Water Heater T&P valve

How long have T&P valves been required on water heaters?

I am replacing an old A.O. Smith in a residence that does not have one anywhere.

This is the first one I have ever come across.

T&p

Forever, although, in the 50's there was a time when electric heaters were considered to be "as safe as light bulbs" and many were installed without T&P valves. Domestic Engineering magazine also had almost weekly stories of them exploding and destroying houses and people. One of the more indelible pictures was of a black boy who was sleeping the room next to one and when it went off he was decapitated.

My guess is it used to have a T&P valve, but someone removed it. Possibly it was removed and capped off because it was "leaking" due to pressure buildup (lack of a pressure tank). Or maybe they removed it so they had an extra port into the water heater for a recirculation system or something? Who knows.

Relief valves used to be a bit different than they are now. Many had lead plugs that would melt at too high a temperature and then run water until someone found it and shut off the main. Often, they were smaller - 1/2" instead of 3/4".

And sometimes, they put the relief valve in a line instead of in the heater itself.

Ancient water heaters often had non-immersible elements that were typically 750 - 1200 watts. They heated very slowly. Still, any electric thermostat could fail and heat the tank indefinitely. (That is, until it explodes . . .)

But today's 4500 watt heaters could put you in trouble very quickly. Water heaters include both the 190 degree electric cut-off and the temperature AND pressure relief valve that will discharge whether the water is too hot or there is unsafe pressure. If they don't work, you've got a bomb. I have walked in on heaters that were spewing live steam from the relief valve because the safety electrical cut-off was not working. Had the relief valve been limed shut, the house would have been gone before I got there. (And we have very hard water in my area.)