Frequently Asked Questions about Solar Systems and Collectors

Solar water heating systems are a topic we get asked about a lot, yet a clear, concise answer to a specific question is often hard to find. Customers keep asking us the same things over and over – what’s the difference between a flat-plate and a vacuum tube collector, why do they need a special solar storage tank, how exactly does the pump control work, what kind of piping should be used for the run from the roof, how are collectors anchored to different roof types, or why does a solar loop need its own expansion vessel. In this article we’ve gathered the most frequent questions about solar sets and collectors and answer them clearly, with links to more detailed topics and specific products.

A solar domestic hot water system (a solar hot-water set) is an investment that pays off especially where the roof faces south, south-east or south-west and there’s enough free area for the collectors. It isn’t a miracle solution that replaces a boiler or a heat pump – it’s an add-on that can significantly relieve the main heat source, especially in the warmer months of the year. That’s exactly why it’s important to understand what to realistically expect from such a set and how the individual components work together.

Below you’ll find answers organized into topic areas – from the operating principle through choosing the collector type, storage tank, controls and piping, to roof mounting and filling the loop. At the end of the article there’s also a separate FAQ section with shorter answers to additional questions and two real-world examples.

What exactly is a solar water heating system and what can I expect from it?

A solar system consists of four basic components that work together.

A solar water heating system (a solar hot-water set) consists of four basic parts: solar collectors mounted on the roof, which capture solar radiation and heat the heat-transfer fluid (a mix of water and antifreeze); a solar storage tank with a heat exchanger; a pump unit, which circulates the fluid between the collector and the tank; and a control unit, which automatically manages the whole process.

It’s important to stress one thing right at the start, which almost every prospective buyer asks about: a solar system supplements the main heat source – i.e. a gas or condensing boiler, a heat pump, or an electric boiler – it does not fully replace it. It can cover the domestic hot water demand most effectively in the summer months, when there’s plenty of sunshine and outdoor temperatures are high. In the winter months, when there’s little sunshine and outdoor temperatures are low, the solar system only contributes partially to water heating, and the boiler or other heat source continues to do the main work.

If you’re considering whether a solar system is worth it for you and which type of set to choose for your house, we recommend reading the detailed guide How to choose a solar water heating system, where you’ll find a complete step-by-step procedure including recommendations based on household size and roof orientation. If you’re considering combining solar heating with an existing boiler, also check out Solar heating as an add-on to a boiler, where we explain how the two devices work together throughout the year.

What’s the difference between flat-plate and vacuum tube collectors, and which should I choose?

This is probably the most common question we hear from customers. There are essentially two types of solar collectors on the market – flat-plate and vacuum tube (tubular) – and each has its own advantages and limitations.

Comparison of the two basic types of solar collectors.

Flat-plate collectors have an absorber surface covered with tempered glass, housed in an insulated aluminium frame. They’re structurally simpler, mechanically more resistant – for example to hail or mechanical impact – and generally cheaper than vacuum tube collectors. In our climate conditions they’re usually fully sufficient for heating domestic hot water and are therefore the most common choice for family houses.

Vacuum tube (tubular) collectors achieve higher efficiency especially at low outdoor temperatures and lower solar radiation intensity – in other words, in periods when flat-plate collectors are already losing output. This feature is particularly useful where solar energy is expected to be used outside the summer season too, for example as a partial contribution to space heating. The downside is a higher purchase price and greater fragility of the construction compared to flat-plate collectors.

So the choice between the two types isn’t a general question of “which is better”, but rather a question of exactly what the system is meant to be used for and what the budget is. You’ll find a complete comparison with specific recommendations in the article Flat-plate vs. vacuum tube solar collectors.

Why isn’t a solar storage tank just a regular water heater with extras?

Another common question is: “Can I use a regular water heater, or do I really need a special solar storage tank?” The answer is clear – compared to a standard water heater, a solar storage tank has at least one additional heat exchanger connected to the solar loop. Very often such a tank even has two exchangers – one for the solar loop and a second for backup heating by the boiler, for when solar energy isn’t enough to finish heating the water.

The principle is indirect heating: the water intended for use (domestic hot water) is heated indirectly, through the exchanger wall. The solar fluid itself circulates separately, in a closed loop, and never comes into direct contact with the domestic water that you, for example, run in the bathroom. This separation also matters for hygiene reasons, since the heat-transfer fluid contains an antifreeze component that has no business being in drinking water.

When choosing the right size and configuration of the tank (number of exchangers, volume based on the number of people in the household), it pays to proceed systematically – you’ll find a detailed guide in the article What solar storage tank do I need.

How does solar loop control work, and when does the pump actually start?

A solar system isn’t running continuously – that wouldn’t make sense, because at night or under an overcast sky the pump would just needlessly circulate cold fluid and draw heat out of the tank instead of adding heat to it. That’s exactly why every solar set has a control unit.

The control (solar) unit uses sensors to compare the temperature at the collector and the temperature in the tank. When the collector is warmer than the tank by a preset difference (typically a few degrees Celsius), the unit switches on the circulation pump of the solar group and heat starts transferring into the tank. When the difference is too small or the tank is warmer than the collector – for example at night – the pump stays off.

Physically, the actual circulation is handled by the pump unit (often also called the solar pump group or solar station). It contains the circulation pump, shut-off valves, thermometers and pressure gauges for checking operating values, and a safety valve in case of excessive pressure. All these components are usually grouped into one compact block, which greatly simplifies both installation and later servicing, since a technician doesn’t have to search for individual parts scattered around the loop.

If you’d like more detail on exactly how control units and pump groups work, what settings they have, and what to watch out for when choosing one, read Control units and pump groups of solar systems.

What kind of piping should be used between the collector and the utility room?

The piping run between the solar collectors on the roof and the utility room, where the tank and pump unit are located, can be handled in two basic ways. The first is classic copper or stainless-steel piping, which is custom-fitted on site and then insulated with thermal insulation resistant to high temperatures and UV radiation (since part of the piping may run along the outer façade or roof).

The second option is a pre-insulated 2-in-1 flexi pipe – a flexible twin pipe in which both the flow and return lines run inside one already-insulated jacket. A major advantage of this solution is a shorter installation time, because nothing needs additional insulation on site, whether on the roof or in a shaft – the pipe is simply pulled from point A to point B and you’re done. On more complex runs with several bends or a longer distance, this saves installers a significant amount of time.

A flexible stainless-steel pipe, the so-called corrugated hose (bellows), is often used as the connecting element between rigid piping and the individual components (collector, pump unit). Thanks to its flexibility it compensates for small installation inaccuracies and is resistant to the high temperatures the solar fluid can reach in summer under full sunlight.

You’ll find more about both solutions, including when it’s worth choosing classic piping and when a flexi pipe, in the article Solar piping and stainless-steel corrugated hoses – how they work.

How are collectors anchored to different roof types?

Another practical question that concerns almost every prospective buyer even before the purchase itself: “Can this be installed on my roof too?” The answer depends on the type of mounting structure, which is selected precisely according to the roof type and collector type.

On a pitched roof, collectors are anchored using a structure adapted to the specific roofing material – roof tiles, sheet metal roofing or beaver-tail tiles each require different fixings, since the way you can safely anchor into the roof truss without damaging the roofing differs. On a flat roof, an adjustable structure is used that sets the tilt of the collector – usually at least 15° – which matters mainly for self-cleaning of the collector surface by rainwater. Without sufficient tilt, dirt could settle on the collector long-term and reduce its efficiency. A third option is mounting on a façade or directly on the ground, which is used where the roof is unsuitable for various reasons (unsuitable orientation, shading, structural load).

One important rule applies in all cases: the mounting structure must precisely match the dimensions of the specific collector type and model. It isn’t a universal part that “fits everything” – when ordering, always check that the structure is designed exactly for your collector model and roof type.

A detailed overview of mounting options for all roof types and façades can be found in the article Mounting collectors – pitched roof, flat roof, façade.

Why does the solar loop have an expansion vessel, and how is it filled and vented?

The solar loop is a closed, pressurized system – similar to a home’s heating loop. Since the heat-transfer fluid can heat up to high temperatures on sunny days, its volume increases as it heats. To prevent this volume increase from causing an uncontrolled rise in pressure in the loop, the system includes an expansion vessel that compensates for this thermal expansion.

Filling and venting the solar loop itself is done similarly to a classic heating loop – through filling valves, using a manual or electric filling pump. The fluid is pumped into the loop under pressure while air is simultaneously pushed out of the loop, since it could otherwise cause air pockets that hinder proper circulation and heat transfer. This is done during initial installation, but also during regular servicing, since the heat-transfer fluid gradually degrades over time and needs to be renewed.

A complete step-by-step installation procedure, including filling and venting, can be found in the article Installing a solar system – step by step. If you’re dealing with an existing system and want to know what to do during servicing or if you suspect a fault, check out Servicing, filling and common faults of solar systems.

Real-world examples

Prices of individual components of a solar set vary significantly.

Example 1 – a family house with an older gas boiler room. A family house for a family of four, gable roof facing south-west, covered with roof tiles. Domestic hot water heating had so far been provided exclusively by the gas boiler, which in summer meant unnecessarily high gas consumption just to heat water for showering and everyday use in the kitchen and bathroom. After adding a solar set with flat-plate collectors, a solar storage tank with two exchangers, a control unit and a pump group, hot water heating in the summer months shifted almost entirely to the solar system – during this period the boiler now switches on only occasionally, for example during a longer spell of overcast weather. Outside the summer season the solar system continues to contribute partially, but the boiler takes on the main load, exactly as this combination is usually designed to work.

Example 2 – a weekend recreational cottage. A cottage used mainly on weekends and during the summer months, without year-round heating, with a flat roof. Since the building isn’t occupied continuously and the owners wanted a simple, reliable solution without complicated controls, they chose a smaller solar set with flat-plate collectors on an adjustable structure for a flat roof (tilt set to the recommended 15° for self-cleaning), a smaller solar storage tank and a compact pump unit. The control unit automatically switches the pump on and off without any need for the owners to intervene – when they arrive at the cottage they simply find hot water ready, from heat accumulated on previous sunny days. Given the seasonal use, it was essential for them that the whole system works reliably even without a regular attendant.

Most common questions about solar sets

Related topics

• How to choose a solar water heating system
• Flat-plate vs. vacuum tube solar collectors
• What solar storage tank do I need
• Solar piping and stainless-steel corrugated hoses – how they work
• Control units and pump groups of solar systems
• Mounting collectors – pitched roof, flat roof, façade
• Solar heating as an add-on to a boiler
• Installing a solar system – step by step
• Servicing, filling and common faults of solar systems

Do you have a question about solar systems or collectors?

Not sure what to decide, or dealing with a specific situation in your household? Write to us – we’re happy to help.

Full name Do not fill in this field:

Email

Message