While performing home inspections we often come across leaking boilers. Most of the time the home owners do not even know that the boiler is leaking. While there are multiple reasons why a boiler could be leaking one of the most common reasons is a leaking pressure relief valve. Below is a picture of a pressure relief valve.
Pressure relief valves can leak due to many issues such as defective valve, buildup of corrosion or high boiler pressure. The most common cause of high boiler pressure is a bad expansion tank. An expansion tank like the one shown below is a tank that has a rubber bladder inside.
On one side of the bladder is air and the other side of the bladder is water from the boiler. As the water inside the boiler is heated it expands into the expansion tank. If the rubber bladder inside the expansion tank fails water will get past the bladder and eventually fill the entire tank. The problem with this is once the tank is full the heated water will have no where to expand into and the pressure inside the boiler will rise. This rise in pressure will cause the pressure relief valve to start leaking.
When the expansion tank fails and fills with water it gets heavy. You can easily tell if an expansion tank is filled with water by tapping on the side slightly with a screwdriver or other small tool. First tap on either the top or bottom of the tank. Take note of the sound and how it felt. Now tap on the opposite end and compare how it sounded and felt. One side should have sounded hollow and felt like an empty tank and the other side should have sounded and felt full. This side of the tank is the water side. If the entire tank felt full and sounded dull then it is most likely filled with water and would need to be replaced. To demonstrate how heavy a full expansion tank can be take a look at the picture below. This is a picture of a defective tank we found while performing a recent home inspection. This tank is completely full. Notice how it is so heavy that it is leaning to the right. The tank is actually bending at the point of attachment.
As you would expect, the pressure relief valve was leaking as shown in the picture below. This pressure relief valve was also missing a discharge pipe which should extend to about 6″ above the floor to help keep hot water from spraying onto someone causing injury.
If you suspect your expansion tank is leaking we recommend contacting a licensed heating contractor to evaluate and correct this issue.