ᐅ Air-to-water heat pump consumption at 30 kWh per day

Hello everyone,

We have an 8 kW air-to-water heat pump from Kermi and a KfW55 house with about 200m² (2,150 sq ft). Currently, the indoor temperature is quite warm at 24.5°C (76°F). Yes, I know that is quite high. The consumption figures are only for the underfloor heating and are separate from the hot water usage.

I understand that the higher temperature leads to increased consumption, but could it possibly be too high?

We live just north of Hanau and in December maintained around 23°C (73°F) with a daily consumption of 6-8 kWh. When it got colder around Christmas, consumption rose to about 20 kWh. It has gotten even colder recently (-6 to 2°C / 21 to 36°F), and we simultaneously raised the temperature to about 24°C (75°F), resulting in a consumption of 30 kWh now.

I read that average consumption is based on 19°C (66°F), and for every degree above that, you need roughly 10% more energy. That would put us at about 60% higher consumption, but is 30 kWh then reasonable? According to the KfW application, we require around 30 W/m² (3 W/sq ft), which totals about 6 kW. But what exactly does that 6 kW refer to — over 24 hours? At what outside temperature? With an indoor temperature of 19°C (66°F)?

We had days in December with consumption as low as 6 kWh, which I think is quite good. However, with a few degrees colder weather and the heating curve increased by 1°C (2°F), the consumption jumps so much? By the way, we heat only via the heating curve, without any additional controllers.

Thanks in advance 🙂

M. Gerd schrieb:

According to the KfW application, we need about 30W/m2 (3 sq ft).

That is just a figure. It has nothing to do with consumption or similar.

What did the room-by-room heating load calculation say? How is the underfloor heating designed? What kind of air-to-water heat pump is it?
Of course, you have sauna temperatures, which definitely costs a lot of money, whether the heating system is well or poorly designed.
Such consumption during subzero temperatures can happen...
But we don’t know enough about the heating system for that.

Hey, here are the details:

We have two calculations: Kermi (engineer)
Heating load calculation by Kermi: 8475 (6650)
Underfloor heating design by Kermi: 20/24 (21/24)

The underfloor heating was installed as close together as possible, with tighter spacing near the windows.

We also have a mechanical ventilation system with heat recovery (also from Kermi). It runs 24/7, and when I turn it off, the room temperature rises by 1°C (1.8°F).

30 kWh per day are absolutely reasonable for the area and temperatures, even rather good!

The question is why 24 degrees Celsius (75°F) is needed at all? In summer, I would activate the air conditioning and cool down to 21 degrees Celsius (70°F). Crazy!

With the temperatures and size, the values definitely stay within a reasonable range.

If you had designed the underfloor heating for 24 degrees from the start, it would have been more cost-effective. Increasing the supply temperature by 1 degree already reduces efficiency by about 2 percent, and this is on top of the additional energy consumption.

Nevertheless, a hydraulic balancing should still be carried out.