Frequently asked questions about solar water heating systems

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Common questions about solar water heating systems – a complete overview of answers

Solar water heating systems are among the most frequently asked solutions that customers ask about when equipping a house or during reconstruction. At the same time, these are systems around which the most myths, half-truths and misleading information circulate – from unrealistic savings "free hot water all year round" to pessimistic "it doesn't make sense in Bratislava". The truth is, as usual, somewhere in the middle, but much closer to the positive side than many people think.

This article collects real questions that customers ask us every day – when choosing a system, during installation, during operation and when solving problems. We answer them as we would explain directly during an inspection of the boiler room or roof space. Without embellishment, with numbers and specific practical experiences.

Basic questions – what it actually is and how it works

How does a solar water heating system work at all?

The principle is simple: solar radiation heats a liquid (solar antifreeze mixture) in the collector on the roof, this liquid flows through the pipes down to the hot water tank, where it transfers energy to the water for bathing and washing through a heat exchanger. Then it returns back up to the collector, where it is heated again. This circuit is closed and completely separated from the drinking water.

We distinguish two basic types of solar systems according to the way the liquid circulates:

Active system (forced circulation): The circulation pump is controlled by a regulator that compares the temperature in the collector and in the tank. If the collector is 5–8 °C warmer, the pump starts. If the temperature drops below this difference, the pump stops. This is the standard method for single-family homes today.
Passive system (thermosyphon): The tank is placed directly above the collector and the liquid circulates by gravity. It has no pump, no regulator. The advantage is simplicity and zero electricity consumption. The disadvantage is limitation to warm climatic zones and aesthetic burden on the roof (tank visible).

Most of the systems we sell and install today are active – that is, with a pump and regulator. Passive thermosyphon systems make more sense in the Mediterranean or for cottages and garden houses, where there is no access to the electrical grid.

What is the difference between a flat and a tubular collector?

This is one of the most frequently asked questions and at the same time an area where customers decide longer. Briefly: flat collectors are more robust, easier to maintain and provide very good performance under direct sunlight. Tubular (vacuum) collectors achieve higher efficiency especially at lower temperatures and diffuse radiation – that is, in winter and in cloudy weather.

Practically this means: if you have a house in a lowland and you care about maximum production in the summer (heating a swimming pool, high HW consumption), flat collectors are an excellent choice. If you are at a higher altitude or you care about year-round production including winter months, tubular collectors have an advantage. More about this decision in the article Flat vs. tubular solar collectors – which type is more profitable, where we present specific performance comparisons.

Questions about performance and savings

How much hot water can a solar system realistically cover?

This number depends on several factors, but practical experience says this: a well-designed solar system for a single-family house covers 50–70% of annual hot water consumption. In summer months (May–September) this can be 90–100%, in winter months (December–February) it drops to 10–25%. The annual average for Slovakia is realistically somewhere between 55 and 65%.

Concrete example: a family of 4 people consumes about 2 000–2 500 kWh of energy annually for heating hot water. A solar system with 2 flat collectors (total area 4–5 m²) and a 300-liter tank covers about 1 000–1 400 kWh annually. The rest is heated by a boiler, heat pump or electric heating element.

How many years does it take for a solar system to pay for itself?

Return on investment is a topic for which there is no single correct answer – it depends on the price of the energy you are replacing, the size of the system, and how well the system is dimensioned. However, approximate figures from practice are as follows:

Cost of a complete solar system for a family house (2–3 collectors + tank + installation): 2,500–4,500 € depending on the type of collectors and tank
Annual savings when replacing electricity: 250–450 €/year (at an electricity price of 0.20–0.25 €/kWh)
Annual savings when replacing natural gas: 120–250 €/year (at a gas price of 0.08–0.12 €/kWh)
Estimated payback period when replacing electricity: 7–12 years
Estimated payback period when replacing gas: 12–18 years

If you add available subsidies (such as the "Green for Homes" program and similar), the payback period is significantly reduced. The technical lifespan of solar collectors is 20–30 years, so even with a payback period of 12 years, you still have at least 8–18 years of pure profit.

Questions about dimensioning and system selection

How many collectors and what size of tank do I need?

A basic rule for an approximate calculation: calculate 1–1.5 m² of collector area per person and 60–80 liters of tank per person. For a family of four, this means 4–6 m² of collector area and a 250–320 liter tank.

Reality is a bit more complicated, however. It depends on the slope and orientation of the roof, the altitude, whether you cook at home and thus have higher consumption, or whether you want to heat a swimming pool. An accurate calculation should take into account solar radiation databases for the specific location. In Slovakia, the differences between the south (Podunajská nížina – approx. 1,300–1,400 kWh/m²/year) and the north (Orava, Záhorie – approx. 1,050–1,150 kWh/m²/year) are quite significant. More about dimensioning can be found in the article What solar collector power do I need for my house.

Is it important which side of the world the roof is facing?

Yes, very much so. The ideal orientation is south (full sun throughout the day), or alternatively southeast or southwest – the performance loss compared to south is only 5–10 %. East or west are still usable, but you lose 20–30 % of annual production. North is practically unsuitable.

The roof slope should be 30–50° – this is the ideal range to maximize annual production. Collectors can also be mounted on a flat roof using a mounting structure with a set slope, or on a façade – but in that case, the performance is significantly lower and the solution is more complicated.

From experience: a customer with a house oriented to the southwest and a roof slope of 38° achieved annual production only 7 % lower than the theoretical maximum for south and 45°. This is a fairly negligible difference. On the other hand, a customer with an east-facing orientation and a slope of 15° achieved only 68 % of the ideal – in that case, we had to dimension the system larger to cover the same need.

Questions about installation and wiring

Can I install the solar system myself?

Technically yes – some systems are sold as DIY kits and people do install them. However, there are several important warnings:

Hydraulic part (collectors, piping, tank) is manageable for a layperson with care and technical skill. You need to know how to solder or crimp copper or stainless steel piping, properly pressure-test the circuit, and fill it with solar fluid.
Electrical installation (connecting the pump, controller, backup heating) must be done by an electrician with the appropriate authorization – this is the law, not a recommendation.
Warranty and inspection: If you do not want to lose the warranty on components and want the system to be insurable, we recommend installation by a professional company with an issued inspection report.
Work at heights: Mounting collectors on a sloped roof is physically demanding and risky – it is not safe without scaffolding or a roof ladder.

More about the installation process can be found in the article Installation of a solar system step by step – what you need to know.

What kind of liquid is used in the solar circuit and why?

In the primary (solar) circuit, water is not used, but a special antifreeze mixture – usually a solution of propylene glycol and water in a ratio that ensures protection against frost down to −28 to −35 °C. Propylene glycol is unlike ethylene glycol (automotive antifreeze) non-toxic for food applications and approved for solar systems.

This mixture has several important properties: it can withstand temperatures up to 180–200 °C without decomposition, has excellent anti-corrosion properties, and a long lifespan. It is recommended to replace or check it every 4–6 years – pH and protective additives degrade over time. Never use regular tap water – it causes limescale deposits in the collector and heat exchanger and freezes in winter.

How is the solar system connected to the boiler or heat pump?

The solar system functions as a "preheating" system – the water in the storage tank is first heated by solar energy, and only if the temperature does not reach the desired level (usually 45–55 °C for regular use, or 60 °C for legionella hygiene), the boiler or heat pump will kick in and heat the water to the desired temperature. This is called a bivalent connection.

A bivalent storage tank usually has two heat exchangers – a lower one for the solar circuit (heats from the bottom, maximizing the space for solar energy), and an upper one for the backup source (boiler, heat pump). Electric backup (heating element) is usually located in the upper part of the tank – it heats only a small amount of water quickly, if needed.

Learn more about this combination in the article Solar system in combination with a boiler or heat pump.

Questions about operation and maintenance

What to do if the system stops working or does not heat water?

First steps in diagnostics when the system is not working properly:

Check the controller: Does it display the temperature in the collector and in the tank? If the collector shows 0 °C or -30 °C in summer, the collector sensor is likely broken. If the tank shows the same temperature as the collector but the pump does not run, check the differential setting on the controller.
Check the pressure in the system: The manometer on the manifold should show 1.5–2.5 bar when the system is cold. If the pressure is below 0.5 bar, the system is under-pressurized – likely there is a leak in the solar mixture or the expansion tank is damaged.
Check the circulation pump: When it should be running (the collector is hot), can you hear it? If not, it may be blocked (stalled shaft) – many pumps have a screw on the front for manual rotation.
Cracked collector or leak: If you see a leak or a noticeable pressure drop without an obvious cause, call a service technician.

More about troubleshooting can be found in the article Common solar system faults and how to fix them.

What is stagnation and is it dangerous?

Stagnation occurs when the storage tank is fully charged to the maximum temperature and the collectors continue to absorb solar radiation – the pump stops, but the temperature in the collector continues to rise. In flat collectors, stagnation temperature can reach 160–200 °C, in tubular collectors even 250–300 °C. This causes the solar mixture in the collector to evaporate and gas to be released into the expansion tank.

This is not directly dangerous if the system is properly designed (correctly dimensioned expansion tank, safety valve, correct type of solar mixture). However, repeated stagnation accelerates the degradation of the solar mixture and can damage seals and rubber parts. A solution is proper system sizing (do not over-dimension the collectors), or installation of a cooling/blocking system.

How often and what should be checked in a solar system?

Recommended maintenance schedule:

Once a year (spring): Visual inspection of collectors (cracks, cover damage, dirt), check the pressure in the system, check the function of the pump and controller.
Every 2 years: Check the pH and concentration of the solar mixture using test strips or a refractometer. If the pH drops below 7.0 or the freeze protection temperature rises above −15 °C, the mixture needs to be replaced.
Every 4–6 years: Replace the solar mixture, check the membrane of the expansion tank (pre-charge pressure), check the safety valve, check all connections and seals.
As needed: Clean the collector glass from dust, bird droppings, etc. – in dry areas with low dust levels, this is usually sufficient to do with rain, otherwise once per season with a damp cloth.

A detailed maintenance schedule can be found in the article Maintenance and service of solar collectors – what and when to check.

Questions about performance under non-ideal conditions

Does a solar system work in winter or on cloudy days?

Yes, it does – but the performance is significantly lower. The system operates whenever the temperature in the collector is at least 5–8 °C higher than in the storage tank. This occurs even on cloudy days when diffuse radiation is sufficient. In December and January in Slovakia, a solar system is actively running on average only 2–4 hours per day, but even these hours contribute something – typically 10–20 % coverage in winter months.

Tubular (vacuum) collectors are better in this regard than flat ones – thanks to vacuum insulation, they lose less heat and use diffuse radiation more efficiently. Learn more about this topic in the article Does a solar system pay off in cloudy weather or in winter?.

What happens to the system when I go on vacation?

This is one of the practical questions that is occasionally overlooked. If you are away for 2–3 weeks in summer and the storage tank is fully charged, the system will start to stagnate (see above). Modern controllers have a "vacation" or "overheat protection" function – when the tank temperature exceeds a set value (e.g. 85 °C), the controller opens the cooling circuit or activates night cooling (lets the pump run at night when the collector is cold).

If your controller does not have such a function, cover the collectors before a long vacation (special tarps are available for this) or at least reduce the maximum tank temperature setting. Alternatively, leave someone at home who can fill the bathtub or run hot water once a week – this discharges the tank and protects the system.

Questions about safety and legislation

Do I need any permits to install a solar system?

In the vast majority of cases, no for a family house. The installation of solar collectors for water heating (not photovoltaics) on a family house is not considered a construction activity requiring a building permit – it is a technological device. An exception is listed buildings and listed areas, where consent from the heritage office is required, and some strict zoning plans.

The electrical installation (pump, controller) must be inspected and certified by a qualified electrician. If you are applying for a grant from state programs, installation by a certified person (SIEA certificate or similar) and technical documentation is usually mandatory. Always read the current conditions before applying for a grant – the rules change slightly every year.

Can a solar system damage the roof or structure of the house?

Properly dimensioned and professionally installed systems do not damage the house structure. Mounting systems are designed to distribute the weight of the collectors (20–40 kg/m²) across a sufficient number of points and not to interrupt the roof waterproofing. Special hooks are used that pass under the tiles and anchor into the rafters – the roof remains uninterrupted.

Problems arise with cheap or incorrect constructions that cut through the roofing and seal only with silicone – which cracks after years. Always insist on a mounting system from a reputable manufacturer with certification and a waterproofing warranty.

Most frequently asked questions (FAQ)

Is a solar system worth it with electric water heating?

Yes – with electric heating, a solar system is the most economically advantageous, as electricity is relatively expensive. A solar system with an investment of 2,500–4,000 € can save 300–450 € per year on electricity, which means a return on investment of 6–10 years. If the storage tank is old and needs to be replaced anyway, the investment costs are reduced to just the collectors and installation – the return on investment is even better.

Can I use a solar system for heating?

Technically yes, but practically it is less efficient. Solar heating systems (combination of hot water and heating) are larger and more complex – they require 3–6× more collector area and a 500–2,000 liter storage tank. The problem is that heating is needed most in winter when solar energy is least available. Solar heating preheating systems make sense as a supplement to low-temperature floor heating, but they cannot replace the main heat source. The main use of solar systems remains hot water heating.

What is the lifespan of solar collectors and the storage tank?

Quality flat collectors can last 25–30 years with regular maintenance. Tubular collectors have a similar lifespan, but the glass tubes are more fragile – after 15–20 years, one or two tubes may need to be replaced. A stainless steel or enamelled storage tank can last 15–25 years – it depends on the material quality and water quality. The circulation pump and controller have a lifespan of 10–20 years and are cheap to replace. Solar fluid is changed every 4–6 years.

Can I expand the system later if I want more performance?

Yes, but with conditions. Collectors can be added if the storage tank and piping system are dimensioned with a reserve. You need to check whether the expansion tank can handle the larger volume, whether the pump is sufficient for a longer circuit, and whether the tank has the capacity to accept more energy. It is usually easier to add a second parallel collector row than to expand the existing row. When planning the system, it is wise to consult in advance if you are considering future expansion.

What happens if the solar fluid freezes?

A properly filled system with propylene glycol fluid down to −28 °C will not freeze even in extreme cold in Slovakia. If, however, the fluid is diluted (due to a leak and water added) or old and degraded, the protection temperature increases and freezing can occur during frost, causing pipes or collectors to crack. This is one of the most expensive problems, so it is worth checking the fluid every 2 years. If you know the system is not used in winter for a long time (a cottage), consider draining the system or checking the fluid concentration.

How can I tell if my solar system is working properly?

Modern controllers display the current temperatures in the collector and storage tank, the pump running time, and in some cases also an estimated energy production. If the controller on a sunny summer day shows a collector temperature of 60–90 °C and the pump has been running for several hours, the system is working properly. If the collector shows the same temperature as the tank (and the tank is not very warm), something is wrong. For accurate measurement of production, it is worth installing a calorimeter (flow meter with temperature measurement) – from experience, only about 20 % of customers have one, but those can precisely tell how many kWh the system produces annually.

Conclusion – what to take away

Solar systems for water heating are a proven, reliable, and economically sensible technology. They are not miracle devices that will eliminate all your energy costs, but they are real working devices with a 20–30 year lifespan that, when properly designed and installed, cover 55–70 % of annual hot water consumption and pay for themselves relatively quickly.

The key to satisfaction is proper dimensioning (not too small, not too large system), quality components, and professional installation. And then just regular, low-maintenance service once a year. If you came across a topic while reading this article that you would like to know more about, read other articles in our Knowledge Center – for example How to choose a solar system for water heating for a family house or How to set up and start a solar system. We have covered all the important topics from selection to service.

Specific products – collectors, storage tanks, controllers, and complete solar sets – can be found directly in the solar systems category on atria.sk, where you can compare parameters and choose the most suitable solution for your home.

Do you have a question about this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us – we are happy to help.