Originally Posted by iamgrowler
It does and it doesn't.
When the entire system is included, there is some room for thermal expansion -- Including back to the main IF the system doesn't have a backflow preventer inline.
BTW, we are now in an era where many municipalities across the US are requiring backflow prevention of the potable water supply -- Making each individual residence in itself a closed system -- Unfortunately, many of these municipalities have failed to make the next logical leap, which is to mandate the installation of an expansion tank.
When it's just the tank closed off, there is no room for thermal expansion -- And if it's a newer tank, say fifteen years or newer, the dialectric nipples in the tank are likely to be one way, which means the expansion tank, if there is one, isn't going to do you a bit of good.
An easy workaround for this shortsighted mandating of closed systems without proper relief would be to require a Reduced Pressure Backflow Preventer, which has a built in double check and relief system -- But nobody really thinks of these things.
Even something as simple as an inline pressure relief valve set to pop off at say 120psi on the cold side of the system and gravity drained to the outside or to an indirect drain would be better than what is currently mandated -- Which is nothing.
I'm not sure I understand what you mean.
Are you asking if the gas or electricity shuts off when the temperature is reached?
If so, where do you think the build up in pressure goes during the heating process if the system is closed and confined only to the tank?
If this "automatic shut off is malfunctioning", then doesn't it stand to reason that the T&P relief valve might also be subject to malfunction and failure?
You also mention 'relieving the water to the drain'.
I've been in a number of homes where the T&P was never run, or run only to a floor drain.
Assuming only the hot side was shut off -- Do you think a floor drain is going to keep up with water gushing out of the T&P from anywhere between 35 to 100psi?