How can water heater pressure be more than water pressure at the inlet? What is the actual function of relief valve?

Generally it is not much different, if at all, and you would not expect it to be.

One has to make a few assumptions on your system to answer this correctly. First, I'll assume you have a closed system supplied by a municipal supply. The majority of these systems in my area require a one-way flow valve after the meter. Second I will assume you are speaking of a residential home heating system.

Based on this, consider the rest of the system in a cold state. All faucets closed, no leaks anywhere. Assume you have a 40 gallon hot water heater filled with cold water and the system initial pressure is 80 PSI.

Next turn on the hot water heater so that it heats the water. In a closed system like this, the overall pressure of the system will increase due to the expansion of the water in you hot water heater. Lets assume it increases to 90 PSI and your pressure relief valve is set at 100 PSI. Nothing happens.

If you should turn on the tap and let the water flow for a moment, then turn the water flow off. the pressure will return to the supplied pressure of 80 PSI.

If you have a well system, and you perform the same experiment again due to one way valves, the pressure will go up however the pressure tank will adsorb the expansion and the pressure will not rise to the 90 PSI suggested above.

So what is the valve for? Let us assume you have a closed system and you are using an electric hot water heater. Generally these are supplied with a 240 volt circuit switched on only one leg of the supply. You can have a condition in which a failed heating element shorts to ground. In this case you are supplying 120 volts to a partial heating element without any form of control since the thermostat only opened one leg of the 240 volt supply.

The water is now able to continue heating and if you do not open a hot water tap, the water will exceed boiling point and the resulting steam build up will cause the pressure to continue to climb. This is when the safety valve should open stopping rupture of the hot water tank or plumbing system.

Hope this helps,

Newton1Law

PS: Was this a test Steve or didn't you really know??

OFT is mostly wrong on this, the cold and hot side equalize every time you open the tap for mixed water. The cold water dip tube sits at the bottom of the water heater tank and is under water, the hot discharge is at the top by the pressure release valve where steam bubbles can raise the pressure on the outlet at the top of the water tank. This is why the pressure release valve is there at the top, to prevent a boil over from over heating. The cold water inlet is shielded from the boil over flash steam pressure by the submerged dip tube that brings the cold water into the bottom of the water heater tank.

Expansion tanks are only used on boilers and high recovery NSA water heaters in commercial 180F gas heaters. There is no check valve on a residential water heater, again, only on commercial installations.

When water is heated, it expands. When it boils, it creates a massive amount of steam.

The safety valve on your water heater is a TPV or temperature and pressure relief valve. If the water heater were to malfunction and over heat the water, the temperature and pressure would be at issue. This valve will open and release the pressure safely BEFORE the tank ruptures dangerously and/or catastrophically.

Most locations require a back flow prevention valve (check valve) on the supply line. If your system sets for a while with no usage then the heating of the water in the water heater can raise the pressure. That is why most houses have an expansion tank to keep the pressure from building up.

Heat.