What is the purpose of the water heater safety tank on top of a water heater?
“Safety Tank”? It looks like a typical expansion tank. Used to allow a space for water to go when heated and expands.
That is an expansion tank.
The main purpose of the expansion tank is to compenate for the expansion of water due to heating it against a closed valve, check valve, or backflow preventor upstream of the water heater.
This expansion tank will basically give expanding hot water a place to go so the increase in pressure brought about by heating the water will be controlled and not cause a failure in the plumbing system. There’a a bladder in this tank that expands as pressured hot water enters, then the water recycles itself back into the plumbing system as it cools down.
I figured it was an expansion tank, but I also figured it never hurts to verify. Thanks guys! It’s good to be NACHI
Expansion tank is needed in a “closed loop” hot water sytem. Hot water from the tank is contiuously pumped in loop that returns to the water heater,usually where the drain valve was.
Rod,
Please see David’s post above as to the reason expansion tanks are in place.
Just remember they have an air fill valve like a bike or car tire and usually need to be set at 40 psi
Temperature Pressure Relief valve vent is wrong.
Hope you noted that in your report.
Looks like CPVC to me, which is allowed. In any event, the post was from 4 years ago, so I think he would be off the hook by now 
Look closer Jeff.
Is it disconnected? It’s hard to tell on my tablet screen…