ᐅ COP / annual performance factor below 1 due to an unsuitable domestic hot water station
Created on: 25 Apr 2025 17:03
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LudwigMucL
LudwigMuc25 Apr 2025 17:03Good day. We built a KfW 40 Plus three-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 a capacity of 13 kW. There is an 800-liter (210-gallon) buffer tank and a domestic hot water station in each apartment.
After moving in, we noticed that our hot water was only 36-41°C (97-106°F) due to hysteresis, with a flow temperature of 52°C (126°F), and it lost another 4 degrees with every shower. The buffer tank temperature was then increased to 60°C (140°F), resulting in a water temperature between 42 and 48°C (108-118°F), depending on hysteresis. After a hot bath, the water temperature dropped to 37°C (99°F). Then I realized that the installed units in the apartments were not the contractually agreed Straba W-FBR but an Oventrop regudis W-HTF, installed without our knowledge.
A comparison of the technical data shows that the Oventrop model performs significantly worse.
Oventrop regudis W-HTF Staba W-FBR
Delta T 15°C (27°F) 3-5°C (5-9°F)
Power output 42 kW 88 kW
Draw-off capacity 17 liters/min (4.5 gallons/min) 21 liters/min (5.5 gallons/min)
As a layperson but a retired engineer, looking at these values, it was clear to me why we have to run with a flow temperature over 60°C (140°F) to get water at 43-48°C (109-118°F). Since then, we have been told that there is no difference between the two stations and that both are equivalent. The boiler was originally planned with a capacity of 1000 liters (264 gallons) but did not fit through the door, so it was reduced to an 800-liter (210-gallon) tank. The company Oventrop also confirmed in writing that their FRIWAST cannot be operated economically in combination with a heat pump.
Looking at the COP/annual performance factor, in January, the value was 1.7. After adjustments of the volume flow, differential pressure, and the position of the temperature sensors on the buffer tank, the COP/annual performance factor dropped to 1.2, and the boiler temperature rose to 64°C (147°F). At a further appointment, a return flow temperature lift was implemented, which resulted in a COP/annual performance factor decrease to 0.81. When questioned about this COP, the Daikin technician said, "The values from the heat pump don't mean anything; you would need to install an electric meter on the heat pump and heat meter in both the supply and return to get accurate figures."
The hot water now lasts for two showers, but running at a flow temperature above 60°C (140°F) 24/7 all year round is not ideal for us. Ideally, we would prefer a 400-liter (105-gallon) hot water tank that heats up 1-2 times per day, while the buffer tank is operated at the usual flow temperature of around 35°C (95°F). The feasibility of this was already 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 enthusiasm for the FRIWAST.
After moving in, we noticed that our hot water was only 36-41°C (97-106°F) due to hysteresis, with a flow temperature of 52°C (126°F), and it lost another 4 degrees with every shower. The buffer tank temperature was then increased to 60°C (140°F), resulting in a water temperature between 42 and 48°C (108-118°F), depending on hysteresis. After a hot bath, the water temperature dropped to 37°C (99°F). Then I realized that the installed units in the apartments were not the contractually agreed Straba W-FBR but an Oventrop regudis W-HTF, installed without our knowledge.
A comparison of the technical data shows that the Oventrop model performs significantly worse.
Oventrop regudis W-HTF Staba W-FBR
Delta T 15°C (27°F) 3-5°C (5-9°F)
Power output 42 kW 88 kW
Draw-off capacity 17 liters/min (4.5 gallons/min) 21 liters/min (5.5 gallons/min)
As a layperson but a retired engineer, looking at these values, it was clear to me why we have to run with a flow temperature over 60°C (140°F) to get water at 43-48°C (109-118°F). Since then, we have been told that there is no difference between the two stations and that both are equivalent. The boiler was originally planned with a capacity of 1000 liters (264 gallons) but did not fit through the door, so it was reduced to an 800-liter (210-gallon) tank. The company Oventrop also confirmed in writing that their FRIWAST cannot be operated economically in combination with a heat pump.
Looking at the COP/annual performance factor, in January, the value was 1.7. After adjustments of the volume flow, differential pressure, and the position of the temperature sensors on the buffer tank, the COP/annual performance factor dropped to 1.2, and the boiler temperature rose to 64°C (147°F). At a further appointment, a return flow temperature lift was implemented, which resulted in a COP/annual performance factor decrease to 0.81. When questioned about this COP, the Daikin technician said, "The values from the heat pump don't mean anything; you would need to install an electric meter on the heat pump and heat meter in both the supply and return to get accurate figures."
The hot water now lasts for two showers, but running at a flow temperature above 60°C (140°F) 24/7 all year round is not ideal for us. Ideally, we would prefer a 400-liter (105-gallon) hot water tank that heats up 1-2 times per day, while the buffer tank is operated at the usual flow temperature of around 35°C (95°F). The feasibility of this was already 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 enthusiasm for the FRIWAST.
A
Allthewayup25 Apr 2025 20:57If you have a separate fresh water station but still run a 35°C (95°F) flow temperature for the heating in a KfW40 Plus house, won’t it get too warm inside? The valves would have to be significantly throttled to prevent that. We ran a maximum flow temperature of 29°C (84°F) at an outdoor temperature of -8°C (18°F). Is the heating system properly balanced? Does your stated seasonal performance factor refer only to January? The seasonal performance factor is given as an average value. In summer, a heat pump can easily achieve 5.x or even better, while in the coldest winter, a value of 1.x is not unusual—depending on the consumption.
Is the single 800-liter (210-gallon) buffer tank used for all three residential units?
Is the single 800-liter (210-gallon) buffer tank used for all three residential units?
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LudwigMuc25 Apr 2025 22:46Hello Allthewayup,
We have a FRIWAST unit in each apartment. Currently, the buffer water arrives there at about 60°C (140°F) and is mixed down as needed for the underfloor heating. For the domestic hot water, the buffer water at 60°C (140°F) is passed through the heat exchanger. Due to the significant heat transfer loss, the system only heats the water up to 45°C (113°F) (delta T). The mentioned 35°C (95°F) was just an approximate value with which we would operate the buffer tank after the retrofit. The advantage I expect is that instead of operating the buffer at 62°C (144°F) all year round, I would operate it at around 35°C (95°F) for about 180 days. With a flow temperature of 35°C (95°F), I hope to achieve a significantly better COP/seasonal performance factor than with a flow temperature of 62°C (144°F). The COP/seasonal performance factor is based on the past two weeks when we had outside temperatures around 20°C (68°F) and nighttime lows down to 10°C (50°F).
We have a FRIWAST unit in each apartment. Currently, the buffer water arrives there at about 60°C (140°F) and is mixed down as needed for the underfloor heating. For the domestic hot water, the buffer water at 60°C (140°F) is passed through the heat exchanger. Due to the significant heat transfer loss, the system only heats the water up to 45°C (113°F) (delta T). The mentioned 35°C (95°F) was just an approximate value with which we would operate the buffer tank after the retrofit. The advantage I expect is that instead of operating the buffer at 62°C (144°F) all year round, I would operate it at around 35°C (95°F) for about 180 days. With a flow temperature of 35°C (95°F), I hope to achieve a significantly better COP/seasonal performance factor than with a flow temperature of 62°C (144°F). The COP/seasonal performance factor is based on the past two weeks when we had outside temperatures around 20°C (68°F) and nighttime lows down to 10°C (50°F).
Hi,
this overall sounds like an unfortunate mix of planning gaps and lack of communication with the construction company. If your contract guaranteed the Straba W-FBR but the Oventrop Regudis W-HTF was installed without any explanation, that is definitely unacceptable—especially in a KfW 40 Plus new build, where efficient heat generation is essential.
Regarding operating costs and the COP: constantly running a heat pump at supply temperatures above 60 °C (140°F) is simply uneconomical—particularly for a system designed for low supply temperatures. And when even the manufacturer of the FRIWAST advises against combining it with a heat pump, that says it all.
A separate domestic hot water storage tank with 400 liters (105 gallons), heated specifically to 60 °C (140°F), while the buffer tank for heating operates at a low level, sounds like a very sensible solution—especially if the heating installer has already deemed it feasible. This would significantly improve the heat pump’s efficiency during heating operation while ensuring hot water comfort at the same time.
Personally, I would not spend more time trying to optimize the current FRIWAST setup—it all sounds very complicated. If you want to focus on long-term cost-effectiveness and functionality, converting to separate domestic hot water preparation is probably the better option.
Best regards,
Chris
this overall sounds like an unfortunate mix of planning gaps and lack of communication with the construction company. If your contract guaranteed the Straba W-FBR but the Oventrop Regudis W-HTF was installed without any explanation, that is definitely unacceptable—especially in a KfW 40 Plus new build, where efficient heat generation is essential.
Regarding operating costs and the COP: constantly running a heat pump at supply temperatures above 60 °C (140°F) is simply uneconomical—particularly for a system designed for low supply temperatures. And when even the manufacturer of the FRIWAST advises against combining it with a heat pump, that says it all.
A separate domestic hot water storage tank with 400 liters (105 gallons), heated specifically to 60 °C (140°F), while the buffer tank for heating operates at a low level, sounds like a very sensible solution—especially if the heating installer has already deemed it feasible. This would significantly improve the heat pump’s efficiency during heating operation while ensuring hot water comfort at the same time.
Personally, I would not spend more time trying to optimize the current FRIWAST setup—it all sounds very complicated. If you want to focus on long-term cost-effectiveness and functionality, converting to separate domestic hot water preparation is probably the better option.
Best regards,
Chris
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