Sizing a Heat Exchanger Help

[Mark987]

Could you please help me with the sizing of the heat exchanger for the hot water storage tank?

The capacity of the buffer tank is 2000 liters (528.34 gallons). I have made 2 HX in the upper zone, one for DHW and the other for wet UFH using Alpex pipes measuring 26x3mm (1"x0.12 inches) and both with a length of 50m (164.04 ft).

I made the test with a temperature of 40°C (104°F) in the buffer tank at the inlet of the heating circuit, I couldn't get more than 30°C (86°F).
There is also a problem with DHW, sanitary water from the supply is around 15°C (59°F). When I turn on the faucet, it reaches nearly 40°C (104°F) until the supply from the heat exchanger is depleted, and then it drops to around 25°C (77°F).

I made a mistake in my calculations somewhere, a correction is inevitable. But I would like to ask for the help of someone more experienced to determine how big of a heat exchanger I need so that I don't make two of them

Consideration can be given to a copper or flexible stainless steel pipe for HX, as well as the installation of a submerged stainless steel boiler tank 80-100L (21-27 gallons) for DHW. What do you recommend?

My climate zone is Dfa/Dfb or 5A (continental humid hot/warm summer). The total area of the house is 150m² (1614.59 sq ft) on 2 floors. The maximum heat losses at an outside temperature of -15°C (5°F) and inside temperature of 22°C (72°F) are 4.5-5 kW/h (15342-17060 BTU/h), and the losses at an average outside temperature of 5°C (41°F) and inside temperature of 22°C (72°F) are around 2.2-2.5 kW/h (7512-8533 BTU/h).

I apologize for listing more units and measures, and I'm from Europe, so we use a different system of units.

 

[Mark987]

I had no choice, in the end, I had to change the pex-al-pex pipes and replace them with flexible stainless steel pipes of 1" diameter. I installed about 160 ft ( 50m ) of this pipe for HX in the upper zone and will be used for underfloor house heating.
Instead of pex-al-pex DHW tube exchanger, I installed a 22 gal ( 100L ) stainless steel boiler.

I still haven't finished the buffer tank and put it into operation, but as soon as I have new information I will let you know