Someone forgot to install a dielectric fitting by to prevent electrolytic action between ferrous and non ferrous metals thus the mineral deposits and failure of the home owner to open the owners manual and do a blow down and test the T&P as required by the NBBI /ASME section IV and GAMA causing the internal diameter of the water outlet to diminish reducing the volume to flow and causing a severe friction loss which results in pressure drop as the water enters and leaves the hot water source

Heard of one case where one of the balls in the heat traps was jambed down into the pipe.

It happended after there was a water line break and it pulled a vacuum on the supply line.

Hi Bill, That is why I make my own heat sink by installing the cold water feed 27" down then bring it up to the heater to prevent stratification of hot water molecules entering the cold water line.

Holby valve (mixing / tempering) has a great book available on how to use their amazing product.

No mechanical device needed thus much less to go wrong with the system