buffalobill wrote:
"Temperature/pressure relief valve (T&P)
This valve is designed to prevent a tank from exploding if temperature
or pressure exceeds safe limits, by opening and venting. Unfortunately,
residential valves are somewhat prone to failure. They should be
checked once a year by pulling up on the handle. Water should flow
freely out and stop when you let go of the handle. If it does nothing,
runs or drips, then the valve should be replaced. Banging on the handle
with something hard, like pliers, sometimes will stop drips or even
runs. If not, replace the valve. Hooking up the drain line with a union
or flex connector makes T&P replacement MUCH easier.

People don't like to test their T&Ps. But then, we don't think it's so
much fun to wake up in the hospital, or to patch a big hole in the
roof, either. When water heaters explode, it's catastrophic. People are
injured or die; buildings are severely damaged. Test your T&Ps! And one
more thing: T&P drain lines should go down and out. Never up. If the
valve opens, water will pool there and corrode it shut. Or freeze in
the line in colder climes. We've seen lines plumbed uphill so many
times we've lost count. But there SHOULD be a drain line, usually to
within about six inches of the floor, or plumbed outside. That's code
around here. It's to prevent you from being scalded if the valve should
open while you're standing next to it.

Finally, if water is running out of your T&P line, look for the cause.
It might just be a bad T&P. But it could also signal high-pressure
problems or a dangerously defective control. Don't ignore it!"

this information and more at:

http://www.waterheaterrescue.com/pag...er-safety.html


Got any facts on how many of these explode, how
many people are injured, how many die?

More important, do you have any facts on how many
TP valves just go bad and how many actually
release pressure because of pressure or
temperature build up?

TP valves are a good safety device. Thinking
about them and testing them is a waste of time
that could be better spent paying attention to
your driving, looking both ways before crossing
the street, getting the ladder steady before
climbing, putting the under counter chemicals out
of the kids/grandkids reach, etc. etc.