COP / Annual Work Number through unsuitable fresh water station less than 1

Good day. We built a KfW 40 plus 3-family house with the company Rötzer Ziegel Element Haus. The construction phase of 8 months went very well. We have a heat pump from Daikin Altherma 3 H 16 HT with 13KW output. A buffer tank of 800 liters and a fresh water station in each apartment.
After moving in, we noticed that our hot water was only 36-41 degrees (due to hysteresis) at a flow temperature of 52 degrees and lost another 4 degrees with every shower. The temperature of the buffer tank was increased to 60 degrees and we had a water temperature depending on hysteresis between 42 and 48 degrees. After a hot bath, the water temperature dropped to 37 degrees. Then I found out that not the contractually agreed Straba W-FBR but an Oventrop regudis W-HTF had been installed in the apartments without our knowledge.
A comparison of the technical data shows that the Oventrop has significantly worse values.

Oventrop regudis W-HTF Staba W-FBR
Delta t 15 degrees 3-5 degrees
KW output 42KW 88KW
Draw-off rate 17 liters/minute 21 liters/minute

As a layperson but an engineer in retirement, looking at the values made it clear to me why we have to run with a flow temperature over 60 degrees to get water of 43-48 degrees. Since then, attempts have been made to convince us that there is no difference between the two stations and both are equivalent. The boiler was originally planned with 1000 liters but didn’t fit through the door anymore, so it was the 800-liter boiler. The company Oventrop also confirmed to us in writing that their FRIWAST in connection with a heat pump cannot be operated economically.
The look at the COP/yearly performance factor showed a value of 1.7 in January. After changes to the flow rate, differential pressure, and the position of the temperature sensors on the buffer tank, the COP/yearly performance factor dropped to 1.2 and the boiler temperature rose to 64 degrees. After another appointment, a return flow raise was implemented which caused a change of the COP/yearly performance factor to 0.81. When asked about this COP, the DAIKIN technician responded, "the values on the heat pump mean nothing; you would have to install a power meter on the heat pump and heat quantity meters in the supply and return flow" to obtain accurate values.
The warm water now lasts for 2 showers but to run 24/365 with a flow temperature of over 60 degrees is not something we find very good. Our preference would be a 400-liter hot water tank that is heated 1-2 times a day and the buffer tank would be operated with the usual flow temperature of about 35 degrees. The feasibility has already been checked by a heating engineer. I am not sure whether we should install a 400-liter hot water tank or replace the heat exchangers in the FRIWAST. Somehow, I have lost interest in the FRIWAST.

If you have a separate fresh water station and still run 35 degrees flow temperature for the heating in a KfW40 Plus house, doesn't it get too warm inside? The valves must be throttled quite a bit to prevent that. We ran a maximum of 29 degrees flow at -8 degrees outside temperature. Is the heating system properly balanced? Does your annual performance factor only refer to January? The annual performance factor is given as an average value. In summer, such a heat pump easily achieves 5.x or even better, and in the deep winter, 1.x is nothing unusual – depending on the consumption(user). Is the one 800L buffer tank used for all three residential units?

Hello Allthewayup,

We have a FRIWAST in every apartment. At the moment, the buffer water arrives there at about 60 degrees and is mixed down depending on the requirements of the underfloor heating. For the hot water, the buffer water at 60 degrees is passed through the heat exchanger. Due to the high transfer loss, the system heats the water only to 45 degrees (delta t). The mentioned 35 degrees was only an approximate value with which we would operate the buffer tank after the conversion. The advantage I expect is that I will operate the buffer with about 35 degrees on around 180 days instead of 62 degrees on 365 days. With a flow temperature of 35 degrees, I hope to achieve a significantly better COP/annual performance factor than with a supply temperature of 62 degrees. The COP/annual performance factor is from the last two weeks when we had outdoor temperatures around 20 degrees and at night down to 10 degrees.

Hi,

overall, this sounds like a pretty unfortunate combination of planning gaps and lack of communication with the construction company. If the Straba W-FBR was promised in the contract and then the Oventrop Regudis W-HTF was installed without comment, that is definitely not okay – especially not in a KfW 40 Plus new building, where efficient heat generation is essential.

As for operating costs and the COP: running a heat pump permanently at over 60 °C supply temperature is simply uneconomical – especially for a system that is actually supposed to be designed for low supply temperatures. And if even the manufacturer of the FRIWAST advises against combining it with a heat pump, that pretty much says it all.

A separate hot water storage tank with 400 liters, specifically heated to 60 °C, while the buffer storage for heating runs at a low level, sounds like a very reasonable solution – especially if the heating installer already classified it as feasible. This would significantly improve the efficiency of the heat pump during heating operation and at the same time ensure hot water comfort.

Personally, I wouldn’t spend any more time optimizing the current FRIWAST solution – it all sounds like a struggle. If you want to rely on economy and function in the long term, converting to separate hot water preparation is probably the better way.

Best regards,
Chris