Studying the household water heater...aspie style

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Why do we call them hot water heaters...we wouldn't need to heat the water if it was already hot would we??? enough of that, this isn't about what we call them!

Okay, I had to replace the ancient water heater in my house that was probably the original one that was put in when the house was built. It's the typical old-school american tank style water heater. The little pressure relief valve on top got my head scratching though. To an NT, the valve makes sense...it's to relieve pressure if the tank oveheats. To me however, it is a bit more puzzling. Yes, these valves were required on heaters to prevent them from exploding into a dangerous missle if pressure builds up too much within.

Yes, but how does the pressure build up??? Okay, if something were to fail in the water heater and the heating element ran continuously, the water would eventually boil and make steam. Okay, so that's where the pressure would come from right? Well, not so fast! Most people I know have water heaters connected to the municipal water system, which is pressurized with about 30-60 PSI of pressure. Since there's not check valve in a water heater, if steam pressure was to build up in a water heater, the pressure would force it's way back through the municipal water system.

The typical old-school tank water heater has the "input" pipe going all the way to the bottom, while the "output" is at the top. If it were to malfunction as I described, the water would boil to steam and rise to the top of the heater. It wouldn't escape unless you turned on a hot water faucet. Instead, the steam pressure would proabably force the water within the heater back up the "input" pipe and into the municipal system. Now, before you start bubbling steam up into your neighbor's faucets, chances are the heating elements would burn out because they aren't covered in water anymore. Your water heater would fail rather quietly.

The only way I see this not holding true is if someone were to close the input valve. Usually though, most people don't close this valve unless they are replacing the water heater, or if they have a leak. If you're my father, he'd close it if you too too long in the shower!

What you are saying sounds very technical, water heaters, particularly gas ones scare me!!

Have you ever tried to fathom how a refrigerator works? We take anything that works flawlessly for granted.

I'll give this a shot.

I'm going to guess that the 30-60psi is the flow pressure on an open valve. The pressure required to force gas or liquid up a water filled pipe say 50 to 100 ft. long will be much higher than 60 psi. Your hot water tank probably would not be able to withstand such pressures.

The other thing to consider is safety shut-offs along the outside pipes to prevent back flow. I'm not all that familiar with municipal water systems but it seems that such a device would be needed.

is there a one way valve in there perhaps?

ok, that'll be the 'check valve' then...

There's a website called How Things Work that can probably answer all your questions. And very good questions they are!