Out of the box, a lot of solar thermal energy installations are not really functioning in a transparent way. It is quite difficult to really know what the installation is doing and how much energy it adds into the warm water reservoir. Our installation, a Buderus gas condensing boiler system (Logamax plus GB162-25 V3) that manages our solar thermal panels, does give some information on the output of the system, but it does not have a unit, thus the numbers are not really directly interpretable, besides seeing if you produced more, less or a lot less/more energy through the solar thermal panels compared to the previous days.

Solar thermal installation’s added energy to the warm water system

After 10 years, our solar thermal installation failed to function for an undefined period of time (we don’t know how the system really failed), including at least one summer. The result was that the solar liquid became very viscous as during the summer months it overheated and boiled in the panels. The result was partially blocked hoses carrying the solar liquid from the roof to the heating system in the basement. Fortunately, some minor flow still was assured so that we could flush the whole system and get rid of most of the solidified liquid that was blocking the hoses. A faulty pump was the reason for this failure.

The Buderus management system showed that the pump was working. What I did not know was, that the display only indicated that it had sent electricity to the pump. It did not get any reliable feedback from the pump indicating if the pump really pumps. So technically, the user is blind and does not really know if the pump is really working or not. After flushing the hoses and panels, as well as installing a new pump, the system was refilled with fresh solar fluid and luckily the system came back to life.

I was then looking for a monitoring solution and find this device from a dutch enthusiast. Its EMS-ESP software helps me render the information from my heating system, including solar thermal panels, visible in my home automation system.

From there started a journey to understand how the heating system works and figuring out how to set it up correctly. To optimise the system, maximise the collection of heat from the sun and minimise gas consumption and heat losses, I worked on the following variables:

• I augmented the maximal temperature of the warm water, when heated by the solar thermal system. I went up to 80°C (beware that such a high temperature also has side-effects, mainly on the longevity of the appliance). Be careful that your warm water recipient does not feed very hot water into your pipes. The risk to burn yourself, especially for kids, is extremely high. Make sure to have a mixing tap installed (at the exit of your warm water appliance and before your pipe system) that only lets water flow to your house system with a pre-defined maximal temperature, for example 50°C.
• I reduced the temperature where the gas condensing fills in to heat up the warm water. The lower you can go in temperature, and still have enough warm water at your disposal, the better it is, as cou can use more energy from the sun. Ideally, the boiler is at 45°C each morning so that I have enough reserve to add a lot of heat to it. My gas boiler jumps in when warm water temperature drops to 45°C.
• I configured the warm water circulation pump so that it only kicks in, when imminent warm water demands from a remote location in the house happen. The circulation pump starts when the light in the shower is witched on. We just have to wait for half a minute to make sure warm water is available in the water collector next to the shower. Having the circulation pump run all day, “extracts” the heat from your warm water reservoir. Without any circulation, my warm water reservoir looses between 0.5 and 0.7°C per hour.
• To avoid excessive gas consumption, I completely switch off the heating circuit of the gas system during summer months (I only keep the warm water circuit switched onI). Our system happens to consume between 1 and 1.5 cubic-metre of gas even if all heating appliances are shut off an d the outside thermostat communicates to the system that there is no heating needed. This might by a configuration issue, but until now I could not find an issue in the configuration parameters.

We found it particularly difficult to find objective real life data on the usage of solar heat panels for hot drinking water. Our solar system (installed for for hot water only, not for heating up the house) is configured to allow for hot drinking water temperatures up to 80°C. A (child-)protection is installed so that the boiler will never deliver more than 60°C at all times. The gas condensing heating system only kicks in when the water temperature in the upper part of the water reservoir drops to 45 °C and will stop heating when the upper part is at 55°C. The following examples are based on real data coming out of our system, which consists of a Buderus Logamax Plus GB162-25 v3 gas condensing heater and a Buderus SM300/1 W warm water reservoir.

To better understand how these water heating systems function, feel free to consult this article (in german). The article also shows a graph of the water reservoir with the upper and lower parts and its corresponding heating systems.

1. Example of heat consumption after showering

The above graph (green line) shows that our warm water reserve is at 75°C at 08:45 in the upper part of our boiler. The first chute at 8:45 is due to a bath tub filling, at 9:05 one person takes a shower, at 09:30 a second person takes a shower. The remaining temperature is around 57°C.

Around 9:40, the sun is providing enough heat through the solar heat panels (yellow line) and the lower sector temperature (blue line) in the boiler begins to rise.

2. Example of the gas heating helping out to provide sufficient hot water

When the water temperature drops in the upper part of the boiler to 45°C, the gas heating will heat this part up to 55°C again. This normally takes around 10 minutes and consumes around 0.2 cubic meter of gas. (To properly measure this, the house heating part of the gas system is manually switched off. The only gas consumption is due to warm water heating.)

The above graph shows on the green line, above the Day 13 when the gas heating jumps in to heat up the upper part of the warm water reservoir. Over the week’s time shown in the graph, this happened 4 times. We are in July and outside temperatures are mid to end 20s°C and it is sunny.

3. How much energy will be lost over night, when nobody uses warm water?

Over 10 hours, from 22:30 to 8:30, the temperature in the upper part (yellow line) of the hot water reservoir drops by 5.5°C. Of cours, the lower part drops quicker, but when using hot water, it is taken from the upper part. So the heat losses are around 0.55°C per hour when the hot water is around 70°C. The losses might vary, if the overall temperature is lower. The higher the overall temperature is (and thus the difference to the room temperature), the higher the loss of heat will be. Around 60°C, we measure 0.51°C losses per hour. These losses cannot be avoided. Any insulation will produce heat losses.