Frequently Asked Questions About Solar Collectors

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Frequently Asked Questions About Solar Collectors – Answers from Practice, No Nonsense

When a customer is deciding to buy a solar collector, time is usually not the main pressure. Questions are. Some are technical, others economic, others purely practical – for example, where to connect it all, what happens in January when the sun hasn't shone for a month, or whether it even makes sense to invest in a solar system when the house is located in northern Slovakia. Over the years we've heard hundreds of such questions, and in this article we've collected them, sorted them, and answered them the way a technician would – someone who has seen it all firsthand: the installation, the selection mistakes, and the satisfying results after years of operation.

This article is long. Intentionally so. Solar collectors are not a simple matter, and anyone who tells you that you'll understand it in five minutes is hiding something from you. Skim the headings and jump straight to the section that interests you – or read it all if you want to truly understand it before you call a technician.

What a solar collector actually does – and what it doesn't

The basic function of a solar collector is simple: it captures solar radiation and converts it into heat, which is transferred to the heat-transfer fluid circulating in the system. This fluid then travels to the hot water tank or to the underfloor heating circuit. Nothing more, nothing less.

What a solar collector does not do: it does not generate electricity (that's photovoltaics – a completely different technology), it does not supply you with heat 24 hours a day throughout the year without a backup source, and it is not self-sufficient in winter when you have three cloudy weeks in a row. This is important to understand right from the start, because these misconceptions are precisely what lead to disappointment after installation.

A solar collector is ideal as the primary source for heating domestic hot water (DHW) in the summer months and as a significant aid for DHW preparation and, partly, for heating during transitional periods (spring, autumn). In winter, especially from November to February, its contribution in central Slovakia is substantially lower – depending on the collector's orientation and tilt, it can cover 10 to 30% of the heat demand during this period.

Which type of collector to choose – flat plate or tube?

This is probably the most frequently asked question of all. And the answer isn't as simple as many would like. In Slovakia, each type has its own place, depending on the specific conditions.

Flat plate collector is robust, proven, and cheaper to buy and service. It has a simple structural arrangement: the absorber (an aluminum or copper plate with piping) is enclosed in a frame with glass on the front and thermal insulation on the back and sides. It is less sensitive to mechanical damage, easier to service, and if problems occur, replacement parts are more readily available. In the summer months it has comparable or better performance than a tube collector at higher ambient temperatures.

Tube (vacuum) collector has better efficiency at lower ambient temperatures and diffuse radiation – i.e. in cloudy weather and winter. The downside is a higher price, more complicated service, and greater sensitivity to hail. In cases where roof space is limited and the customer needs to get as much energy as possible from a smaller area, a tube collector may be the better choice.

For a typical family house in Slovakia, where the main goal is DHW heating and possibly heating support, we recommend a flat plate collector in most cases. You'll find a more detailed comparison in the article Flat Plate vs. Tube Collector – Which Type Is More Worthwhile.

What do designations like AlCu mean for solar collectors?

The absorber material designation is important because it directly affects efficiency, lifespan, and price of the collector.

AlCu means the absorber is made of aluminum (Al) with copper tubes (Cu). This is currently the most popular material combination for flat plate solar collectors. Aluminum has excellent absorption properties and is relatively light, making it easier to handle during installation. Copper tubes ensure excellent thermal conductivity and long service life without corrosion – provided the correct heat-transfer fluid is used.

Our Flat Plate AlCu Solar Collector with Structural Glass is a typical example of this solution – an absorber made of an aluminum lamella with copper tubes and a proven selective coating that maximizes absorption of solar radiation and minimizes radiation losses.

When choosing, always check which selective coating is used on the absorber. Modern collectors have a PVD (Physical Vapour Deposition) coating with absorptance α ≥ 0.95 and emissivity ε ≤ 0.05. These values are found in the collector's technical data sheet and should be compared.

What is the difference between structural and anti-reflective glass?

The glass on the front of the collector is not ordinary window glass. It is special solar glass with a high iron content, achieving a transmittance of τ ≥ 0.88 (compared to about 0.80 for ordinary glass). However, the difference between glass types is important, and customers rightly ask about it.

Structural glass has a fine pyramidal or prismatic texture on its surface, which scatters incoming light and reduces its reflection. The advantage is a lower price and higher scratch resistance. The downside is that at a more oblique angle of sunlight incidence (low above the horizon in winter months), its efficiency is lower than that of anti-reflective glass.

Anti-reflective glass has a special surface treatment (a coating of silicon dioxide or similar compounds) that minimizes light reflection. Transmittance can reach τ ≥ 0.94. This means that under the same conditions, 5–8% more energy reaches the absorber. Over a year, this can represent a real contribution to energy yield – for a typical area of 2–3 collectors, an estimated extra 50–150 kWh per year.

If you're interested in this topic in more detail, we dedicate an entire separate article to it: Structured vs. Anti-Reflective Collector Glass – What's the Difference. Here we'll just say that for more demanding customers with a well-oriented roof and the ambition to maximize yield, we recommend the Flat Plate AlCu Solar Collector with Structural Anti-Reflective Glass, which combines the advantages of both technologies – a structured surface that reduces reflection plus an anti-reflective coating.

How many collectors do I need?

The golden rule for DHW heating in a family house: allow 1.0 to 1.5 m² of collector area per person. For four people, that's 4–6 m². A standard flat plate collector has an area of approximately 2.0–2.5 m² (aperture area, not the outer frame dimension), so 2 units are optimal for a four-person family.

If you want to combine the solar system with heating support (so-called solar combi-systems), the number of collectors increases – typically to 8–15 m² of aperture area for a family house. It depends on the energy demand of the house, the type of heating (underfloor vs. radiators), and geographical location.

For an accurate calculation, you need to know:

Number of people in the household and average daily DHW consumption (approximately 40–60 liters per person at 45 °C)
Planned tank volume (minimum 50–80 liters per person, optimally 80–100 liters)
Roof orientation and tilt (south ±30° and tilt 30–60° is optimal)
Any shading (trees, neighboring buildings, dormers)

You'll find a detailed calculation guide in the articles What Solar Collector Output Do I Need for My House and Collector Dimensions and Area – How Many Units Do I Need.

How long is the payback period for investing in solar collectors?

A question everyone wants a precise answer to, but the precise answer depends on too many variables. Nevertheless, we'll try to give a realistic estimate.

A typical installation for a family house – 2 flat plate AlCu collectors, a 200–300 liter tank, circulation pump, expansion vessel, controller, installation – currently costs 1,800–3,500 EUR depending on the chosen components and installation complexity. If you have an easily accessible roof, direct connection, and no need for a long pipe run, you'll be closer to the lower limit. If you need to deal with a complicated staircase connection, tank replacement, and electrical wiring, you'll be closer to the upper limit.

Annual savings depend on what you were previously using to heat water:

With an electric boiler at an electricity price of 0.22 EUR/kWh and an annual system yield of 1,200–1,500 kWh: savings of 264–330 EUR/year
With gas heating at a natural gas price of 0.09 EUR/kWh and boiler efficiency of 85%: savings of 127–159 EUR/year
With a heat pump (COP 3.5) for DHW heating: smaller savings, approximately 75–100 EUR/year

Payback with electric heating: 5.5 to 13 years. With gas: 11–28 years. The service life of a quality collector is 25–30 years. So the math works out favorably in many cases, especially considering rising energy prices and available subsidies.

Subsidies: At the time of writing this article, options exist in Slovakia to draw contributions through the Environmental Fund (Envirofond), or regional programs. Always check the current status before purchasing, as conditions change. A subsidy can shorten the payback period by 20–40%.

What happens to a solar collector in winter?

Winter is a topic where we encounter the most myths. First myth: solar collectors don't work at all in winter. That's not true. They do work – just with a substantially lower yield. Second myth: collectors freeze and crack if the system is left unattended during a vacation. With proper installation using anti-freeze heat-transfer fluid, this isn't an issue.

Heat-transfer fluid (solar fluid or propylene glycol) is designed to withstand frost down to at least -28 °C, commonly down to -35 °C. The concentration must be maintained, and the fluid should be replaced every 4–6 years, as it degrades due to heat and UV radiation.

Stagnation is a phenomenon that occurs when the collector produces more heat than the system can dissipate – for example, in summer with an empty tank and full sun. In flat plate collectors, the fluid temperature reaches 120–200 °C, and in tube collectors it can reach up to 250 °C. The system must be designed for this situation – the expansion vessel must have sufficient volume, the safety valve must be correctly set, and the pipe connections must withstand these temperatures.

You'll find a detailed look at winter operation in the article Winter Operation of Solar Collectors – What You Need to Know.

What are the optimal roof orientation and tilt?

The ideal orientation is south (azimuth 180°). A deviation of ±15° causes a yield loss of less than 5%, while a deviation of ±30° causes a loss of 10–15%. Collectors oriented southeast or southwest are still economically worthwhile.

As for tilt (angle from the horizontal plane):

For maximum annual yield: 30–40°
For maximum summer yield (DHW only): 20–30°
For a combination of DHW + winter heating support: 45–60°
Facade mounting (90°): only in very specific cases, lower annual yield but better winter yield and less of a stagnation problem in summer

Real-world example: A customer in the Banská Bystrica region had a gable roof with a 35° tilt, oriented southwest. The yield was 12% lower than with ideal orientation, yet they still achieved annual savings of over 280 EUR with electric heating – the system will pay for itself in less than 9 years. A north-facing facade? Not worthwhile there. A facade facing exactly south? Works surprisingly well, especially during winter months with a low-lying sun.

More details in the article Collector Tilt and Orientation – How to Maximize Energy Yield.

Is a building permit needed for installing collectors?

In Slovakia, installing a solar collector on an existing family house generally does not require a building permit, provided it does not fundamentally alter the exterior appearance of the building and the house is not in a heritage reservation or a protected zone around a heritage site. Most installations on a pitched roof fall into the category of a minor construction notification or a construction modification notification.

However, it's always advisable to check the situation with the local building authority in advance, as each municipality may have its own requirements, and conditions are stricter for houses in protected areas. The heritage inspectorate, for example, requires a permit in several historic town centers and may set conditions for the placement and appearance of collectors.

Do I need a tank for the collector? Can I use my existing boiler?

You almost always need a tank with a solar heat exchanger (a so-called bivalent tank or solar tank). A classic boiler with an electric heating element usually does not have a built-in heat exchanger through which the solar fluid from the collector could circulate. Therefore, it cannot be directly connected to the solar circuit.

A bivalent tank has a coil heat exchanger for the solar circuit in the lower part, and either an electric backup heater or an inlet from a boiler in the upper part. This arranges the energy logically: the solar circuit pre-heats the water in the tank from the bottom, and the backup heat source (boiler, electricity) only finishes heating it when needed.

Tank volume: 150–200 liters is enough for two people, 250–300 liters for four people, 350–400 liters for six people. A tank that's too small fills up quickly and the collector stagnates – wasting potential yield. A tank that's too large is uneconomical – the water ages and unnecessarily large backup heating is required.

What does the installation itself look like and what needs to be prepared?

Installing a solar collector is not a weekend project for an inexperienced enthusiast. It's a pressurized installation with hot fluid that must undergo a professional pressure test and be properly deaerated. Most tank and collector manufacturers make the warranty conditional on professional installation.

Typical installation procedure:

Step 1 – Project preparation: Calculation of collector area, tank selection, pipe route design, placement of the distribution unit (station with pump, expansion vessel, and controller)
Step 2 – Mounting the support structure: Anchoring into rafters or into the concrete surface of the roof (flat roof), static load check (a flat plate collector weighs 20–35 kg, with wind loads being higher)
Step 3 – Collector installation: Placement, interconnecting the collectors, connecting the pipes – we recommend copper or stainless steel piping, not galvanized (solar fluid reacts with zinc)
Step 4 – Installing the tank and solar station: In the utility room or boiler room, connecting to the existing heat source
Step 5 – Filling and deaeration: Filling with anti-freeze mixture, pressure test (at least 1.5 times the operating pressure, typically 5–7 bar), deaerating the entire circuit
Step 6 – Controller setup and commissioning: Setting the differential temperature controller parameters, setting the temperature limiter

The entire installation typically takes 1–2 days for a simple installation. More complicated jobs (facade mounting, combi-system, multi-story house with a long pipe run) can take 3–4 days.

You'll find a detailed procedure in the article Installing Solar Collectors on a Roof – Procedure and Requirements.

How many years does a solar collector last and what damages it?

A quality flat plate collector with an aluminum frame and tempered solar glass has a designed service life of 25–30 years. In practice we've seen collectors from the 1990s that still work – with replaced heat-transfer fluid and serviced seals. Of course, their efficiency is lower than that of new collectors, but the basic function remains intact.

What damages a collector the most:

Hail: Structural solar glass has resistance to hailstones of approximately 25 mm at a defined speed. Larger hailstones or extreme weather can break the glass. Anti-reflective glass is generally equally resistant, but it depends on the specific product.
Degraded heat-transfer fluid: Acidic fluid (pH below 6) corrodes the copper tubes and aluminum. Therefore regular pH checks and fluid replacement are necessary.
Insufficient deaeration: Air bubbles in the system increase noise, reduce flow, and can locally overheat the piping.
Mechanical damage to the absorber: During service access to the roof, care must be taken that no one steps on the collector.
Condensation inside the collector: With poor seals, moisture can get under the glass and reduce transmittance. Quality collectors have ventilated frames that prevent this condensation.

What certifications should a solar collector have?

When choosing a collector, always ask for these certificates and documents:

Solar Keymark – the European quality certification for solar collectors according to EN 12975. This is one of the key parameters, and without it you cannot apply for some subsidies.
CE marking – mandatory for sale in the EU
Technical data sheet with measured parameters: optical efficiency η₀, linear heat loss coefficient a₁ [W/m²K], and quadratic coefficient a₂ [W/m²K²]
Warranty certificate – standard 5–10 years on collectors, depending on the manufacturer

The lower the a₁ and a₂ values, the less heat the collector loses to the surroundings – meaning it performs better at low ambient temperatures (winter) and at higher operating temperatures.

Can I connect a solar collector with a heat pump?

Yes, a solar collector and a heat pump are compatible and in many cases complement each other very well. There are two common configurations:

Series connection (source regeneration): The solar collector lowers the temperature of the ground collector circuit or the heat pump's heat source, thereby increasing its COP. This is a technically more demanding configuration, more suitable for new buildings with a comprehensive design.

Parallel connection (shared tank): The solar collector and heat pump are connected to a shared tank. The controller ensures that whichever source is economically more advantageous at a given moment is the one that runs. Solar energy always has priority – the heat pump only kicks in when solar isn't enough.

This combination makes the most sense with older heat pump installations, where DHW is not their strong suit (air-to-water heat pumps with unsatisfactory COP when heating to 55–60 °C). In summer, the solar collector takes over this work entirely.

Most Frequently Asked Customer Questions (FAQ)

Is a solar collector worthwhile if we have a lot of cloudy days?

Most areas of Slovakia have 1,600–1,800 hours of sunshine per year, which is sufficient for economically viable operation of a solar system. Even with diffuse radiation (cloudy sky without direct sun), quality collectors achieve 15–30% of their nominal output. From practice: a customer in the Žilina region (one of the areas with a higher proportion of cloudy days) with 2 collectors and a 250-liter tank achieves 55–65% coverage of annual DHW demand – which is quite a standard result.

What happens when I go on vacation and the tank is full?

The system enters stagnation. The heat-transfer fluid evaporates from the collector into the expansion vessel, and the collector temperature rises to 150–200 °C. If the system is properly dimensioned (sufficient expansion vessel, safety valve, heat-resistant piping and seals), this is a safe condition. After returning from vacation, the system automatically resumes normal operation. We recommend filling the tank to maximum before leaving for an extended summer vacation – that thermal capacity "slows down" the onset of stagnation. Some controllers have a night cooling function (night bypass) that minimizes stagnation.

Can I mount collectors on a flat roof or on the ground?

Yes, both options are common. On a flat roof, special support structures with a 30–45° tilt are used, anchored to the roof or ballasted with concrete slabs (a system without penetrating the waterproofing). On the ground, more robust frames are needed, suitable where there's enough space and the roof isn't suitable (unfavorable orientation, weak structure, heritage protection). You need to account for a longer pipe run and possibly greater heat loss in the piping.

How do I know if my solar system isn't working properly?

The clearest sign: the tank isn't warm even on sunny summer days around 2 PM. Other symptoms: a noisy pump (air in the system), a drop in expansion vessel pressure (fluid or gas leak), visible corrosion on the collector or joints, poorly set differential control (the pump doesn't start even though the collectors are hot). The controller will always show you the collector temperature and tank temperature – if the collector is significantly warmer than the tank and the pump isn't running, something is wrong. You'll find a detailed diagnostic overview in the article Common Solar Collector Faults and How to Fix Them.

How do collectors differ in price, and is more expensive always better?

Not necessarily. Price differences are mainly due to frame material (aluminum vs. steel), glass type (structural vs. anti-reflective), quality of the absorber's selective coating, insulation thickness, and certification. A cheap collector without Solar Keymark certification and with an unknown selective coating can have 10–15% lower yield and a shorter lifespan. However, the difference between mid-range and premium price categories among reputable manufacturers isn't always large. Always compare parameters from the technical data sheet, not just price and appearance.

Do I need to service the solar system every year?

Annual servicing isn't essential, but we recommend a check every 2 years. What to check: system pressure (should be 1.5–2.5 bar when cold, according to the design), pH and density of the heat-transfer fluid (pH 6.5–8.5, density corresponding to protection down to -28 °C), joint tightness, condition of the expansion vessel (membrane pre-charge pressure), and controller functionality. Replacing the heat-transfer fluid every 4–6 years is more economical than later dealing with a corroded pipe or tank heat exchanger.

Conclusion: Solar collectors are a reliable technology – when properly selected and installed

Solar collectors are a mature, proven technology with predictable yield and a long service life. Most problems we encounter in practice don't stem from poor technology, but from poor selection (undersized tank, north-facing collectors, missing backup heat source) or from poor-quality installation (air in the system, bad seals, undersized expansion vessel).

If you're considering a purchase, start by choosing the right collector. For most family houses, we recommend a flat plate AlCu collector – either the version with structural glass for standard conditions, or the version with anti-reflective glass if you want a higher yield and have a well-oriented roof surface. Then check out the other articles in this Knowledge Center – How to Choose a Solar Collector – What to Watch Out for Before Buying is a good next step – and make your purchase thoughtfully. A well-designed solar system can serve you for 25 years or more without major complications.

Do you have a question about this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we're happy to advise.