The Daring Dufas wrote:
John wrote:
Tom Horne wrote:

Putting a valve on the output line of a hot water heater can lead to
tank damage from water hammer.

How could a valve on the output line possibly cause tank damage?
Maybe if I were to close that valve fast for some reason, but that
would be the case if I closed another hot valve fast.

Many water pressure regulators for homes have check valves
that prevent water from going back to the water meter and
water heaters have check valves in the short pipe nipples
you may see on top of a tank type water heater. The check
valves allow water to flow only in one direction. Water in
the heater expands and the typical home plumbing has enough
room for the expanding water. If not, an expansion tank is
called for. If you were close valves on both hot and cold
connections to the water heater, the T&P safety would open
and flood the floor if it's not piped to a drain. Many water
heaters have a pipe connected to the T&P valve that stops
just above the floor next to the heater. Look at this website
and read the paragraph titled "Thermal expansion". The site
has some good information about water heaters that is easy to understand.

I understand that *operating* a water heater with the valves closed
would be unwise, but one doesn't install a valve on a water heater so
that it can be operated while closed. The valve is for closing when
the water heater is offline to be serviced or replaced. This is like
having a drain plug on a boat, you don't have a drain for when you use
the boat, it is for when the boat is offline/out of the water!