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Boiler Keeps Blowing Relief Valve? Here’s Why (+ How To Fix)

A boiler has a relief valve rated for its heating capacity (BTU) and maximum allowable working pressure (MAWP). This relief valve blows when the pressure inside a closed-loop boiler system exceeds the maximum psi or bar. So, what to do when a boiler keeps blowing its relief valve?

When a boiler keeps blowing its relief valve, check the pressure and temperature in the closed-loop system. The relief valve may be faulty if it is blowing at allowable pressures and temperatures. Also, check the expansion tank, pressure regulator valve, and aquastat. 

A few typical issues are generally responsible for a boiler that keeps blowing its relief valve. These problems are relevant to gas and electric boilers with any type of expansion tank, compression, bladder, or diaphragm. Read on to know what you can do to detect the cause and how you can fix it. 

Why Your Boiler Is Blowing Its Relief Valve

Most residential boilers use a relief valve rated for 30 psi (~2.07 bar). So, this temperature and pressure relief valve should blow or allow water to leak through its outlet when the closed-loop boiler system exceeds 30 psi (~2.07 bar).

Thus, you should check the temperature and pressure gauge for your boiler to confirm if the relief valve is doing its job correctly. If it seems to be working properly, you can investigate the other probable causes in this guide. 

Defective Relief Valve

However, the relief valve can blow despite the pressure gauge reading below 30 psi (~2.07 bar).

The pressure inside a standard residential boiler system can reach ~25 psi (1.72 bar) when you call for heat. A relief valve should not blow at this pressure, but it can fail due to wear, sediment buildup, corrosion, or damage. Hence, the relief valve is faulty, and you should replace it. 

Failing Expansion Tank

Suppose your boiler blows the relief valve when the pressure gauge reads 30 psi (~2.07 bar) or greater. Thus, the relief valve is working alright, and you must address the pressure spike in the boiler system.

A common reason for this pressure spike is a failing expansion tank. Boilers need an expansion tank that allows the water to expand as the heating system functions. 

The pressure in the boiler system will increase if this expansion tank is already filled with more water than it should hold. Also, a diaphragm or bladder expansion tank should not have water leaking through the air chamber valve. 

The compressed air in an expansion tank is separated from the water chamber by a bladder or diaphragm. So, any water leaking through the valve under the tank means the diaphragm or bladder is broken. Thus, the relief valve keeps blowing. 

Furthermore, you need to check the air pressure inside the expansion tank. Even if you don’t have water leaking through the valve, the air pressure inside the tank may have dropped below its rated requirement. Hence, the tank won’t do its job properly. 

Faulty Water Pressure Regulator Valve

Most residential boilers need a water pressure regulator valve. The mainline for domestic water supply has a much higher pressure than what a boiler requires. 

For instance, the standard water pressure in a boiler when it is cold is 12 psi (0.83 bar) in most systems. So, the main supply line or feed water pressure cannot be 40 to 60 psi (2.75 to 4.13 bar). Therefore, boilers need a water pressure-reducing valve.

If the pressure regulator or reducing valve is failing or leaking, the psi or bar at the relief valve will spike, and it can blow, causing water to leak or drip. 

Excessive Temperature and Pressure

Generally, residential boilers don’t reach temperatures higher than 180 °F (82 °C). Most boilers will shut if the temperature spikes to 190 °F (~88 °C). Also, the highest pressures are usually in the low 20s psi (~1.4 bar).  

However, if the gauge on your boiler displays a temperature of around 200 °F (93 °C) and 30 psi (~2.07 bar) pressure or more, the aquastat is probably bad. The only exceptions are boilers with manual high limit settings that allow you to select 220 °F (~104 °C). 

4 Easy Fixes for a Boiler That Blows Its Relief Valve

It can be frustrating if your boiler is constantly blowing its relief valve. Luckily, it’s a fairly easy problem to fix once you have identified the root cause.

1. Replace a Defective Relief Valve

You can open the relief valve and let some water drip out to clear any gunk buildup on the seat. However, years of corrosion and repeated blowing up may impair the seat and the spring that holds the tension to sustain the pressure. 

Despite its crucial function and safety implication, a relief valve is a simple device with a spring and a metal cap. This spring can lose its tension in due course. Also, the metal cap and its seat may have structural deterioration. Thus, replacing it is the only viable and durable solution. 

Always match the part number, pipe diameter, boiler capacity, and rated psi or bar when you buy a new relief valve. 

2. Repair or Replace the Expansion Tank

There are three types of expansion tanks used in residential boilers:

  • Bladder
  • Diaphragm
  • Compression

Irrespective of the type, your boiler’s expansion tank has a specific size and rated air pressure. This rated air pressure can be anywhere from 12 psi (0.83 bar) to 40 psi (2.75 bar). Hence, you need an air gauge to measure the pressure inside the expansion tank.

Here are the steps:

  1. Turn off the power source for the boiler.
  2. Allow the boiler system to cool.
  3. Shut the feed, return, and supply valves.
  4. Disassemble the expansion tank.
  5. Remove the valve cap from the tank.
  6. Get a screwdriver and press the valve.
  7. See if water leaks through the valve.
  8. If water leaks, the tank is unusable. 
  9. Else, use a gauge to measure the air pressure.
  10. Check if the air pressure is lower than the rated psi or bar for the tank.
  11. If the pressure is low, refill the tank using a manual or powered air pump.
  12. Adjust the refill and let some air out as required to get the rated psi/bar.

You can restore the air pressure in an expansion tank if there is no water leaking through the bladder or diaphragm. Otherwise, you need a new one.

Also, check if the expansion tank is an appropriate size for your boiler. Furthermore, review the orientation of the tank. Expansion tanks with a bladder or diaphragm should be installed upright, not upside down. Compression tanks are installed horizontally.  

3. Replace the Water Pressure Reducing Valve

Like a defective relief valve, you cannot do much about a failing or leaking feed water pressure regulator. You need to replace it if it is faulty. However, check the water pressure at one of the downstream feed ports to confirm whether or not the regulator valve is indeed beyond its prime. 

4. Get the Boiler Aquastat Inspected by a Technician

A boiler reaching temperatures and pressures greater than the high limits for both is a hazard. If you suspect this is causing your boiler to blow its relief valve, you should consult a technician and get the system inspected. 

Also, exercise caution if you use the boiler meanwhile. Call for low or medium heat. Avoid high heat until the issue is fixed. 

Final Thoughts

Whatever you do, never cover the boiler’s relief valve outlet to prevent water from leaking or dripping. When a boiler keeps blowing its relief valve, the safety fixture might not be in an impeccable condition. So, check if you can safely use it despite other issues being the problem. 


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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