ᐅ Underfloor Heating and Solar Thermal Systems

Hello and good day,

I would like to ask about your experiences regarding heat loss.
I have 6 tube collectors on my south-facing roof. The output and the system itself are working very well. Recently, I started using the heating partially. The supply temperature of the underfloor heating is between 24-28 degrees Celsius (75-82°F), depending on the outside temperature. I don’t consider this to be very high.
On sunny days, the storage tank (for domestic hot water and underfloor heating, 1000 liters (264 gallons)) is heated up to 60 degrees Celsius (140°F). When I look at the display in the morning, the storage tank has lost over 30 degrees Celsius (54°F) of heat (not counting the domestic hot water). What are your experiences with this? Am I worrying unnecessarily, or is this quite normal?

Best regards.

P.S.: I will gladly introduce myself and complete my profile soon.

I don’t have solar thermal, so I can only comment on the actual flow temperature (VLT) of our heat pump.

At an outside temperature of 20°C (68°F), I have the flow temperature set to 24°C (75°F) with a slope of 0.2.
This means that at 10°C (50°F) outside, the flow temperature should be 26°C (79°F), at 5°C (41°F) it should be 27°C (81°F), and at 0°C (32°F) it should be 28°C (82°F). I have disconnected most of the bypass valve (ERR), so the heating circuits are continuously open.
I am currently testing again. Last year, I kept the curve 1°C (2°F) higher throughout the heating season.

So your flow temperatures of 24–28°C (75–82°F) are nothing unusual.

Hello Musketier, thanks for the feedback.
As a "layperson," I would also consider the flow temperature quite normal. What puzzles me a bit are the 30 degrees Celsius (54 degrees Fahrenheit) of heat loss per night.
This results in the following: On a very sunny day like yesterday and today, the system barely manages to heat the temperature up to 53 degrees Celsius (127 degrees Fahrenheit) from 3:00 PM, meeting only the domestic hot water demand. ;-) The next morning it’s back down to 28 degrees Celsius (82 degrees Fahrenheit)… Is the heat being lost somewhere due to a wrong setting, or does the underfloor heating really consume that much?

I can only help you to a limited extent with this question. However, I don’t think heat is being lost, and the heating system should actually not turn on at flow temperatures above 28°C (82°F).

I don’t have any practical experience with solar thermal systems.

I suspect that the pump for the underfloor heating might not run at all or only rarely during the day, or that the thermostatic radiator valves (TRVs) close on most of the heating circuits due to solar radiation. When temperatures drop at night, the TRVs might open, mixing all the water and resulting in a lower average temperature.

Neubau_2016 schrieb:
On sunny days, the storage tank (domestic hot water and underfloor heating, 1000 L (264 gallons)) is heated to 60 degrees Celsius (140°F). When I check the display in the morning, the tank loses over 30 degrees Celsius (54°F) of heat (not including domestic hot water).

The temperature difference of 30°C (54°F) in 1000 L (264 gallons) of water corresponds to about 35 kWh of energy, which is roughly equivalent to 3.5 m³ (124 cubic feet) of natural gas.

Best regards

Hello and good morning,
these values are exactly right, at least regarding solar. For example, the system generated about 30 kWh yesterday and 33.5 kWh the day before. I think this is a good figure. Does anyone happen to have comparison values?
My main concern was whether this "loss/requirement" is completely normal or if I should be worried. To be honest, I’m currently testing something with the heating to find the right settings. Last night, I changed the temperature adjustment value from originally 1.0 to -1.0. As a result, the loss dropped to 9.0 degrees Celsius (48°F) per night instead of about 30. Let’s see how this affects the room temperature now. Best regards