Solar system in combination with a boiler or heat pump

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Solar system in combination with a boiler or heat pump – complete technical guide

When designing heating and hot water preparation in a single-family house, you will sooner or later come across a crucial question: solar collectors are not sufficient to cover the annual heat demand on their own. In summer months, their performance easily exceeds consumption, while in winter they produce only a fraction of what the household needs. The solution is an intelligent combination of the solar system with another heat source – a gas, pellet or wood boiler, or a heat pump. This combination forms the basis of modern, energy-efficient systems that have proven themselves in practice as the most economically and comfortably advantageous solution.

In this article, we will take a detailed look at how such combined systems work, what technical conditions are required for their proper connection, what you need to know when designing them, and what you must not forget during implementation. A large part of the practical information comes directly from practical experience – we have seen dozens of installations, from those that have been working flawlessly for years to those where the poor connection of the solar circuit to the existing boiler caused problems already in the first year of operation.

Why a solar system alone is not enough and needs a backup source

A solar collector produces heat only when the sun is shining – and even then it depends on the angle of incidence of sunlight, cloud cover, collector contamination, the season, and geographic location. In Slovakia, you can expect 5–7 hours of effective solar irradiation per day in summer, while in December and January it is often only 1–2 hours, or even less. The annual production of solar energy on a flat collector with an area of 2 m² in Central Europe is around 700–1 000 kWh, which covers approximately 50–70 % of the annual hot water consumption for a family of four.

This means that the remaining – at least 30–50 % of the energy – must be supplied by another source. Without a backup source, the family would have no hot water at all in winter, or only cold water from an unheated tank. Therefore, every properly designed solar system includes a bivalent tank with two heat exchangers or another way of integrating the backup source. This article is entirely about that.

Basic types of combined systems

In practice, we encounter several typical combinations. Each has its strengths, technical requirements, and suitable conditions for use. Let's go through them systematically.

Solar system + condensing gas boiler

This is the most common combination in Slovakia. A condensing gas boiler is reliable, relatively cheap to install, and a gas connection is available in most towns and villages. The solar system complements the boiler mainly in the warm half of the year, when it can cover the entire hot water consumption without turning on the boiler. The boiler thus does not play any role at all in the summer months – which is not ideal for it, but modern condensing boilers are adapted to this operation.

Technically, the combination is implemented via a bivalent hot water tank (TÚV). It has a lower heat exchanger connected to the solar circuit and an upper heat exchanger (or electric booster, or direct connection) to the boiler. The boiler only takes over when the temperature in the tank drops below a set limit – typically 45–50 °C. The regulation is automatic via a solar controller that communicates with the boiler.

It is important to set the controller hysteresis correctly. If the boiler turns on too early, it "steals" the work from the sun – the tank is heated to 55 °C at night (by the boiler), in the morning the sun comes, but the tank is already warm and the solar pump circuit does not activate because the temperature difference between the collector and the tank is not sufficient. The result is low utilization of solar energy. Correct setting: the boiler only heats the upper third of the tank and only when the temperature in the upper part drops below 42–45 °C at the end of the day or almost in the morning.

Solar system + pellet or wood boiler (biomass)

Biomass boilers, specifically automatic pellet boilers or wood briquette boilers, are interesting from the perspective of combination with a solar system, but require a more complex design. A pellet boiler works most efficiently at a stable output – it does not like frequent switching on and off. Therefore, when combining with a solar system, an accumulator tank (puffer) with a sufficient volume is essential.

Typical scheme: solar collector → solar TÚV storage (bivalent) → buffer (accumulation) → pellet boiler. The pellet boiler charges the buffer, and the buffer supplies heating and TÚV. The solar circuit preheats the TÚV storage from the bottom, thus reducing the need for supplementation from the buffer and, consequently, the frequency of boiler activation. The result is a higher number of hours when the boiler operates in a stable mode at optimal performance – combustion efficiency increases and emissions decrease.

The buffer volume for a pellet boiler with a solar system should be at least 500–1,000 liters for a family house with a 15–25 kW boiler. When combined with a solar system with 8–12 m² of collectors, an ideal buffer is 800–1,500 liters. Do not forget that every liter of buffer costs money and takes up space in the boiler room – this is a real compromise that must be resolved in practice.

Solar system + heat pump

The combination of a solar system with a heat pump (HP) is technically the most interesting and, when properly designed, also the most energy-efficient. At the same time, it is a combination where design errors are most frequently made – because both sources are "green" and the customer or installer sometimes assumes that the more renewable sources, the better, regardless of their mutual interaction.

In reality, an important rule applies: a solar system and a heat pump can compete with each other or complement each other very well – it depends on how they are connected and set up. The heat pump operates most efficiently at low output temperatures (35–45 °C for floor heating). The solar system produces heat at temperatures of 40–80 °C. If the solar system heats the storage to 65 °C and the heat pump to the same storage, it only activates when the storage is cold – this is the correct connection.

Hydraulic connection – the key to the functionality of a combined system

The most common cause of malfunction in combined systems is not bad technology, but incorrect hydraulic connection. We have seen this repeatedly: the customer has a new heat pump, new solar collectors, a new storage – but the combination does not work because the return from the HP goes to the top part of the storage, where the solar heat exchanger is also working, and both sources steal energy from each other.

Bivalent storage – stratification principle

The basis of a correct hydraulic connection is the understanding of temperature stratification in the storage. Hot water rises to the top, cold water sinks to the bottom – this is a physical law that should be used, not suppressed. A properly connected bivalent storage has:

Lower heat exchanger (solar) – located in the lower third of the storage, where the water is the coldest. The solar collector works more efficiently at a lower input, the temperature at the collector output is higher, and the temperature difference (ΔT) between the collector and the storage is greater → better performance of the solar circuit pump.
Upper heat exchanger (boiler/HP) – located in the upper third of the storage. The boiler or HP heats only the upper part to the desired temperature (e.g., 50–55 °C for TÚV). The lower part remains colder for the solar system.
Hot water outlet – from the top of the storage.
Cold water inlet – to the bottom of the storage (via a check valve).

If these rules are not followed, the system will still work, but with significantly lower efficiency. Practical example: a customer had a correct bivalent tank of 300 l, but the plumber connected the solar circuit to the heat exchanger in the upper part and the boiler to the lower part. Result: the boiler heated the bottom of the tank, the solar controller did not detect sufficient ΔT (the tank was "warm" from the bottom), and the solar circuit almost never started. After swapping the heat exchangers, solar production increased by 60 %.

Three-way valve and circuit priority

In systems where the boiler is used for both heating and DHW preparation, it is necessary to address the priority of the DHW tank over the heating circuit. When the temperature in the tank drops below the set value, the controller switches the three-way valve to charge the tank – heating is temporarily turned off. This is called DHW priority and is standard for all modern boiler controllers. The solar circuit can charge the tank simultaneously with the boiler during this time – the upper and lower heat exchangers can operate together without problems.

Dimensioning – what collector area and what tank size?

For combination with a backup source, collectors are dimensioned to cover 50–70 % of annual DHW consumption. Oversizing (aiming to cover 90–100 %) is economically disadvantageous – the last 20–30 % of annual production costs twice as much for installation, and in summer overheating occurs when the tank reaches maximum temperature and the collectors "stagnate".

Guidelines for dimensioning (valid for Central Slovakia):

Flat collector area per person: 1.0–1.5 m² (for DHW)
Tubular (vacuum) collector area per person: 0.7–1.0 m²
Tank volume: 60–80 liters per person, minimum 150 liters for a 2-person household
For a 4-person family: 4–6 m² flat collectors, 250–350 liters tank
For combination with heating (solar heating support): 8–15 m² collectors, tank/buffer 500–1 500 liters

If you plan to add collectors in the future, choose a tank with sufficient reserve – changing the tank is much harder and more expensive than adding collector area. It is always better to buy a tank one size larger than you need, because a larger tank means greater thermal storage and shorter backup source activation times. More on dimensioning can be found in the article What solar collector power do I need for my house.

Control and management of the combined system

Control is the heart of every combined system. Modern solar controllers (e.g. Resol, Steca, Tecnec and others) have multiple inputs for temperature sensors and multiple outputs for pumps and valves. For combination with a backup source, these functions are key:

Differential control – the solar pump is activated when the temperature on the collector is ΔT higher than the temperature in the tank (typically ΔT start = 8–10 °C, ΔT stop = 4–5 °C).
Protection of the tank from overheating – when the tank reaches the maximum (e.g. 90 °C), the controller turns off the pump and the collector enters stagnation. It is necessary to take into account that the heat transfer fluid will evaporate into the expansion tank – therefore, the dimensioning of the expansion tank for solar systems is critical.
Anti-legionella function – the controller can be set to command the tank to reach a temperature of 60–65 °C once a week even by activating the backup source – prevention against the Legionella bacteria.
Tank cooling function – in summer, when the tank has accumulated too much heat, the controller can start the pump at night and transfer the heat back to the atmosphere via the collector. This function significantly reduces the frequency of stagnation.
Connection with the boiler controller – via digital buses (OpenTherm, eBUS, ModBus) or simple contact inputs, the solar controller can prevent the boiler from starting when the tank is sufficiently heated by solar energy.

In practice, we recommend always choosing a controller with at least 4 temperature inputs when combining with a backup source: collector, lower tank, upper tank, boiler return. Basic two-input controllers are insufficient for combined systems.

Solar heating support – real possibilities and limits

Many customers ask whether solar collectors can help not only with DHW but also with heating. The answer is: yes, but with realistic expectations. Solar heating support (so-called solar combisystem or combisystem) makes sense only with low-temperature heating – that is, floor heating or large-area heat air units. Radiators dimensioned for 70/55 °C (old standard) are unsuitable for solar support – they would require too high a temperature that collectors cannot reach in the transitional period.

Floor heating operates at temperatures of 30–45 °C, which is a range in which solar collectors can work efficiently even in spring and autumn – precisely when heating is needed. In January and February, solar heating support functions minimally (contribution 5–15 %), but in March, October and November it can reach 30–50 % of daily heat demand. Annual contribution of solar heating support is around 10–20 % of total heating demand – which for a house with consumption of 10 000 kWh/year means a saving of 1 000–2 000 kWh.

For a combisystem, a large buffer tank (puffer) with a volume of 500–2 000 liters is essential. Without sufficient accumulation, collectors overheat the tank quickly on sunny days and produce nothing for the rest of the day – the heat is "wasted" through stagnation. A larger buffer allows the collectors to operate longer at an optimal temperature. More about selecting collectors can be found in the article Flat vs. tubular solar collectors – which type is more cost-effective.

Economic evaluation – when does the combination pay off?

The question of economic return is always on the table. Let's look at specific real-life numbers that are realistic for Central Slovakia in 2024–2025.

Example 1: Family house, 4 people, gas boiler + solar collectors

Without a solar system: gas consumption for DHW is approximately 700–900 m³/year = 420–540 € annually (at a price of 0.60 €/m³). A solar system with 5 m² flat collectors and a 300 l tank covers approximately 60 % of DHW consumption → annual savings of 252–324 €. Investment in a solar system (material + installation): 3 500–5 000 €. Simple payback period: 11–16 years. With a subsidy (Green for Home and similar programs) covering 30–50 % of the investment: 6–10 years. After repayment, the saved money accumulates as pure profit.

Example 2: Family house, 4 people, air-to-water heat pump + solar collectors

A heat pump with a COP of 3.5 consumes electricity for DHW preparation (same house) worth approximately 180–220 € annually. A solar system with 4 m² tubular collectors covers approximately 55 % of DHW → annual electricity savings of 100–120 €. Investment in a solar system (to an existing HP): 2 800–4 000 €. Simple payback period: 24–33 years without subsidy – this is at the edge of economic sense. The combination of HP + solar makes the best economic sense when the HP is already necessary and solar only complements it – not as a primary investment for DHW.

From the perspective of energy price changes and inflation, the economic return improves over time. Systems installed 10–12 years ago, when gas was cheaper, are now showing as excellent investments – gas prices have doubled or tripled. This should be considered when making decisions.

Most common mistakes when combining a solar system with a backup source

Over the years of practice, we have identified several recurring mistakes:

Too small a tank – a 100–150 l tank with 4 m² of collectors. The tank heats up to maximum almost by midday, and the rest of the day the collectors stagnate. Solution: always dimension the tank to 50–80 l/m² of collector area.
Incorrect ΔT regulator setting – too low a ΔT start (e.g., 2 °C) causes the pump to run almost constantly even when temperatures are almost the same – the pump wears out and the tank does not heat efficiently.
Lack of thermal insulation of pipes – uninsulated solar circuit pipes lose heat on the way from the collector to the tank. On a summer day, the outer pipe can be 70 °C – this is unnecessary heat loss to the environment.
Wrong heat transfer fluid – using regular antifreeze for cars instead of certified solar fluid. Solar fluid must withstand temperatures above 150 °C (stagnation) without decomposition and deposit formation. Automotive antifreeze cannot do this.
Too large a collector area without a buffer – overdimensioning for DHW without accumulation. In summer, the collector overheats the tank to 90 °C by 11:00 and the rest of the production is wasted. Solution: a buffer or solar support for heating.
Lack of monitoring – without measuring production and consumption, customers do not know if the system is working properly. Modern controllers with Wi-Fi monitoring allow real-time tracking of production and quickly detect faults.

If you are planning an installation or solving an existing system with problems, try to find help in the article Common faults of solar systems and how to eliminate them or How to set up and commission a solar system.

Combination with a photovoltaic power plant (FVE) – triple synergy

More and more customers today are considering the combination of three technologies: solar thermal system (collectors) + heat pump + photovoltaic power plant (FVE). This triple combination is very interesting from an energy point of view: FVE produces electricity, part of it powers the HP (water heating and heating), solar collectors preheat the DHW tank and reduce the running time of the HP. Excess electricity from FVE can power an electric resistance rod in the tank (boiler), which is another way to use surpluses without the need for a battery.

This is today's probably the most intelligent solution for new buildings: FVE 5–8 kWp + air/water HP 8–12 kW + solar collectors 3–5 m² + bivalent tank 300 l. The investment is higher (overall 25,000–40,000 € depending on the size of the house), but dependence on external energy drops to a minimum and the system can pay for itself in 8–12 years at current energy prices when considering grant schemes.

Practical tips when choosing and buying a system

If you have decided to invest in a combined system, here are a few practical tips from practice:

Always have an energy calculation prepared, not just an approximate offer. Every house is different – orientation, roof slope, shadows from surrounding buildings, number of people, existing equipment.
Give preference to verified manufacturers of collectors and tanks with Solar Keymark or EN 12975 certification. These certificates guarantee measured technical parameters.
The expansion tank for the solar system must be specifically dimensioned for the solar circuit – not a standard pressure tank from the heating system. The reason is high temperatures during stagnation (fluid evaporation).
Piping in the solar circuit must be made of copper, stainless steel or approved plastic materials resistant to high temperatures. PEX pipes without UV protection must not be used in the solar outdoor circuit.
Always install a flow meter and heat meter in the solar circuit. Without measurement, you cannot assess performance and verify the return on investment. The cost of a thermometer is negligible compared to the total investment.

For inspiration and further details on choosing specific products, visit the category solar systems on atria.sk, where you will find tanks, solar pump units, controllers and accessories.

Frequently asked questions (FAQ)

Can I connect solar collectors to an existing gas boiler without replacing the tank?

Yes, but with the condition that the existing tank has a suitable connection point for the solar heat exchanger – most old tanks do not have it. If the tank does not have a lower heat exchanger for the solar circuit, it must be replaced with a bivalent tank. Some systems solve the situation with an external heat exchanger (external tank in series before the existing tank), which is a functional solution, but reduces efficiency. Always consult the hydraulic connection with an experienced designer before installation.

Is a solar system compatible with every type of heat pump?

Yes, in principle it is compatible with every type – air/water, water/water, brine/water. The differences are in the hydraulic connection and regulation settings. Heat pumps air/water with an integrated DHW tank are sometimes more difficult to integrate with a solar system, because the tank is part of the HP unit and does not always have a standard heat exchanger connection for the solar circuit. Always check the technical documentation of your HP before ordering a solar system.

What happens to the solar system in summer when the tank reaches maximum temperature and no one uses hot water (e.g. during a holiday)?

The collector enters stagnation – the heat transfer fluid evaporates and the steam leaves into the expansion tank. Collectors overheat to 180–220 °C. Modern systems are designed for this situation, but repeated stagnation shortens the life of the fluid and seals. The recommended solution is the night cooling function in the controller (fluid circulates through the collector at night and removes heat), or covering the collectors before going on holiday. More information can be found in the article Maintenance and service of solar collectors – what and when to check.

Is a solar system worth it if I have a heat pump that also has a DHW heating function?

It depends on the specific numbers. A HP with COP 3.5 for DHW heating consumes less electricity than direct electric heating, but a solar system produces heat for free (only pump operating costs ~30–50 W). In summer months, the solar system saves HP work – the HP does not have to run for DHW and its lifespan is extended (the compressor has a limited number of working hours). The economic benefit is smaller than when combined with a gas boiler, but the technical benefit (HP relief, extended lifespan) remains.

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'll be happy to advise you.