HvH combines air heat pump with panel radiators - temperature values?

Hello community,

I just had a conversation with my responsible regional manager from Heinz von Heiden. As standard, the houses now have an air source heat pump with conventional radiators operating at a flow temperature of 55°C.

Isn't a seasonal performance factor (SPF) of less than 2 inevitable there?

From a gut feeling, I would say that a flow temperature of 40°C is sufficient for almost all days of the year. But that is actually still way too high for a heat pump.

What do you think?

Brief research on the internet: it is possible - at least up to 45° (albeit with somewhat higher costs, but probably not a complete disaster), but if you don't have to, why should you? Furthermore, there seem to be special systems designed even for higher flow temperatures - whether that makes sense?

Hi BH
sure, I was already at that point. But just because the heat pump companies say a 45° flow temperature is okay, that doesn't necessarily have to be the case. From what one reads, the practical experiences of many users of a WPL are rather sobering, especially with underfloor heating, mind you.
The pumps designed for higher flow temperatures also cost significantly more, and of course there are special radiators for low temperature, but those are even more expensive, have fans, etc. So no alternative to gas.

By the way, it would be a Rotex CPU 308/or 508 with a 300 l layered storage tank on regular 22 mm convectors.

If the heaters are poorly designed and everything has been optimistically calculated, and the 6 kW heater then does not deliver the power at -10°, causing the heating rod to switch on, it will be expensive - yes. If the system is properly calculated and the moisture is gone after the first 1-2 years, the nasty awakening should actually be avoided. Simply put: 2.5% more electricity per degree of flow temperature (I picked that up somewhere)... so 25% more -> so not 80 euros per month anymore but rather 100 euros ... so 240 euros more per year. If you get the house 5000 thousand euros cheaper for that, it’s worth it - but underfloor heating is usually not that expensive. Since your basic tone is already extremely critical, you won’t be happy with this. Choose underfloor heating or gas (or both).

By default, the houses now operate with an air heat pump and conventional radiators at a supply temperature of 55° C.

Honestly, I would not want to build a house like that today. (In any case, take underfloor heating designed for a supply temperature of a maximum of 35 °C. Which system then generates the heat is irrelevant (whether heat pump or gas, liquid gas, oil, pellet or ...). This way you always have the option to install a heat pump later.)

We also only have normal flat radiators. This is because my wife and I lived with underfloor heating for years, and for various reasons, we simply don’t like it. With the house construction, it was possible to change this heating situation for us.

Modern flat radiators have the advantage over older radiators that 70-80% of the heat is emitted through radiation. After more than one heating season, I can say: They work very well: a pleasant warmth is created.

Now I am already making sure that the flow temperature is as low as possible. The heating installer set it to 75 degrees at -15 degrees back then. That is, of course, madness. That makes the return temperature too high, and the advantages of my gas boiler no longer come into effect, I lose efficiency in the range of 3-5%, so money.

In our KfW 70 house (in terms of heating energy requirement, the house is actually closer to KfW 55), flow temperatures between 35 and 42 degrees are sufficient up to outside temperatures of +8 to about 1-2 degrees. At further dropping temperatures (-12 degrees was the top last winter), I then need 47-53 degrees in the flow for indoor temperatures of 21.5-22 degrees.
You operate an underfloor heating there in the range of 32-35 degrees flow... There is already a visible difference.

Regarding your question: I personally would have serious doubts about whether a solution with an air-to-water heat pump and flat radiators is optimal.

Due to the necessary hot water preparation, you anyway noticeably lower the annual performance factor of an air-to-water heat pump.

In my opinion, the air-to-water heat pump requires underfloor heating in order to still achieve a reasonably usable annual performance factor.

Conclusion: If I were the builder, I would never agree to the described solution.

PS: My supervisor has an air-to-water heat pump in his house. We do compare consumption values and costs from time to time. The ones for his air-to-water heat pump are significantly higher per sqm/year. In addition, no hot water warmer than 43-45 degrees is achievable if the auxiliary heating rod is not to turn on constantly (and the auxiliary heater makes you poor, it really consumes electricity without end). It’s no coincidence that my supervisor says that from about 0 to -5 degrees he more or less has a pure electric heater when the thing then runs at 1:1.4 or so…
Personally, I wouldn’t install an air-to-water heat pump anyway. Especially not in the cold and windy region where I live. Because it is hard for it to pay off, it is only widely calculated favorably (which works well with decreasing primary energy factors for electricity). At best, it achieves 1.1, and technically it is nice, yes.
Warm water heat pump or brine-to-water heat pump: brilliant, but expensive, brine-to-water heat pump sadly not possible here for me.