What performance does our heat pump need?

Good day,

we will start building a single-family house at short notice. We have decided on a ground source heat pump with deep drilling as the heating system.
Only the type of heat pump and its capacity make us more than uncertain.

The house will be 143 m², underfloor heating, insulation in the Kfw70 range, 5-person family, 200 liter buffer tank.
The offers range from 5.9 kW or 7.7 kW (Viessmann Vitocal 222) up to 8 kW. The capacity required for the heat pump, calculated by the companies, ranges from 5,x to 7,x kW.

We often hear and read that the correct dimensioning of the heat pump is the key to low heating and hot water costs. We are now quite ... confused due to the many different recommendations and calculations.

Who can help? What capacity should the heat pump have?
Is 5.9 kW sufficient? Better 7.7 kW?
Or something completely different?

I can't really say much about the heat pump right now, but 200 liters of buffer storage seems a bit too little for 5 people.

What is the buffer tank for? Hot water or heating?

The buffer tank is for the heating. The hot water tank has a capacity of 170 liters.

Hello,

..... We have decided on a geothermal heat pump with deep drilling as the heating system....

Sometimes a more inexpensive trench collector is also sufficient.

.... The offers range from 5.9 kW or 7.7 kW (Viessmann Vitocal 222) up to 8 kW. The required power of the heat pump calculated by the companies ranges from 5.x to 7.x kW…..

Usually everyone calculates with their own, mostly sales physics. ;-)

.... We often hear and read that the correct dimensioning of the heat pump is the key to low heating and hot water costs.…

Correct! For an exact dimensioning, the actual demand (power, energy) for heating and hot water must first be calculated (basic determination). The room heating loads must also be known in order to be able to design the heating surfaces and hydraulics! Furthermore, for the dimensioning of the source (geothermal heat), one must finally know how many kWh/year it must deliver in total. The imposed cooling capacity should also be known.

.... Who can help?

An independent building services engineer with appropriate professional experience. Otherwise, there is only guessing, estimating or assuming. For this, however, the investment would be too high for me.

Best regards

170 liters of hot water for 5 people is definitely far too small. At least 300 liters. The buffer tank for the heating system is technically not necessary and rather counterproductive for consumption. However, a heating system without a buffer tank is quite difficult to adjust and balance properly if you don’t want to accept a loss of comfort. The heating engineer cannot manage that because he would have to live with you for a year to do so. Therefore, heating engineers tend to generally equip systems with buffer tanks. The size of the heat pump should actually be calculated by the house manufacturer or the architect. There is finally a heating load calculation for that. I can only provide you with a rough comparison: For me, 6.7 kW were calculated for 225 sqm gross area ( = 174 sqm living area + gallery and other spaces) and KFW70 from the house builder. As a safety margin, I added a little and installed 7.8 kW. That was a small mistake. The 6.7 would have been correct. However, there are only two of us. By comparison, I would say that for you the lower value is probably more appropriate. If the heat pump is somewhat oversized, it can be well disguised by various settings. That is not the end of the world. Much more important is that the pipes of the underfloor heating are laid at close intervals of about 10 cm. House builders like to save on that.