ᐅ Fresh water station with highly variable domestic hot water temperature

Hello,

I have installed a fresh water station (Buderus FS 2) on a buffer tank (Buderus PNR750-C).

Unfortunately, the temperature of the hot water produced by the fresh water station is fluctuating significantly!
Attached is a picture showing the large temperature variations.
The fresh water station produces hot water (with a buffer tank supply temperature of 70°C (158°F)) ranging from 62°C (144°F) down to around 25°C (77°F) at times.
Showering becomes very unpleasant when the hot water temperature uncontrollably varies by up to 40°C (72°F).

Possible causes such as too low buffer tank temperature can be excluded (it remains consistently high at about 70°C (158°F)).
The flow rate is also well below the capacity of the Buderus FS 2: 8 liters per minute (2.1 gallons per minute) versus 22 liters per minute (5.8 gallons per minute) rated.
The incoming cold water temperature is stable as well. There were no other simultaneous draw-offs.
The primary pump in the fresh water station shows no error.

(The entire system consists of a Buderus Logaplus package S94 together with the Logaplus package K33/2.)

What else could be causing this? I would appreciate any ideas. As it fluctuates now, the Buderus fresh water station is unfortunately completely unusable :-(

Best regards!

At which point was the water taken for the test?
How long has the problem existed?
Is there a circulation system installed and active?

A faulty valve with a thermostat could be the cause.
It does not necessarily have to be the one where the water is taken from, but any other valve as well.
The thermostat allows uncontrolled cold water to enter the hot water pipe.

buderus_friwa schrieb:

The temperature of the domestic hot water produced by the fresh water station fluctuates significantly!
Attached is an image showing these extreme fluctuations.
The fresh water station produces hot water (with a buffer storage supply temperature of 70 degrees Celsius (158°F)) ranging from 62 degrees Celsius (144°F) down to about 25 degrees Celsius (77°F) at times.
Showering is a nightmare with this kind of hot water temperature variability.

RotorMotor schrieb:

At which point was the water sampled for the test?
Since when has the problem existed?
Is there a circulation system installed and active?

A faulty thermostatic mixing valve could be the cause.
It doesn’t necessarily have to be the valve at the tap but could be any other one.
The thermostat can allow cold water uncontrollably into the hot water line.

Hello, thank you very much for your questions and suggestions!

The measurement of the produced hot water and the buffer storage supply temperature was taken directly via the sensors installed on the fresh water station and read out through the Buderus control panel RC320. Additionally, the hot water temperature was measured at the tap point for validation (which matches the sensor readings).
There is no thermostatic mixing valve at the tap. The heating system is about 1.5 months old. The problem has existed since the system was commissioned. Since the system is brand new, we can rule out any malfunction of the fresh water station or buffer tank caused by scaling. The domestic hot water circulation was deactivated for the test to avoid interference with the measurements.
The significant fluctuations in hot water temperature produced by the fresh water station can therefore be observed directly at the hot water outlet of the fresh water station.

My question is whether it is actually possible for a fresh water station, despite more than sufficient sizing, a high buffer supply temperature, and a constant draw-off flow rate, to have such variations in output. A fluctuation of plus or minus 30 degrees Celsius (54°F) seems unacceptable to me. I also can’t imagine that all fresh water stations fluctuate this much, can they?

Hello,

an update:

The cause of the fluctuating domestic hot water temperatures was apparently air trapped in the storage buffer tank.

Solution: Release the air from the storage buffer tank (via the ventilation, usually located at the top of the buffer tank) and refill the water using the refill station. Of course, keep an eye on the maximum pressure indicated by the pressure gauge.

Hope this helps some of you next time!

Best regards