ᐅ Fresh water station causes temperature fluctuations

Good day.
I came across this forum seeking advice about my domestic hot water system.
I hope this is the right category (heating or plumbing?).

Problem:
My domestic fresh water system is causing temperature fluctuations, especially while showering.
The temperature varies by up to 5 degrees from the set level on the single-lever mixer valve. You can notice a slight drop in temperature, requiring frequent readjustment. This is obviously annoying. The temperature is not stable.
PS: There are no thermostatic mixing valves installed! I need to identify the cause of these fluctuations, even if thermostatic valves might solve the issue.

It is a multi-family house with 5 units, 4 years old. The problem has existed since construction, sometimes more pronounced, sometimes less.
Fresh water flow rate approx. 50 liters per minute (13 gallons per minute).
Supplied by a 1000-liter (264 gallons) buffer tank, with 50% reserved for domestic fresh water. So, a large volume!
The buffer is charged at 70°C (158°F) at the top. Buffer temperatures range approximately from 70°C (158°F) at the top to about 60°C (140°F) in the middle of the tank.
Temperature limiter set at 65°C (149°F) for the domestic fresh water. Fresh water draws feed water from the top buffer connection—the hottest spot. Hence the manual temperature limiter set to 65°C (149°F).

Target hot water temperature is 57°C (135°F). Buffer supply temperature is 65–68°C (149–154°F), sometimes pushing above 65°C despite the limiter.
Circulation line temperature is 52°C (126°F).
The domestic hot water generated initially starts at around 60–62°C (140–144°F) but drops to about 55°C (131°F) under full load.
At the consumption points in the apartments, temperature is approximately 55 to 57°C (131–135°F).

Supply: 5 residential units (not all showering or bathing simultaneously—usage is well distributed).

Please do not suggest lowering the fresh water temperature to 50°C (122°F) or similar. Producing water at 57–60°C (135–140°F) is desired and should remain. Even if this leads to increased scaling (water hardness about 10 °dH).

The question is: Why does the domestic hot water system produce fluctuating temperatures with the conditions described above, which is annoying when showering?
During summer, with solar assistance, this problem is less noticeable.

Perhaps someone has experienced a similar issue with a domestic hot water station.

Thank you very much for your help!

Has the fresh water pressure in the house ever been slightly adjusted to see if this change might improve the situation? Could it be that unwanted turbulence in the buffer tank is causing imperfect layering?

Temperature fluctuations in fresh water stations seem to occur occasionally in some systems here.

I also suspect turbulence, which then temporarily causes issues with the flow rate. The output capacity of the fresh water supply should actually be sufficient.

Joedreck schrieb:

I also suspect turbulence, which then temporarily causes issues with the supply flow. Considering the flow rate of the domestic hot water, it should actually be fine.

Hello.
Yes, the flow rate should be more than sufficient. A high-performance domestic hot water system.

Regarding layering:
According to the thermometer and boiler sensor, the layering should actually be fine.
Top of the buffer tank approx. 65-70°C (149-158°F) (this is where the supply line to the domestic hot water is connected. A simple temperature limiter is set at 65°C (149°F) in between).
Middle layer of the storage tank approx. 63°C (145°F) (about 500 liters between 70 and 63°C (158 and 145°F)). This is where the underfloor heating withdrawal is, which runs with a supply temperature of about 35-38°C (95-100°F).
Bottom layer of the buffer approx. 53°C (127°F).
Return flow of domestic hot water approx. 52°C (126°F) (almost at the bottom).

The buffer charges when the temperature at the bottom reaches about 57°C (135°F), and then charges up to 57°C (135°F) in the middle of the buffer.
Special note: Since the upper buffer sensor (buffer "off") is located in the middle of the buffer tank (instead of at the top), which allows for a larger buffer volume for the domestic hot water, it is set to switch off at 57°C (135°F) in the middle of the buffer. That corresponds to a temperature range from the top down to the middle of the buffer of about 70-63°C (158-145°F). Although relocating the upper sensor to the top is possible, it would significantly reduce the buffer volume. I’ve noticed that the sensors are not installed in the usual way. The reason for this is simply to increase the buffer volume available for domestic hot water.

However, whether this is the cause of the fluctuating domestic hot water temperature still needs to be determined...

Yes, when the water is still, the layering stays intact. The question is, what happens when water is drawn? Does the incoming fresh water cause turbulence throughout the entire buffer, leading to fluctuations?

Joedreck schrieb:

Yes, if the water is still, the layering is stable. The question is, what happens when water is drawn? Does the inflow of fresh water cause turbulence throughout the entire buffer tank, leading to temperature fluctuations?

Yes, that could be true.
But this would apply to all such systems.
The fresh water return always enters at the bottom, and the fresh water is drawn from the top of the tank, where the boiler inlet is also located.
Underfloor heating is also returned at the bottom and drawn from the middle for the underfloor heating circuit.
However, the temperature differences between the layers are not very large.
The fresh water return is around 52°C (125°F).
Such temperatures at the bottom of the tank are normal.

If that were the case, the principle of buffer tanks and fresh water systems would not make sense.
Therefore, I suspect there is another issue.