ᐅ Is the hot water storage tank for an air-to-water heat pump too small?

Hello everyone,

We are planning a solid construction house (semi-detached, two full floors, approximately 140 sqm (1507 sq ft)), not KfW certified, with an air-to-water heat pump and underfloor heating, as well as a central controlled residential ventilation system with heat recovery.
For the air-to-water heat pump, we have the following system offered:
Split heat pump system by Viessmann, model “Vitocal 200-S 201.D10”
The system achieves a coefficient of performance (COP) of 4.0 at A2/W35 according to EN 14511 during heating operation. The energy efficiency class according to EU Regulation No. 811/2013 is A++ for both low-temperature application (W35) and medium-temperature application (W%%).
Additionally, a hybrid storage tank for the heat pump is installed, white in color, with 300 liters (79 gallons) for domestic hot water and 100 liters (26 gallons) for heating volume.

My questions are:
1. The heating volume of only 100 L (26 gallons) seems very low to me (two adults and one small child, with another child expected in the future). Or is the system designed in such a way that 100 L are sufficient?
2. Is it possible to simply install a larger domestic hot water storage tank, or would that make it impossible for the air-to-water heat pump to operate properly, requiring a different system with higher capacity instead?

Thank you for your recommendations!
Best regards,
Nils

NilsHolgersson schrieb:

My questions are:
1. The heating buffer volume of only 100 L (21 gallons) seems far too small to me (two adults and one small child, and another child expected in the future). Or is the system designed so that 100 L will be sufficient?
2. Is it possible to simply install a larger hot water storage tank, or would that make it impossible for the air-to-water heat pump to handle, thus requiring a different system with higher capacity?

You need to separate two things:
Buffer tank for heating – which is usually completely unnecessary and, if anything, reduces efficiency. So get rid of it.
Hot water storage tank – this is for the domestic hot water supply. You have 300 liters (79 gallons), and I can assure you that will be enough. Of course, it won’t be sufficient if two bathtubs are running back-to-back or both children are showering extensively at the same time (I speak from experience with three kids), but under normal circumstances, it works well.

What you are usually offered is a combined storage tank that covers both points above. If the heating technician agrees, have only a domestic hot water tank installed.
Besides that, the heat pump has no problem heating 300 liters (79 gallons). Even 500 liters (132 gallons) is not a problem – it just takes longer, uses more electricity, and you have 500 liters (132 gallons) losing a few degrees overnight. In my opinion, that would be oversized.

Oh, and before I forget. Looking at the heat pump’s performance data, I would call it massively oversized. With that heat pump, you could easily heat a 300 m² (3,230 sq ft) house. Who did the design? How is the room-by-room heating load calculation?
That would be the question I would ask, not the one about the buffer tank. Although with such an oversized heating system, you might actually need the buffer tank...

Many thanks for such a detailed response! So, does "drinking water" possibly mean "hot water" as well?

That’s my concern – that if everyone showers one after another or simultaneously in the morning, it will take a long time for the hot water to return.

I don’t see a buffer tank at the moment, but I’m aware of it for further discussions.

The offer will come from the general contractor’s building consultant and is included in the fixed-price contract for the house; so far, we have only expressed our preferences regarding the heating system.

I have a 300-liter (80-gallon) solar water heater and a 200-liter (53-gallon) electric water heater, both separate, which easily cover the needs of four people, even with a large bathtub. The solar system can provide hot water for several days, but the temperature gets higher. You can still shower at under 40°C (104°F), but for bathing, the water should be around 50°C (122°F).