A relief valve in a residential boiler system is set at 30 PSI and there is a maximum allowable temperature of 250 degrees farenheight. The reason that the relief set point is 30 PSI and the maximum allowable working pressure is 50 PSI is so that the maximum is never reached, and no damage is done to the boiler in the event of a rapid increase of pressure. One thing that many people have a tough time understanding, is water under pressure without heat applied, is not anywhere as dangerous as water under pressure with heat. And water under pressure, heated past its corresponding boiling point is very dangerous.

Water that is in a closed system, that has an increase of pressure, but not an increase of heat, would simply flow out, if a valve were to open or a leak would occur. Yes, if the pressure was really high the rate at which the water would flow out would be greatly increased, but the water would still come out on in liquid form, leaving little danger. To give a comparison, there is about 30 PSI when water comes out the average sink, and it is comfortable to put your hand in this liquid water. A home garden hose can be adjusted so that the water comes out at about 2500 PSI, and it is less comfortable, maybe even stings, to put your hand in this liquid water.

Now consider the effect of adding pressure and heat.

If water was in a closed system (with a surronding one atmosphric pressure) and heat is applied to bring the water pressure to 10 PSI, the water could be heated to 242 degrees farenheight before it would start to boil. Lets say the water temperature was heated and reached a pressure of 10 PSI and a temparture of 225 degrees farenheight. Inside the closed system this would be fine because the increase in pressure on the water molecules increased the boiling point of the water. If you were to open a valve to the atmosphere at this given time, it would be very dangerous. The water that is heated and under pressure would immediately flash to steam, since it is past it's boiling point of 212 degrees at 1 atmosphere. When water flashes to steam instantly, there is a great deal of danger, This is because 1 cubic foot of water flashes to 16 cubic feet of steam. Inside of a closed system this happens so fast that the piping will shatter causing an explosion.

A relief valve, with pop off at the set pressure of 30 PSI, will relieve the pressure in the system before it becomes too dangerous to operate.

The next thing that might come to mind is how the water temperature is maintained in a boiler, and how normal expansion of heated water is handled? Well, the water temperature is maintained through a control called an aqua stat, and the thermal expansion is handled through an expansion tank.