Real Plumbing Help Plumbing and Heating Answers


The boiler relief valve or the blow off valve is a safety valve that protects your heating system from building up too much pressure and possibly blowing up. It's a very important part of your heating system. Some times the relief valve or blow off valve will leak, and naturally people assume that it is because the relief valve is defective. Sometimes this is true, and if it is leaking you should replace it even though it is leaking for other reasons. But it may just be doing it's job. There are 5 reasons a relief valve leaks.

The first reason a boiler relief valve leaks is:

Nothing lasts for ever, and a relief valves or blow off valve will go bad. A boiler relief valve needs to be tested every three months to be sure it is operating properly. if you have not checked yours out for a long time, you should have it checked out ASAP. 
A boiler relief or blow off valve, that has not been tested in a long time may become rusted closed, causing it to be as dangerous as not having one at all. You can test it yourself but it can be dangerous you can get scalded by the 180 degree to 210 degree water, Water this hot will cause a very serious burn. and in most cases once you pop the lever, the valve will not seat again, and the valve will need to be replaced. if you want to do it your self turn the power to the boiler off and leave it for about 2 hours before pulling the lever. Check the temperature first.

blow off valve

The Second Reason a boiler relief valve leaks is:

The expansion tank on a boiler allows the water in the boiler to expand. Water can not be compressed and when it is heated it expands. the expansion tank has a diaphragm with 12 pounds of air pressure in it. This allows the the water a place to go when it is expanding. over time the tank can become water logged or the air leaks out of the tank. when this happens the water has no room to expand, so the pressure builds up and the relief valve will start to leak. the best way to test to see if this is the reason your relief valve is leaking is to watch the pressure gauge (see stuck pressure gauge) when the boiler is heating if the pressure builds while the boiler is heating, the tank is bad and needs to be replaced see expansion tank

Boiler Expansion Tank

The third reason a boiler relief valve leaks is:

The pressure reducing or fill valve for a boiler is set to only allow 12 psi in to the boiler. If this valve fails it will allow the pressure in the boiler to reach 30 psi or higher. causing the relief valve to leak. if your gauge goes over 30 psi and the relief valve does not leak, turn the boiler off and call a plumber ASAP. This is a very dangerous situation. To test for this problem, turn the boiler off, wait till the boiler is cold, drain off some water till the pressure drops below 10 psi, then with out turning it back on, wait to see if the gauge starts to go up again if this happens it is probably the fill valve (it may also be Reason Four)) to eliminate the fifth reason turn the water to the boiler off, if the pressure does not raise with the water off, it's the fill valve. if it still raises with the water off, it is reason four.

Watts 1156 boiler relief valve

The fourth reason a boiler relief valve leaks is:

Some boilers have a hot water coil, sometimes called a tank less water heater. Some times a coil will develop a pin hole in it. If this happens it will cause the house pressure to seep into the boiler causing the the relief valve to leak. to test this turn the water to the coil off and see if the pressure stops raising. An indirect water heater. can become defective and cause the same thing as the coil. to test this turn the water to the tank off and see if the pressure stops raising.  

Boilermate with Burnham boiler

The Fifth reason a boiler relief valve leaks is:

The aquastat or aquastat relay, This is a very important part of your heating system. the item in the picture to the left is a secondary aquastat, it's purpose is to back up the main aquastat, if it fails. Most area codes require this aquastat be installed on your boiler. if you don't have one you should have one installed. Now if the aquastat fails, the temperature of the boiler can raise to a very dangerous point, causing the relief valve to blow off, this is an extremely dangerous situation and must be corrected immediately, turn the power and gas to the boiler off and call a plumber.

Secondary Boiler Aquastat

Boiler gauge settings:


The pressure should be from 12 psi to 18 psi
The Temperature should be between 160 and 180 degrees f. but can be up to 200 degrees f.

To replace a relief valve. Turn the power to the boiler off, let the boiler cool, turn the water to the boiler off, remove the drip tube from the outlet side of the relief valve. (this must be put back after the relief valve has been replaced) Drain the pressure off the boiler, remove the relief valve, make sure you replace the relief valve with a boiler relief valve of the same rating, DO NOT USE A WATER HEATER RELIEF VALVE ON A BOILER. after you replaced the relief valve and put the dip tube back on, turn the water back on and purge the system, then turn the power back on.
Don't want to try this yourself? Use the
24/7 help line   See a stuck boiler gauge