Air-to-water heat pump sizing in new construction

Anhydrite screed is generally considered well suited for underfloor heating due to its good thermal conductivity

So, I now have the offer from the heating engineer. Looks quite reasonable. However, I am now in a dilemma.

He wants to install the Vaillant Arotherm VWL 105/6 A, which with 10kW heating capacity is significantly larger than our heating demand of 5.9 for heating + 0.8 (estimated) for hot water = 6.7kW according to NAT.

"Actually," the smaller VWL 75/6 A should easily be sufficient and even have reserves. Based on what I have read here, it would still be too big.

However, I am now unsure about the pump flow rates.

VWL 105/6 A: 1418 l/h (788 mbar)
VWL 75/6 A: 790 l/h (640 mbar)

Heat load calculation resulted in:

Pressure loss in the most unfavorable circuit: 7879 Pa
Pump head: 0.79 m
Total flow rate: 22.2 l/min
Total flow rate: 1331.3 l/h

As a layman, I conclude that while the heating capacity of the smaller heat pump would easily suffice, the circulation pump would then be too weak to pump the warm water through my many and long heating circuits – I would have to bite the bullet and take the larger one with the risk of short-cycling, etc.

Did I understand that correctly or am I making a conceptual mistake?

Can I see anywhere how far the large Vaillant can modulate down?

Vaillant states a maximum volume flow of 1205 liters for the 75. You are then a bit above that.

The 55 goes down to 2.10. The 75 goes down to 3 The 105 goes down to 5.40

As a layman, I would even tend towards the 55, but the volume flow no longer fits there.

Thank you for the feedback, may I ask where you found the information? I usually don’t have much trouble googling, but when it comes to all the [Haustechnik] topics, it seems there are different circles....

According to the annual performance factor calculator, the 55 model does not achieve 4.5 at least.

According to the annual performance factor calculator, the 55 model doesn't achieve 4.5.

Thanks for the hint. With my supply temperature and hot water proportion, I already reach an annual performance factor of 4.7 with the 75.