ᐅ Improving the Efficiency of Solar Thermal Systems

We have a 300L (79 gallons) buffer tank, and our heating is supplied by district heating. Currently, the heating boost is set to run from 5:30 a.m. to 10 p.m. I have read that it is better to start the heating boost from around midday to optimize the efficiency of the solar thermal system, since the water is colder and can absorb energy more effectively. That sounds reasonable to me.

My question is: How much can I reduce the district heating boost time for domestic hot water production? At the same time, I want to ensure that Legionella prevention is adequately addressed. Currently, my domestic hot water temperature is set to 65°C (149°F). Would it theoretically be sufficient to run the heating boost for about 1 hour per day to raise the tank temperature to 65°C (149°F) and thus prevent Legionella?

What would be the most efficient approach regarding the solar thermal system in this context?

Is the buffer tank for heating or hot water?

The buffer tank is used exclusively for domestic hot water preparation.

Are you sure that this is a 300-liter (79-gallon) tank and not a 150+150-liter (40+40-gallon) tank? We have one like that, and the conventional heating system only heats the upper 150 liters (40 gallons). It heats its partial tank once in the morning—before getting up—and during the morning (at least in summer), the lower part is charged via solar thermal energy. We don’t have district heating, but I am satisfied with the energy costs.

300 L (79 gallons) is the total volume of the tank. In principle, it works the same way for me as you described: the lower part is heated by solar energy, the upper part by district heating. Do you only heat it once in the morning with the heater and then rely on solar energy for the rest of the day? What temperature do you set the water to?

That’s exactly how it is (in summer). The target temperature is 55 degrees Celsius (131°F), which is completely sufficient for all applications. The solar thermal system reaches significantly higher temperatures in good weather anyway (around 120 degrees Celsius (248°F) at the collector), so the lower part of the storage tank usually stays at about 65 degrees Celsius (149°F). (The legionella prevention cycle still heats the upper part of the tank to a higher temperature once a week.)

In winter, the lower 150 liters (40 gallons) are generally not used, which is why I give the heating system a second warm-up phase in the late afternoon. (Then the tank temperature is usually just below 50 degrees Celsius (122°F).) This works perfectly.