In purely mathematical terms, the decision to use a solar thermal system to generate heating energy is preferable to installing a PV system. On average, you can achieve 2.5x the energy yield compared to a PV system. However, maintenance must also be taken into account.

As the owner of a solar thermal system, you have to service and refill it at regular intervals. You can find out how to do this in this guide, which explains the individual steps involved in filling.


The right moment

It’s easy to determine when to change the fluid inside the solar system and replace it with fresh one. It is recommended to change the solar fluid every four to seven years, as it is no longer usable by then. In rare cases, the fluid may need to be replaced sooner, especially if it starts to “crack” early. This term refers to the degradation of propylene glycol (1,2-propanediol) under the influence of high heat (from 160°C to 400°C), which causes the pH value to drop quickly. This is reflected in the following points:

  • crumpled
  • Deposits in the pipe system
  • viscose
  • smells bad
  • brown coloring
  • pH below 7.0 (measure with test strips)

Antifreeze performance is dramatically reduced by this condition and the solar fluid should be changed as soon as possible. If you check the fluid and you find only one of the above points yourself, it is time to change the solar system fluid. However, the point in time can also be determined earlier. It is particularly easy to measure the tube content using a refractometer. To do this, take some of the pipe content and measure it with the device. The following values ​​are then displayed, which make it clear that the change may be necessary:

  • density
  • antifreeze content
  • Amount of dirt particles contained

Ideally, the system should be checked annually by a specialist to prevent this problem from occurring. If the liquid degrades over several years, you have to reckon with significant performance losses.


In order for the replacement and refilling of the solar system to work, you need the right utensils. A suitable pump with which you can flush and bleed the pipes is particularly important. This is the only way to fill the solar system yourself. A pump of the following types is sufficient for your own four walls:

  • drill pump
  • garden pump

Make sure that the pump offers a pressure of at least 2.5 bar, otherwise no liquids can be sent into the circuit. Ideally, you should use pumps with 3 bar, as these work even more effectively due to the higher pressure. In addition to the pump, you also need the following items to put the project into action:

  • 2 x water hoses (heat resistant)
  • 1 x collection container (heat-resistant)
  • 1 * Mixing Container
  • Rohrzange
  • stick for mixing
  • Solar Fluid Concentrate
  • solar system cleaner
  • slotted screwdriver

The amount of concentrate and cleaner depends on the dimensions of your solar system. Please refer to the manufacturer’s instructions for the mixing ratios, as each product is different. However, the cleaner has a guideline you can refer to:

  • Amount of cleaning agent in l = line volume in l + 5 l of cleaning agent

Make sure the collection container is large enough. Especially if you need to change the solar system fluid, a large container is an advantage. This must have at least the volume of the pipe system. Keep in mind that you will also need to clean the lines, which again creates as much liquid as waste. Twice the volume is therefore recommended for the collection container. The volume of the system is completely sufficient for the mixing tank. Suitable for this:

  • canister
  • big bottles
  • mortar bucket
Tip: In addition to electrically operated pumps, you can also use hand pumps that are operated with pure muscle power. These are available for prices of around 50 euros.

Drain solar fluid

The first step after measuring the quality of the liquid is to drain the solar system so that you can refill it fresh. Here it is important to know how many liters the pipe system of your solar system holds so that you can determine the size of the collection container. These can be found in the product description of your system, as these can differ greatly depending on the size and equipment. Of course, if it’s a fresh install, this step is unimportant, as is cleaning. When draining the old fluid, proceed as follows:

  • Connect the hose to the drain cock
  • Place hose with opening in collection container
  • open the reverse and forward gravity brakes
  • 45° position is sufficient
  • open the drain cock
  • drain liquid

Depending on the size of the system, the duration of draining the solar system fluid differs. You can shorten the duration by opening one of the vent valves at the top of the facility. This allows the liquid to get out of the lines faster. As soon as no more liquid comes out, you can continue with the cleaning. You don’t have to change the container just yet, unless it’s already full.

Clean line system

Cleaning the pipe system is an important step to ensure that no residue from the old solar fluid remains in the system. This would reduce the frost resistance of the fresh liquid, which has a negative impact on the performance of the entire system. Feel free to leave the drain hose attached to the system, as the resulting waste products must not go down the drain. They also contain too many pollutants and should be disposed of in the same way as the solar system fluid. Now connect the pump to inject the cleaner into the pipe system. This works as follows:

  • Connect the pump to the KFE cock/filling connection
  • is located at the top of the facility
  • Mix cleaning agent in bucket
  • Place pump tubing in cleaning solution
  • turn on the pump
  • let it run for about 30 minutes

After half an hour, the remains of the used solar fluid should no longer be in the pipes. This also applies to possible residues. Depending on your needs, you can let the cleaner work in the lines a little longer if the first pass was not sufficient. Then clean the pump thoroughly so that no residue of the solar fluid remains in the device or the hoses.

Rinsing the solar system: instructions

Would you like video instructions? Then we recommend this video to you .

One of the most important steps in filling your solar system is flushing the line. It doesn’t matter if you are starting up your system for the first time or if you need to change the liquid, flushing is always necessary. The main thing to remember about this step is that it is done very thoroughly so that you can get the system up and running again afterwards without any problems. Flushing removes the following deposits:

  • solar fluid residue
  • cleaning residues
  • installation residues

After rinsing, the lines are free of deposits and can be filled without any problems and without performance weaknesses occurring. For rinsing, remove the previously used collection container and close the pump. Lead the drain hose into the drain. Don’t worry, the rinsing water can go down the drain. Connect the feed hose to a water tap, for example the house connection, so that you have enough water available for rinsing. Then follow these instructions:

  • Open the screw on the sight glass
  • crosswise (3 o’clock)
  • open the faucet
  • let water run through
  • Duration: 10 min or until water is completely clear

You don’t have to wait any longer. Make sure that the temperature of the collectors does not exceed 40°C during the flushing.

Note: You don’t have to worry about residual water after rinsing. This is normal and does not have a negative effect on the refilling of the system.

Mix solar fluid

After rinsing, you must mix the solar fluid in order to be able to fill the system with it. To do this, use the mixing container that you have prepared. Mixing is not difficult and can be implemented without much effort. This is excellently managed while the residual water is sucked out of the system’s pipes, if there is any left there. Proceed as follows when mixing:

  • Fill container with water
  • ideally use demineralised water
  • optimizes antifreeze function
  • add concentrate
  • mix well

You can use the liquid directly. If you want to be on the safe side with the amount, feel free to mix up a little more. You don’t have to worry about overcrowding the system. Too much liquid is simply drained away and is not used.

Tip: If you don’t want to mix the solar fluid yourself, you can purchase ready-to-use mixtures. For these you have to reckon with costs of four to ten euros per liter, which is slightly more expensive than the comparable amount of concentrate.

Filling the solar system: instructions

The next step is to fill the solar system with the solar fluid. After you have mixed, place the container in front of the solar system so that the mixture does not have to travel long distances. That would only put unnecessary strain on your pump. The pump must now be connected to the solar system via the supply hose so that you can fill the system with sufficient solar fluid. Please note the following points:

  • Lay the drain hose in the mixing bucket
  • should be completely covered with liquid
  • Place hose from mixing bucket to pump
  • The hose must also be completely submerged
  • Set a pressure of at least 2.5 bar
  • turn on the pump

The pump now circulates the liquid within the pipes so that it reaches all corners of the plant. Over time, the amount in the bucket reduces because the liquid sits inside the pipes and can no longer escape. This makes it easy to see how long the filling is going on.

Properly vent

Bleeding is closely related to filling. You have to remove unnecessary air from the lines, because this has a negative effect on the heating performance and aggravates the cracking effect of the carrier liquid. If you do not bleed sufficiently, you will have to replace the solar system fluid much sooner. Since venting follows directly after rinsing, it is not necessary to clean something in between. You proceed as follows:

  • keep the pump running
  • Regularly check the liquid in the container for air bubbles
  • leave the hose below the surface

Bleeding is complete once you no longer see any air bubbles in the mixture. All air has now been purged from the lines, which takes between ten and twenty minutes. Then close the valves on the system so that the water stays in the line. Then you can finally clean the pump. If there is still liquid left in the container, dispose of it together with the contents that have already been emptied.

Conclusion: set the system pressure

Last but not least, you must not forget to set the system pressure correctly again. This is important for the system to run efficiently. Orientate yourself on the previous settings that were used. If you can’t remember the values, ask a professional for advice. In most cases, a pressure of 1.5 bar is sufficient, but can be higher in multi-storey houses.

Dispose of solar fluid properly

If you were able to effectively replace the solar fluid, refill the system and put it back into operation, you are not done yet. You still have to dispose of the liquid from the system. The liquid is hazardous waste and should never be poured down the drain. The mixture falls under the disposal regulations for antifreeze and must therefore be disposed of properly. This also includes any lumps resulting from cracking. They may only be disposed of at the following locations that belong to your community or city, otherwise fines (amount depending on place of residence) will be incurred for incorrect disposal:

  • Recyclinghöfe
  • pollutant mobile
  • recycling depots

Find out in advance whether the site has a collection system for harmful substances. In most cases, it is not possible to send solar system fluid back to the manufacturer when it has been used up. For this reason, you must contact the specified bodies to dispose of the liquid professionally and in an environmentally friendly manner. It is also possible to dispose of the liquid via specialist companies. In comparison, you have to pay more for disposal for these. The disposal costs are otherwise dependent on the location and vary greatly for pollutants. In many municipalities, for example, quantities of up to 50 liters are free, while in the capital a maximum of 20 liters can be dispensed free of charge. When you dispose of the liquid, you should also note the following points:

  • place in tightly sealed containers
  • eg plastic bottles, paint buckets
  • larger quantities can therefore be stored for a long time
  • necessary for small purchase quantities (without additional costs)
Note: In most cases, you will be issued with a receipt when you dispose of solar system fluid. You should keep this in a safe place in case the authorities contact you afterwards about disposal.

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