How to Set Up and Commission a Solar System

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How to set up and commission a solar system

Correct commissioning of a solar system is just as important as its correct selection and installation. Even the highest quality solar collector with a perfectly dimensioned storage tank can operate inefficiently or even be damaged if the first start-up is neglected or done hastily. This article will guide you through the entire process – from pre-filling checks through the first start-up, regulator setting, air venting, to verifying correct operation and setting up protection. I base this on what I see in customer projects – most problems with new installations do not arise due to poor materials, but due to skipping one of the first start-up steps.

Preliminary checks before start-up – what to check before filling

Before you even reach for the solar fluid or switch on the pump group, it is essential to perform a visual and mechanical inspection of the entire installation. Many installers skip these steps because the system "looks good" – and that is often the source of later problems.

Mechanical inspection of pipes and collectors

Check all connections in the solar circuit – whether the collectors are correctly mounted in the frame, connecting hoses or copper/stainless steel pipes are tightly fastened and without visible damage. On the roof, check whether the collectors are tilted at the correct angle (the optimum for Slovakia is 30–50°, with 35–45° being ideal for year-round heating). Also check the orientation – collectors must face south, a deviation of 15–20° to the southeast or southwest is still acceptable, a larger deviation reduces performance.

Check whether the correct fittings are mounted on the pipes – check valves, drain valve, pressure gauge, safety valve, and air vent. These components are not optional – they are basic safety elements without which the system must not be put into operation.

Pressure test before filling

The solar circuit must be pressure tested before filling with fluid – most commonly with nitrogen or dry air. The test pressure is usually 1.5 times the maximum operating pressure of the system, which for most residential solar systems is 6 bar (operating pressure 4 bar). Fill the system to the test pressure, close it and let it stand for at least 30 minutes. If the pressure does not drop (or drops by no more than 0.1 bar, which may be only thermal expansion), the installation is tight.

If the pressure drops, systematically search for the leak – usually with soapy water on all connections. In practice, leaks most often occur at the collector connections on the roof and at the wall penetrations into the boiler room. Do not underestimate the pressure test – a leak in a system filled with solar fluid is much more complicated and expensive to fix.

Filling the solar circuit with solar fluid

Solar systems are not filled with ordinary tap water – special solar fluids based on propylene glycol antifreeze (usually 40–50% concentration) with additives against corrosion and cavitation are used for this. Using tap water causes limescale deposits in collectors and heat exchangers, accelerated corrosion, and dangerous boiling in a closed system at temperatures above 100 °C.

Step-by-step filling procedure

The most reliable way to fill the solar circuit is with a filling pump (manual or electric) connected to the service valve of the pump group. The system is filled from the bottom – from the pump group – while air escapes through the air vent at the highest point of the circuit (usually on the collector or just behind it).

Before filling, open all closing valves in the circuit (except the safety valve, which is always open).
Connect the filling pump to the filling valve of the pump group.
Open the air vent on the collector (or at the highest point of the route).
Start pumping the fluid at a slow pace – too fast filling traps air bubbles.
Once the fluid starts flowing out of the air vent (without bubbles), close it.
Continue filling until the pressure gauge shows the required operating pressure (usually 1.5–2.5 bar in a cold system).
Close the filling valve and disconnect the filling pump.

After filling, it is important to check the pressure in the expansion tank before filling the system. The expansion tank must be pre-pressurized to a value 0.2–0.3 bar lower than the static pressure of the system at the point of connection of the tank. For a typical family house with a collector height of 6–8 m above the boiler room, this results in a pre-charge pressure of the expansion tank of around 0.8–1.0 bar. If you neglect this value, the expansion tank will either not work properly or the safety valve will repeatedly release pressure when the system is heated.

Deaeration of the solar system – the most important step of the first start-up

Air in the solar circuit is the biggest enemy of proper function. Air bubbles slow down or completely block the flow of liquid, cause pump cavitation, noise in the pipes, and mainly – reduce system performance. After the first filling, there is always a certain amount of air in the system and deaeration can take several hours of operation.

Deaeration procedure

After filling the system, start the circulation pump (manually, if the controller is not yet set up). Let the system run for 15–30 minutes at full pump capacity – air moves upward and accumulates at the highest points of the system. At each deaeration valve, briefly loosen the closure until clean liquid without bubbles flows out, and then close it again. After deaeration, check the pressure and possibly inflate the system to the desired level.

In practice, deaeration is a three-step process: the first deaeration right after filling, the second after the first heating of the system by the sun (when the liquid circulates and air is released from the pipes), and the third after several days of operation. Automatic deaeration valves make this process easier, but it is not good to rely on them as the only solution – manual inspection remains necessary.

Flow and setting of the flow rate

Each solar system has an optimal flow rate of liquid in the primary circuit. For flat collectors, it is standard 40–50 liters per hour per m² of collector area (low-flow operation) or 60–80 l/h·m² (matched-flow). For vacuum tube collectors, the recommended flow is lower, typically 20–30 l/h·m². The correct flow is set using a regulating valve on the pump group – the rotameter (flow meter) will show the current value. For a system with 4 m² of flat collectors, set the flow to 160–200 l/h.

Why is flow important? Too low flow causes overheating at the collector outlet and inefficient heat transfer. Too high flow shortens the dwell time of the liquid in the collector, so it does not warm up sufficiently, and also overloads the pump. When setting the flow, monitor the temperature difference between the supply and return (ΔT) – at the correct setting, it should be 8–15 °C under normal operating conditions.

Setting up the solar controller

The controller is the brain of the entire system. Most modern solar controllers work on the principle of a differential thermostat – they compare the temperature at the collector (sensor T1) and the temperature in the storage tank (sensor T2) and start the pump when the temperature difference is sufficient. This is called the turn-on differential (ΔT-on) and the turn-off differential (ΔT-off).

Basic controller parameters for the first setup

ΔT-on (turn-on differential): The standard value is 8–10 °C. The pump starts when the collector is 8–10 °C warmer than the storage tank. Setting too low a value (e.g. 4 °C) causes frequent unnecessary pump starts – short cycles occur where the system pumps on and off repeatedly without real heat gain.
ΔT-off (turn-off differential): Standardly 4–5 °C. The pump stops when the temperature difference drops below this value. The difference between ΔT-on and ΔT-off (so-called hysteresis) is important – it prevents unnecessary switching.
Maximum storage tank temperature (T-max tank): Set to 60–65 °C. Above this temperature, the system will not allow the pump to operate, even if the collector is hot. This protects the tank from excessive heating and prevents Legionella growth (at 60 °C, hygienic protection is sufficient).
Maximum collector temperature (T-max collector): Set to 120–130 °C. At this temperature, the pump stops (or starts, depending on the setting), to prevent overheating of the liquid and evaporation.
Anti-freeze protection: Activate the antifreeze function – when the collector temperature drops below 4–5 °C, the pump briefly starts to prevent freezing of the remaining liquid in the collector. Even if the system contains antifreeze, this function is an additional protection.

Placement and inspection of sensors

The accuracy of temperature measurement depends on the correct placement of temperature sensors. Sensor T1 (collector) must be in a housing at the collector outlet (on the supply pipe just after the collector), protected from direct sunlight and insulated from the outside air, so that it measures the liquid temperature, not the environment. Sensor T2 (storage tank) must be in the lower temperature housing of the storage tank – not at the top, where the water is hottest. If the sensor is placed at the top of the storage tank, the controller will think the tank is already sufficiently warm and will not start the pump even if the lower part of the tank is cold.

Setting up the circulation pump and checking hydraulic balance

Most pump groups for solar systems have adjustable pump speeds – typically three levels or electronically controlled speeds (ECM motors). The first setting is usually done at the medium level, then you verify the actual flow on the rotameter and adjust the level or balancing valve accordingly.

If the installation is properly designed, the total pressure resistance of the circuit should match the hydraulic parameters of the pump. The easiest check in practice is as follows: start the pump, set the desired flow using the regulating valve on the rotameter, and check whether the pump "cavitates" or "cracks" – which would indicate too low flow and cavitation. If the pump makes noise at minimum flow, check whether any valve is only slightly open.

If you also have a bivalent tank in your heating system (with two heat exchangers – one for solar and one for the boiler), check whether the primary circuit (solar) is connected to the lower heat exchanger and the boiler circuit to the upper heat exchanger. The tank must be heated from the bottom – this ensures stratified storage of hot water and maximum solar coverage efficiency. More about combined connections can be found in the article Solar system in combination with a boiler or heat pump.

First actual start-up and function verification

An ideal day for the first start-up and function verification is a clear sunny day – best before noon, when the sun is high enough. The procedure is as follows:

Set the controller to manual or test mode – most controllers have a "test" or "manual start" button that starts the pump regardless of temperatures.
Monitor the temperatures on the controller display: the collector temperature T1 should rise within a few minutes (on a sunny day, you can see an increase of 10–20 °C in 5 minutes with the pump stopped).
Start the pump and observe whether the collector outlet temperature begins to drop and the tank inlet temperature rises – this means the system is actually transferring heat.
Check the temperature at the tank inlet and on the return pipe – ΔT should be 8–15 °C with proper flow.
Switch the controller to automatic mode and let it run – monitor the first automatic pump on/off cycles.

What to monitor during the first days of operation

In the first 5–7 days of operation, it is recommended to check several values daily: system pressure (must not fluctuate significantly), tank temperature at the end of the day (with good sunshine it should be 55–65 °C), condition of the air vents (whether they indicate air), and function of the safety valve (must not discharge during normal operation). If the safety valve regularly discharges, it means either too high operating pressure, too low or improperly pressurized expansion tank, or incorrect setting of the maximum tank temperature.

Verification and setting of system protection

Every solar system must have properly set safety protections. Ignoring them not only leads to inefficient operation but can also cause damage to equipment or safety risks.

Pressure relief valve

The pressure relief valve in the solar circuit is typically set to an opening pressure of 6 bar (for most residential systems). Never replace it with a pressure relief valve higher than the maximum allowed pressure of the components in the circuit (collectors, tank, piping). The pressure relief valve must have a prescribed discharge – a hose or pipe to a safe location or container where any liquid/vapor leak does not cause injury or damage. Verify the initial setting by manually opening it once by pulling the lever (only briefly) – this confirms it is not seized or blocked.

Overheat protection (stagnation)

During summer overheating (when the tank reaches maximum temperature and the sun is still shining), the system enters stagnation – the pump is turned off and the fluid in the collector overheats. Good controllers have a "cooling" or "legionella protection" function – at night the pump briefly starts and cools the tank by circulation, thus preventing long-term stagnation and fluid oxidation. Make sure this function is activated. More about system behavior under extreme conditions can also be found in the article Is a solar system worth it on cloudy days or in winter.

Anti-freeze protection check

Check the concentration of the antifreeze in the solar fluid using a refractometer or hydrometer. A standard 40% concentration of propylene glycol antifreeze protects down to -24 °C. If you have diluted the solar fluid or you don't know the original concentration, measure it. In Slovakia, where -15 °C frosts are common and occasionally below -20 °C, a 40–50% concentration is the correct choice. Too high a concentration (over 60%) paradoxically worsens the thermal properties of the fluid and unnecessarily increases viscosity.

Commissioning record and first documentation

After a successful first start-up, it is important to document all settings and measured values. This record will allow you (or the service company) to quickly identify deviations from the original state in the future. Record the following:

Date of commissioning and weather at start-up
Operating pressure in the cold state (bar)
Expansion tank pre-charge pressure (bar)
Set flow rate (l/h)
Controller set parameters (ΔT-on, ΔT-off, T-max tank, T-max collector)
Measured temperatures T1 and T2 during first full operation
Type and concentration of solar fluid
Manufacturer and model of all main components

This record is also important for any possible warranty claim or insurance claim. Some manufacturers condition the warranty on the commissioning being documented by a professional company. If you are unsure about the extent of documentation, contact atria.sk solar system page directly, or refer to the article Installation of a solar system step by step – what you need to know.

First month of operation – what to pay attention to

During the first 4 weeks, it is recommended to monitor the system more intensively than later. What to specifically check:

System pressure every 2–3 days: Small drops (up to 0.2 bar) may be caused by air being released during venting – this is normal. A larger drop indicates a leak.
Regulator function: Monitor whether the pump starts and stops at the correct times. A daily record of temperatures T1 and T2 will show whether the system is working as expected.
Storage tank temperature at the end of a sunny day: With a 4 m² collector area and a 300-liter tank, you should achieve a temperature increase of 20–30 °C after 6 full hours of sunlight. If you achieve less, look for the cause (air in the system, incorrect flow, faulty sensor).
Noise: A normal pump start is quiet or slightly humming. Squeaking, cracking, or bubbling sounds indicate air or cavitation.

If problems occur during the first month of operation, I recommend also reading the article Common faults in solar systems and how to fix them, where typical real-life scenarios are described in detail with specific causes and solutions.

Selecting the right equipment and where to find more information

Proper commissioning naturally depends on the system being correctly designed and dimensioned even before installation. If you are still in the selection stage, I recommend reading the articles How to choose a solar water heating system for a family home, What solar collector power do I need for my home, and Flat vs. tubular solar collectors – which type is more cost-effective. These topics together form the basis for deciding on the right system, without which even the best commissioning will not yield results.

The full range of available solar systems can be found at atria.sk – from complete sets to individual components. We will also be happy to help you choose the correct size of the storage tank, collector area, and suitable regulator for your specific case.

Most frequently asked questions (FAQ)

What pressure should a solar system have after filling?

In the cold state (at room temperature), the operating pressure should be 1.5–2.5 bar – typically 1.5–2.0 bar for systems with collectors placed up to 6 m above the boiler room. When heated to operating temperature (fluid 60–80 °C), the pressure will increase by another 1.5–2 bar, which is still significantly below the opening pressure of the safety valve (6 bar). If the system reaches a pressure above 5 bar after heating or the safety valve opens, check the expansion vessel – its volume may be too small or it has an incorrect pre-pressure.

The pump starts and stops every minute – is that normal?

No, frequent short cycles (so-called chattering) are a sign of incorrect regulator settings. The most common cause is too small hysteresis – that is, too small a difference between ΔT-on and ΔT-off. Set ΔT-on to 8–10 °C and ΔT-off to 4–5 °C. Chattering can also be caused by poor placement of the T1 sensor – if the sensor measures ambient temperature instead of fluid temperature, the regulator receives distorted values.

How much black solar fluid can I add to the system and where?

Solar fluid is added via the filling valve of the pump group using a filling pump – never directly by opening the system to the air. The fluid must be the same type and concentration as already in the system. Always measure the concentration of the existing fluid with a refractometer before adding. If you need to add more than 10 % of the system volume, it is advisable to analyze the fluid – in the case of a larger leak or after a long time, a complete fluid change may be necessary. More about maintenance can be read in the article Maintenance and servicing of solar collectors – what and when to check.

Why doesn’t the storage tank reach the desired temperature even on sunny days?

There can be several reasons – air in the system (the pump "sucks" but does not transfer heat), incorrect placement of the T2 sensor (the tank appears warm, but the pump does not start), too high flow rate (fluid does not have time to transfer heat), or a clogged heat exchanger in the tank (in older systems with water). Systematically check each possible point. If the problem persists after checking, contact a service company or read the article Common faults in solar systems and how to fix them.

Can I commission the solar system myself, or must a professional do it?

Legislatively, in Slovakia, there is no obligation that the first start-up must be carried out by a certified technician – unlike, for example, gas appliances. In practice, however, I recommend professional first commissioning for several reasons: proper setting of the regulator and hydraulics requires experience and measuring instruments (pressure pump, refractometer, thermometer), and in addition, many manufacturers of collectors and storage tanks require professional installation and first commissioning with a protocol for warranty purposes. If you decide to do it yourself, carefully read the instructions for all components and follow the procedures outlined in this article.

What to do if the system freezes and does not work in winter?

If the system freezes, never use hot water or fire to thaw the pipes – the risk of cracking is high. Let the system thaw naturally (in a heated boiler room or during weather warming). After thawing, check whether any component has deformed or cracked, measure the pressure and fluid. If freezing occurred despite the presence of antifreeze, immediately measure its concentration – if the concentration is insufficient, replace the entire volume. Prevention is always cheaper: correct antifreeze concentration, functional frost protection in the regulator, and regular checks. More tips can be found in the article Common questions about solar water heating systems.

Conclusion: First start-up as an investment in long-term performance

Proper commissioning of a solar system is not a one-time formality – it is the foundation on which the efficiency, reliability, and lifespan of the entire system depend for decades. From experience, I know that systems with a precise first start-up and correct regulator settings achieve 15–25 % higher annual yields than identical systems started hastily without checking flow, pressure, and sensors. Every hour spent on a thorough first start-up is returned in energy savings and the prevention of faults that would otherwise require service interventions. If you want a solar system that works reliably and efficiently year after year, do not rush its commissioning – proceed methodically, measure, record, and verify each step.

Do you have a question about this topic?

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