Why is the relief valve leaking?
A common sight in a hydronic boiler room is water dripping from the discharge pipe of the boiler relief valve. While it may appear to be inconsequential, it could cause extensive damage to the heating system.
Some boiler rooms have a bucket under the relief valve discharge pipe to mask the problem. The following are some suggestions if you would like to resolve the problem.
Leaking water from a sealed hydronic system can reduce the life of the system by introducing untreated makeup water containing oxygen and solids to the system. The oxygen can attack and pit the boiler and piping, causing corrosion and leaks.
The solids can affect the efficiency and safety of the system. Solids form scale on the hottest surfaces in the boiler lowering the heat transfer ability and efficiency of the system. A leaking relief valve can allow solids to form on the seat of the relief valve, increasing the rate of the leak.
A worse situation occurs when the solids form on the spring side of the relief valve as it could alter the opening pressure. A relief valve was a contributing factor in a fatal boiler accident as scale formed on the relief valve, prohibiting it from opening properly.
The relief valve, rated for 30 psig, was tested after the accident and did not open until the pressure reached 1,500 psig. Diagnosing the cause of the leaking relief valve is time-consuming and sometimes frustrating. I like to explain this to the customer to prepare them when the diagnosis and repair may take more than one visit.
Troubleshooting the valve
The first thing I check is the system pressure. Most hydronic boilers have a gauge called a tridicator, or PTA (pressure, temperature, altitude) gauge. How much pressure do we need for the system? Each pound of system pressure will raise water 2.3 ft. The way to calculate how much pressure you need is to determine the height of the tallest radiator and divide the height by 2.3.
For example, we have a radiator on the sixth floor, and the elevation is 60 ft. high. When you divide the height (60 ft.) by 2.3, we get 26.09 pounds. We should add 3 to 4 pounds to that number to allow for better air removal and to limit the chance of the hot water flashing to steam. This takes our normal system pressure to 29-30 psig.
The next step is to verify the pressure rating of the relief valve. The pressure rating of the relief valve should be at least 10 psig higher than the operating pressure of the system, but less than the maximum allowable working pressure (MAWP) of the boiler. Many hydronic boilers are shipped with a 30 psig relief valve from the factory. In this example, the relief valve should be at 40 psig or higher. If the system pressure is 30 psig and the relief valve, rated for 40 psig, is leaking, the relief valve is most likely defective.
Another troubleshooting task I perform is watching the tridicator (or PTA gauge) while the boiler is firing and heating the water. When water is heated from 65° F to 180°, the water volume expands by 3%. If the pressure gauge starts creeping up as the water heats, I would suspect a flooded compression tank or plugged piping to the compression tank.
In some instances, it may take several days for the pressure to build and open the relief valve and these are the most difficult to troubleshoot. The first place I would look is the compression tank. If the tank is flooded, there are a couple of reasons.
The most common causes are leaking gauge glass fittings above the water line of the tank, excessive system pressure, undersized tan, or the tank has a leak above the water line. If the system has been operating correctly for years, I would be hesitant to believe the tank is undersized.
A pinhole leak on top of the tank may be impossible to find and one of the ways to test the integrity of the tank is to valve off the water feeder to the system and check the tank in a few days to see if it flooded. If the tank is flooded, you might have to replace the tank.
If the compression tank has an enclosed bladder, check the pressure on the bladder side of the tank. To do this, you have to reduce the system pressure to 0 psig and check the bladder pressure using a tire pressure gauge. The internal pressure should be that same as the system pressure. From above, we know the system pressure is 30 psig.
Another culprit that can cause the pressure to rise and open the relief valve is if the boiler has an indirect water tank which uses the boiler water to heat the domestic water using a water-to-water heat exchanger. A leaking heat exchanger could allow the higher city water pressure to enter the space heating side and increase the system pressure. To test this idea, shut the valves from the domestic water side and see if the pressure still rises.
The last item to check is the pressure-reducing valve (PRV). This is a brass valve with an adjustment screw. Some models have a quick-fill feature, which allows you to pull a lever and quickly fill the system. A stethoscope is sometimes used to troubleshoot the PRV to detect if water is leaking through the valve.
Another way to test for leaking is to feel the downstream pipe and see if it is cold. In many instances, the water is fed slowly and difficult to detect. Another way to test to see if the PRV is leaking through is to shut off the valves on the feed water pipe and see if the pressure still rises. If it does, I will suspect the piping to the compression tank is restricted or the tank is flooded. If the pressure does not rise, it could be the pressure-reducing valve.