No hot water? Replace the backflow preventer!

The shower no longer gets warm?
Hot water pressure too low?
The solution is to replace the backflow preventer (non-return valve) in the cold water inlet of the hot water tank. The calcified backflow preventer no longer opens and cold water cannot flow into the storage tank.
Only the backflow preventer top part needs to be replaced.
Required material:
Angled seat backflow preventer, brass, according to DIN-DVGW
here: DN 20 - internal thread 3/4" (alternatively only the upper part)
If necessary, fitting grease for drinking water
if necessary, descaler/citric acid