Expert wanted, defective heating systems, several repair attempts

Hello,
does anyone here have experience with heating systems in connection with ventilation systems and [Schwimmbad]? Or do you know a competent company or expert? Our heating system has been operating with major defects for 3 years, despite numerous repair attempts no improvement was achieved. Now nothing works at all, [Schwimmbad] cold, hot water and heating. No one is able to fix it. We would be grateful for any advice.:confused: The developer is now shirking his responsibilities!!!

....Now nothing works anymore, pool cold, hot water and heating. No one is able to fix it.

The latter is rather unlikely. However, it must first be clarified whether it is actually a developer or, as is usually the case, a general contractor. Legally a significant difference.

Best regards.