Temperature/pressure-relief or (TPR) valves
are safety devices installed on water heating appliances, such as boilers and domestic water supply heaters. TPR valves are designed to automatically release in the event that pressure in the water tank exceeds safe levels.
If the thermostat in a water heater malfunctions
the heating coil may not turn off at its predetermined temperature. This would cause the water to become super-heated and reach a boiling point when under pressure. The result would be the pressure in the water heater tank would rise beyond the design levels of the tank resulting in its catastrophic failure and explosion. The TPR valve is designed to release before the pressure in the tank reaches dangerous levels and thus stabilizing a dangerous condition. Water heater explosions are rare because a number of simultaneous conditions must occur and redundant safety systems fail at the same time. These conditions usually only align in cases of extreme neglect.
The TPR valve is typically located
on the top or side of the water heater and is vented to the exterior or a safe location where a person would not be exposed to the venting and scalding water. On the top of the TPR valve there is a test lever that the home owner can manually lift to open (release pressure) and test the valve. When this is done some water will be expelled and when the lever is released the valve will close and reseal.
Due to common minerals in our water,
over time the TPR valve may develop some calcification and scale on its seat and mating washer surfaces where it seals. If the valve has not been tested for a period of time this may cause the valve to stick when tested manually. Think of how your eyelashes may be stuck in the morning due to salts excreted overnight. The valve may still function because the emergency release pressure is much greater than the force a homeowner or inspector will apply when testing the valve.
If the inspector were to apply more pressure
and force the valve to open the small mineral crystals which collected on the valves mating surfaces may dislodge and relocate on the mating surfaces preventing the valve from resealing. Sometimes if the valve is tapped, turned and jiggled its mating surfaces will clean and reseal themselves. If the valve does not reseal, it will need to be replaced immediately as it will leak water until it is repaired. This degree of testing is beyond the scope of a Home Inspectors responsibility. The potential purchaser of the home will have to decide if he wishes further testing to determine if the valve will function properly in an extreme condition. It is suggested that home owners test the TPR valve regulary to keep them from scaling.
