ᐅ Air-to-Water Heat Pump – With or Without a Buffer Tank?

Hello everyone,
for our new single-family house (160 m² (1722 sq ft) living area, 4 people), we received a quote from a general contractor. The planned air-to-water heat pump is:
Vaillant VWL 115/2 aro Therm air/water 8.4 kW.
We are choosing a 300 L (79 gallons) storage tank.
According to our general contractor and their heating installers, a buffer tank is not necessary.
A heating engineer friend thinks that a buffer tank is advisable.
Opinions found online are also very mixed regarding the necessity.
How have you handled this?

If any information is missing, please let me know briefly. Thanks.

Which energy efficiency rating are you building for? I find the 8.4 kW heating system for 160 m² (1722 ft²) quite large.

I consider the heating system to be oversized.
It would be better if others with more expertise comment on the buffer tank...

I also find a 300L (79 gallons) tank unnecessarily oversized for 4 people.
We use a 190L (50 gallons) tank for 5 people and are completely satisfied, with no shortcomings. A 300L (79 gallons) tank requires additional space in the utility room and more energy to heat, while most of the stored water remains unused most of the time. You might want to reconsider this.

A buffer tank almost always comes at the expense of efficiency. However, it fundamentally depends on the overall system and its usage. A buffer in the return line is usually fine and ensures there is enough water during defrosting.

We also have a 300L (79 gallons) tank. It often has the advantage of allowing good use of photovoltaic electricity. However, it also has the drawback of making it harder to control Legionella bacteria.

The model mentioned is, as already noted, very large, quite loud, and almost certainly oversized, which leads to frequent cycling. According to the manufacturer’s specifications, it is even 11 kW, which is really a lot for a 160 m² (1722 ft²) new build.

RotorMotor schrieb:

We also have a 300L (79 US gallons) storage tank. It often has the advantage of allowing good use of photovoltaic electricity.

That was also the only advantage that came to my mind. However, this usually does not apply during winter and transitional months because there isn’t enough photovoltaic electricity available...

Recommended storage size for the DHW tank was 150L (40 gallons), which we found to be too small after researching online and consulting friends. Therefore, we decided to go with 300L (80 gallons), as we often have overnight guests.
Space is not an issue since the equipment will be installed in the attic.