Frequently asked questions about solar water heating systems
Frequently asked questions about solar water heating systems – expert answers from practice
Over the years we have been working in the field of sales and installation of solar systems, we have noticed that customers repeatedly ask the same twenty questions. Some are simple, others are quite technically sophisticated. In this article, we have gathered the most important ones and answered them as we would answer directly during a personal meeting – specifically, without unnecessary circumlocutions and with real numbers you can rely on in practice.
If you are looking for a more specific view on system selection, we also recommend the article How to choose a solar water heating system for a family house, or if you want to calculate the performance right away, see What solar collector power do I need – calculation based on the number of people and consumption. Here we focus purely on answers to specific questions that customers ask most often.
How does a solar water heating system actually work?
The basic principle is simple: a solar collector on the roof captures the thermal energy of the sun and transfers it to a heat transfer fluid (a mixture of water and antifreeze), which circulates through a closed loop to a hot water tank in the house. There, it transfers the heat through a heat exchanger and returns to the collector cooled down.
The entire loop is closed under pressure and operates as a thermosiphon controlled by a regulator. The regulator measures the temperature at the collector (using a sensor placed directly on the absorber surface) and the temperature in the tank (using a probe in the tank's well). When the collector is a set number of degrees warmer than the tank – typically 5 to 8 °C – the circulation pump turns on. When the difference drops below the lower limit (e.g., 2 °C), the pump turns off to prevent unnecessary energy loss from reverse transfer.
In practice, this means that in summer months you can have almost 100% solar hot water preparation, while in winter the system contributes only partially and a backup source (gas boiler, heat pump, electric heating) covers the rest.
Is solar water heating suitable for Slovak conditions?
This is the most common doubt we hear, and it is completely understandable. Slovakia is located in Central Europe and our climate is not the same as in Spain or Greece. But – and this is key – the annual solar potential of Slovakia is sufficient for a solar system to be economically worthwhile even without any subsidized benefits.
Specific numbers: the average annual total of solar radiation on a horizontal surface in Slovakia ranges from about 1,050 kWh/m² (Orava, northern areas) to 1,250 kWh/m² (southwestern Slovakia, Danubian Lowland). On southern slopes with a 35–45° tilt and a southeast to southwest orientation, a flat collector with an area of 2 m² can capture 800–1,100 kWh of usable heat annually. For a household of 4 people, which consumes about 2,800–3,200 kWh annually for hot water heating, this means a solar coverage of 30–40% without summer surplus and up to 55–70% in combination with summer operation.
A detailed comparison of the performance of flat and tubular collectors for our conditions can be found in the article Flat vs. tubular solar collector – comparison of types for Slovak conditions.
How many collectors do I need and what size?
The basic rule that works in the vast majority of cases: 1.5 to 2 m² of collector aperture area per person, a 50–70 liter tank per person. For a family of four, you therefore need 6–8 m² of collector area and a 200–300 liter tank.
In practice, this means that for a family of four in a standard family house in Central Slovakia, we recommend two units of flat frame solar collector IVAR.SOLAR 210 M5, which together give about 4.2 m² of aperture area at the beginning. If you have higher consumption or a lower inlet water temperature (cold groundwater), we choose three panels.
Do not blindly rely on online calculators that tell you "two panels are enough." Each project is different. The result is influenced by roof slope, orientation, shading from neighboring buildings, trees or a chimney, altitude, type of storage tank, and whether the family showers in the morning or in the evening (this significantly changes the daily demand curve).
Is the investment worth it? What is the return on investment?
Economic return depends on the price of the energy the solar system replaces and on the total installation costs. A typical installation for a family of four (2–3 collectors, 300 l storage tank, pump unit, piping, installation) currently costs 2,500–4,500 € including VAT, depending on the quality of components and roof accessibility.
If you replace electric water heating at an electricity price of 0.22 €/kWh and save 1,200 kWh annually, you save about 264 € per year. Return on investment = 3,500 / 264 = 13.3 years. If you replace gas (0.09–0.12 €/kWh, but with a boiler efficiency of 90 %), you save less – about 110–160 €/year, which extends the payback period to 22–32 years. Therefore, solar heating is most quickly profitable where it replaces electricity, or in households with a heat pump, where the price of heat is lower, but the systems complement each other well.
Important: energy price growth (historically 4–7 % annually) and collector lifespan (20–30 years with regular maintenance) should also be included in the calculation. With such an analysis, solar heating comes out positive almost always.
What is solar fluid and should it be changed?
The heat transfer fluid in the solar circuit is not just plain water. It is usually a mixture of propylene glycol (less toxic) or ethylene glycol with corrosion inhibitors, in a ratio that ensures freeze protection down to –28 °C to –35 °C. This mixture is necessary for two reasons:
- Frost protection: in winter, temperatures in the collector drop well below zero, and with regular water the piping would crack.
- Protection against boiling (stagnation): in summer, when the storage tank reaches maximum temperature and the pump stops, the temperature in the collector can rise to 180–200 °C. The special fluid can handle this without permanent damage.
Solar fluid degrades over time: it oxidizes, loses inhibitors, its pH drops and it becomes corrosive. Rule of thumb from practice: check every 2 years, replace every 5–7 years – or sooner if pH drops below 7 or glycol darkens. Checking is simple using pH strips and a refractometer to measure concentration.
Why is proper regulation and sensors important?
Regulation is the heart of the whole system. If the sensors measure inaccurately or are placed incorrectly, the system either does not pump when it should (you miss solar gains) or pumps unnecessarily (the pump cools the tank). In both cases, the customer complains about a "bad solar system," even though the problem is elsewhere.
The sensor on the collector must be placed at the hottest point – on the outlet pipe of the collector, or directly in a special well on the collector. The tank sensor must be located in the lower third of the tank, near the outlet from the solar heat exchanger. If the tank sensor is too high, the system measures hot water that has not yet been heated by the solar system, but by old residual energy from the boiler, and the pump unnecessarily stops.
We recommend using high-quality sensors with sufficient durability – for example, temperature sensor into a well, resistant up to 180 °C with a 2 m cable, which will easily withstand stagnation temperatures in the collector. For tanks at a greater distance from the controller, a well sensor with PVC cable 4 m long, resistant up to 95 °C is suitable – it is dimensioned for tank temperatures, where we do not exceed 95 °C.
More about regulation, regulator functions, and when to replace sensors can be found in the article Sensors and regulation of a solar system – how they work and when to replace them.
What pipe to use for the solar circuit?
This is an area where customers often save in the wrong place. Common copper pipe with plastic insulation is not ideal for a solar circuit – plastic degrades at high temperatures and UV radiation, and in an outdoor environment (on the roof, under the roofing) standard insulation lasts 5–8 years. After its breakdown, heat losses increase so much that the system loses up to 30–40 % of the total annual gain.
We recommend pre-insulated stainless steel pipe in rubber insulation double with cable (2× pr. 16, length 10 m). The rubber insulation (EPDM) can withstand temperatures up to 150–180 °C and UV radiation without degradation for the entire lifetime of the system. The stainless steel pipe is resistant to glycol mixtures and to stagnation temperatures. The cable inside the insulation allows the wiring for the sensors without the need for a separate route – which saves time during installation and prevents leaks through additional penetrations through the roof.
The pipe diameter depends on the system's capacity and the length of the route. For systems with 2–4 collectors and a route length up to 15 m, DN 16 (outer diameter 16 mm) is usually sufficient. For longer routes or larger collector assemblies, DN 20 is more suitable. A more detailed calculation can be found in the article Dimensioning solar piping and pump unit – how to do it.
What is a solar pump unit and why is a regular pump not enough?
A solar pump unit is a compact block that integrates several components into one unit: a circulation pump, safety valve, pressure gauges, check valve, air vent, filling/drain valve, and often a flow meter. All these components are essential for reliable operation of a closed solar loop.
A regular circulation pump (used in central heating) is not suitable for a solar loop for several reasons:
- Seals and plastic parts are dimensioned for a maximum of 110–120 °C, not for stagnation temperatures of 180–200 °C.
- It is not resistant to glycol mixtures (some plastic types may swell).
- It lacks integrated safety components – safety valve, air vent.
- It does not have a control input for a solar controller (PWM or on/off control).
A good example of a complete solution is the solar pump unit IVAR.SOLAR K with regulation IVAR.SOLAR IMTDC. This unit has integrated regulation built directly into the body – there is no need to buy a controller separately and to solve the wiring between the unit and an external controller. The IMTDC controller supports differential regulation (T1 vs. T2), maximum tank temperature, collector frost protection function, and tank night cooling function.
What happens to the system in winter? Will it freeze?
This is one of the most common concerns, and our answer is: with a properly designed and filled system, freezing is not a risk. Solar fluid (propylene glycol mixture) is dimensioned to provide protection down to –28 to –35 °C, which is more than enough for even the coldest locations in Slovakia. Nevertheless, it is important to check the concentration – over the years, the fluid may slowly lose its concentration (for example, when topping up the system with pure water after a leak).
Modern controllers also have a frost protection function for the collector: if the temperature on the collector drops below a set threshold (e.g., +4 °C), the controller briefly starts the pump and circulates warmer fluid from the tank into the collector. This is a safety function – it does not replace proper fluid concentration, but it prevents damage in borderline situations.
In winter, the system is active whenever the sun is shining. Even in January, at temperatures of –10 °C, the collector can reach 40–60 °C on a clear day and deliver 2–4 kWh of heat to the tank per day. It is not a lot, but it is free and the system operates without your involvement.
Do I need a building permit for installation?
According to the Slovak Building Act (Act No. 50/1976 Coll. in the version amended by later legislation) and Decree No. 453/2000 Coll., small solar systems on family houses (collectors on an existing roof, without changing the building structure) are usually classified as a "simple construction" or "minor construction" and do not require a building permit, but only notification to the building authority. In practice, it depends on the specific building authority and whether it is in a heritage zone. We always recommend checking the situation in advance at the local building authority – the consultation is free and will save you problems later.
If you want to be sure, ask the installation company for an opinion, as experienced installers have experience with your local authority and can tell you what procedure is usually required in the given municipality.
Can I connect the solar system directly to an existing water heater tank?
In principle, no – at least not without modification. A standard hot water tank heated by an electric heating element or boiler does not have a solar heat exchanger (pipe in the lower part of the tank). A solar system requires a tank with two heat exchangers or at least a solar heat exchanger in the lower zone.
There are three common solutions:
- New bivalent tank: a tank with two heat exchangers – lower for the solar loop, upper for the boiler or electric heating. The cleanest and most efficient solution.
- External heat exchanger (plate heat exchanger): the solar fluid passes through an external plate heat exchanger, which heats the potable water loop from the existing tank. Less efficient (higher temperature losses at the heat exchanger), but usable for retrofit installations.
- Open container: in some older systems with an atmospheric water heater, direct connection is used – this is, however, unsuitable from a hygiene and safety perspective and is not allowed in new installations.
What is stagnation and is it dangerous?
Stagnation occurs when the tank has reached the maximum temperature, the controller stops the pump, but the sun is still shining on the collector. The temperature in the collector starts to rise rapidly – in a flat collector typically to 160–200 °C, in a tubular collector even to 250–300 °C. The solar fluid begins to boil and turn into vapor (or glycol vapor), which pushes the fluid back into the expansion tank.
Stagnation is not immediately dangerous, but repeated stagnation (every day all summer) degrades the fluid, causes sedimentation in the pipes, shortens the lifespan of the pump seals, and may lead to micro-leaks. The solution is to correctly set the maximum tank temperature (70 °C), enable the tank cooling function (night cooling – the controller runs the pump at night and transfers excess heat to the collector, where it is lost to the environment), or set the "antileg" function (antilegionella heating to 70 °C once a week, which, however, does not solve the summer surplus).
How long does the installation of a solar system take?
A standard installation for a family house (2–3 collectors, 200–300 l tank, pump unit) takes 1–2 working days with an experienced team of two to three people. On the first day, the load-bearing structures and collectors are usually installed on the roof, and the piping is laid. On the second day, the internal installation is completed: the tank, pump unit, connection, filling of the system, bleeding, setting of the controller, and test operation.
Complications that prolong the installation: access to the roof (steep gable roofs, missing scaffolding), long pipe routes through living rooms, simultaneous work with other renovation tasks, or the need to wait for masonry work (penetrations through perimeter walls). More about the entire process can be found in the article Installation of a solar system step by step – from the collector to the tank.
Does a solar system require regular maintenance?
Yes, and this question is more important than it may seem at first glance. A solar system is a long-term investment, and its reliability depends on regular inspection. Recommended frequency:
- Annually: visual inspection of the collectors (cracked glass, corrosion of the frame, damaged seals), check of the pressure in the circuit (correct pressure 1.5–3 bar in a cold system), inspection of the pipe insulation condition.
- Every 2 years: chemical inspection of the solar fluid (pH, concentration, color, content of inhibitors), inspection of the expansion vessel (nitrogen overpressure).
- Every 5–7 years: replacement of the solar fluid, inspection of the pump seals, possibly replacement of the pump.
A complete overview of all inspections can be found in the article Maintenance and service of a solar system – what to check every year.
Can I use a solar system to support heating?
Technically yes, but economically and in terms of performance, it is a compromise. Solar combination systems (combination of hot water heating and heating support) require a larger collector area (usually 8–15 m² for a family house), a larger tank (400–1,000 l), and more complex hydraulics. The problem is that when heating is most needed (November–February), solar potential is lowest. During the transitional period (October, November, April), the system can significantly help, but it should not be the primary argument for purchase.
For most Slovak households, it is economically more advantageous to install a solar system primarily for hot water heating (with the possibility of summer surplus for a swimming pool) and to solve heating with a heat pump or condensing boiler. If you are interested in a combination system, we recommend consulting a designer who will prepare an energy balance specifically for your conditions.
What is the difference between a flat and a tubular collector?
A flat collector (flat framed) has an absorber surface protected by a glass cover in a framed body. It is more robust, more resistant to hail, easier to integrate into the roof, and more affordable. A tubular collector (vacuum tubes) has higher efficiency at low temperatures and diffuse radiation, but it is more expensive, more fragile, and during stagnation it reaches extremely high temperatures.
For standard hot water heating in a family house in Slovakia, a flat collector performs very well in a comparative analysis – easier installation, lower cost, fewer problems with stagnation. A detailed comparison with numbers for our conditions can be found in the article Flat vs. tubular solar collector – comparison of types for Slovak conditions.
Frequently asked questions (FAQ)
Is a solar system worth it even if I have a gas boiler and gas is cheap?
With current gas prices in Slovakia (0.08–0.12 €/kWh), the payback period of a solar system as a supplement to a gas boiler is longer – it ranges around 18–28 years. This is at the edge of economic attractiveness if we do not take into account subsidies and the assumed increase in gas prices. However, if the primary motivation is environmental (reducing CO₂ emissions) or energy independence, it is worth it even with gas. The situation also changes if you consider a solar system as a supplement when replacing an existing tank, since you would be replacing the tank anyway.
Can I install a solar system myself, without a professional?
Mechanical installation (support structures, collectors, piping) is not technically inaccessible for a skilled DIY enthusiast. More problematic are works on the pressure circuit (filling, pressure tests) and the electrical installation of the controller – these should be performed or at least checked by a qualified professional. From the perspective of component manufacturer warranties: most manufacturers condition the warranty on professional installation with a certificate. Therefore, if you want full warranty on the collectors and pump unit, the installation should be done by a certified installer.
What happens if it is cloudy for a long time – will I run out of hot water?
No, if the system is properly designed with a backup source (backup). A solar system is always dimensioned as a supplementary system – the existing boiler, heat pump, or electric heating element in the tank covers what the solar system cannot achieve. A bivalent-type tank has an electric or boiler heating element in the upper zone, which turns on automatically when the water temperature drops below the set limit (usually 50–55 °C). You will not even notice the difference.
How can I find out that my solar system is working properly and not losing performance?
The most reliable way is a heat energy meter (calorimeter) on the solar circuit – it records the flow and temperature difference between the supply and return and calculates the delivered kWh. By comparing with expected values (according to the size of the collector area and annual solar potential for your region), you can determine whether the system is working properly. A cheaper but less accurate method is to monitor the temperature difference: if the controller shows T1 (collector) significantly higher than T2 (tank) on a sunny day and the pump is not running, something is wrong. It could be a stuck pump, controller failure, or sensor malfunction. More about causes and solutions can be found in the article Stuck or weak solar pump – causes and solutions.
Can I connect a swimming pool to solar collectors?
Yes, and this is one of the smartest ways to use the summer surplus of solar energy, which would otherwise cause stagnation. The pool circuit is connected via a three-way valve and a special hydraulic circuit so that it has lower priority than the hot water tank – the pool is heated only when the tank has reached the desired temperature and the collector is still producing heat. An advantage is that the pool water can be run directly through the collector (without a heat exchanger, if the pool has water treatment suitable for low-alloy steel), which increases efficiency. Solar coverage of the pool for 3–4 months a year is realistic without any additional costs.
Is it necessary to change the fluid in the system if the system is still working?
Yes, even if the system appears to be "working". Degraded fluid with low pH corrodes metal parts of the system (collector absorber, tank heat exchanger, pump body) and corrosion deposits clog fine sections of the system. The result is reduced flow, lower performance, and in the worst case, a cracked absorber or heat exchanger. The cost of fluid replacement is 80–150 € including labor – compared to the cost of a new tank or collector, it is a trivial investment. Do not skip it just because the controller still shows correct values.
Conclusion: what to take away from this article
Solar systems for water heating are a proven and relatively simple technology that, when properly designed and installed, reliably functions for 20–30 years with minimal maintenance. Most of the problems users encounter do not stem from faults in the collectors themselves, but from underestimated control, faulty sensors, unsuitable piping, or neglected fluid maintenance.
If you have decided to invest in a solar system, do not save on components that determine reliability – that is, on the pump unit, regulation, sensors and piping. For collectors, it is meaningful in a reasonable range to look for the price/performance ratio, but other system components should be dimensioned with a reserve and made from verified materials.
Further details on specific topics can be found in other articles of the Knowledge Centre – we recommend starting with the article How to choose a solar system for water heating in a family house, if you are at the beginning of the decision-making process, or How to properly place and mount solar collectors on the roof, if you are dealing with a specific installation.
Do you have a question regarding this topic?
Having trouble deciding or dealing with a specific situation in your household? Write to us – we will be happy to advise you.