ᐅ How does my heating system work with solar thermal energy?

Hello everyone,

Our heating system consumed about 8,700 kWh of natural gas between 12/16 and 06/17, even though the house is still being renovated and unoccupied. All radiators were set to frost protection only, and no domestic hot water was used.

The system was installed in 12/15 by the previous owners and mainly consists of:

• Viessmann Vitodens 200 gas condensing wall unit 19 kW
• Austria Email KWS 800 combined stratified storage tank
• Flat plate solar collectors 9.36 m² (100.8 ft²)
• ESR 31 solar controller
• Delphis solar station FV 70 (with Grundfos circulation pump on the supply side)

This summer, I noticed that the pipe from the solar collector (supply line) was only hot up to the pump, while the return line was almost cold. From this, I concluded that the pump was not running. At the same time, the controller showed 999°C (1,830°F) for the temperature sensor mounted at the top of the collector, which the technician said is the maximum display value indicating no plausible reading.

Testing the temperature sensor (a PT1000) was successful, so the technician concluded that the controller itself must be defective (he believed the pump was still operational).
So, a new controller was ordered, which I am still waiting for as of today, but that’s another story...

Now, with the cold season approaching, we have temporarily installed some radiators to heat the construction site a little and, most importantly, to ensure frost protection.
I was able to reduce the natural gas consumption by lowering the target temperature for domestic hot water to 10°C (50°F); however, I would say the consumption is still quite high (since Saturday, so in 4 days, about 30 m³ [1,059 ft³]... :O).
Is it possible that due to the defective controller, the 800-liter (210-gallon) water volume of the solar thermal system is being kept warm using natural gas, and usually the ESR 31, depending on whether heat is available from the roof or not, activates or deactivates a heat exchanger between the domestic hot water tank and the solar thermal water system, which in our case might currently always be open or something like that?
Maybe someone, even as a layperson, can explain how a combined stratified storage tank works or is constructed?

Many thanks in advance!

Best regards

PS: The heating technician is hard to reach; unfortunately, I have to work with him since this is obviously a warranty case. Once this issue is resolved, he definitely won’t touch our heating system again… -.-

Why do you have to work with him? Set a written deadline and then hire someone else. Send him the invoice. Preferably do this with a lawyer involved and that’s it.

First, I would turn off the domestic hot water system completely, if possible. Then you can check whether the consumption is noticeably affected by this.

To my knowledge, a combined stratified storage tank is designed so that the temperature is not uniform throughout the entire water tank. For example, it can be 50 degrees Celsius (122°F) at the top and only 30 degrees Celsius (86°F) at the bottom. A specialist should explain the exact details to you.

markus2703 schrieb:
First, I would completely turn off the domestic hot water system, if possible. Then you can see if the consumption is noticeably affected.

The Vitodens 200 has only 3 operating modes: "Heating and Domestic Hot Water," "Domestic Hot Water Only," and "Shutdown Mode." Unfortunately, there is no "Heating Only" mode. The feedback from Viessmann, which I have already followed, is to lower the set temperature of the domestic hot water to 10°C (50°F).
€dith also adds: If you turn off the fresh water supply, the system goes into a fault state, so that is not possible either...

markus2703 schrieb:
To my knowledge, a combined stratified storage tank is designed so that the entire water tank does not have the same temperature throughout. For example, the top can be 50°C (122°F), while the bottom is only 30°C (86°F). A specialist would have to explain exactly how this works in more detail.

Thank you very much for your explanation. If anyone can or would like to explain in more detail, I would be very grateful!
I am especially interested in how heat loss from the domestic hot water towards the solar thermal system is prevented during winter when little or no heat is produced by the solar collectors on the roof. (Otherwise, the system would be rather pointless...)

When it comes to the roof, nothing can be done... it's a one-way street. So either heat moves from the roof into the attic or it doesn’t at all...

Heat definitely does not rise upwards. This is clearly visible from the fact that snow remains on the panels (until it slides off) and ice can form. So, they are cold during winter.