Spanish #plumbers almost exploded the whole house and themselves today.
They were installing two replacement expansion vessels (white containers) in the photo. They had a very ill-conceived theory that the expansion vessels had broken because hot water flowed back from the very powerful three-phase current water heater. That wasn't the reason; they broke the vessels themselves with water pressure. Anyways.
They installed a reverse flow check valve on the position marked with a red line, to prevent hot water from flowing to the expansion vessels.
This not only prevents the expanding water from going into the expansion vessels, but also to the emergency overpressure valve.
The expansion vessels are there, as is standard, to support the water heater, by flexibly allowing the water to expand when it's heated.
They also closed down the valve on top of the water heater.
High school physics question: What happens to water which is heated in a closed container? It makes essentially a huge pipe bomb.
Luckily I noticed their errors and got them to remove the extra valve. They again left without opening the top valve, which if I hadn't noticed, would have broken the expansion vessels once again, the same way they broke them the last time...
