. The condition is sufficient solar radiation, which does reach the collector even in frosty, clear weather. In practice, such cases occur most often in January and February during anticyclones with significant frost and no clouds.

How much energy does a solar system produce in winter compared to summer?

December and January typically reach 15–25% of the summer (June) yield. February is significantly better – reaching 30–40% of the June yield. March approaches 50–60%. In absolute numbers: a system with 2 collectors will produce 20–35 kWh in January, and 140–180 kWh in June. Winter months therefore cannot fully replace a backup heat source, but they do reduce its consumption.

Do I need to shut down the system before winter if I'm leaving for 3 months?

If the system is filled with antifreeze mixture at the correct concentration and the controller is functional (has a power supply), there's no need to shut down the system. The controller automatically monitors temperatures and handles necessary protective pump starts on its own. However, if the house has no electricity, the controller doesn't work and the system is passive – in that case it's safer to drain the system or ensure the correct glycol mixture concentration for the entire absence period.

What to do if snow stays on the collector for days and doesn't fall off?

Don't climb onto the roof and don't try to pull the snow off – risk of falling and damaging the collector. If the tilt is sufficient (above 35°), the snow will fall off by itself on the first sunny day, because the glass heats up quickly and the snow slides. If the tilt is lower or the layer freezes into ice, you need to wait for warmer weather. When designing systems for flat roofs (tilt below 20°), a longer winter pause must be expected and properly factored into the payback calculation. A longer-term solution is to consider changing the tilt of the mounting structure.

How do I know when the glycol in the system needs to be replaced?

Measure the concentration with a refractometer – if it's lower than the protective value, top up or replace it. Other symptoms: dark or sludgy discoloration of the liquid (instead of clear greenish), sour smell at the drain valve, pH below 6.5 (test strip), visible sediment or sludge when draining. The typical lifespan of a glycol mixture with inhibitors is 5–7 years, shorter with frequent stagnation.

Is a tube or flat-plate collector better in winter?

Tube (vacuum) collectors have better thermal insulation at low outdoor temperatures and can achieve slightly higher output than flat-plate collectors on clear, frosty days. Flat-plate collectors, on the other hand, are more resistant to stagnation and snow falls off them more easily. In practice, it depends on the specific location, tilt, shading, and required reliability. In most cases, for family houses in Slovakia, quality flat-plate collectors are sufficient even for winter, and their total annual yields are comparable to tube collectors. You'll find a detailed comparison in the article Flat-Plate vs. Tube Collector – Which Type Is More Worthwhile.

Conclusion: winter is not the enemy of the solar collector

A properly designed, installed, and maintained solar system works reliably all year round – including the coldest months. Winter brings lower yields, but not zero operation. The key to trouble-free winter operation is the correct antifreeze mixture with a checked concentration, functional control with protective functions, quality insulation of outdoor piping, and regular (at least once a year) inspection of the whole system.

Investing in a solar collector with good thermal insulation properties and quality glass – whether with standard structural glass or with an anti-reflective treatment – pays off in the long run precisely because the system works not only in July, but also in February and March, when preheating hot water is most valuable. The winter months determine whether a solar system actually achieves its projected annual energy savings – and therefore deserve just as much attention as hot summer days.

If you're dealing with faults or unusual behavior of your system in winter, take a look at the article Common Solar Collector Faults and How to Fix Them, where you'll find an overview of the most frequent problems and their symptoms.

Do you have a question on this topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we'll be happy to help.