Help! My Boiler is Letting Out Steam

If you use a boiler to heat your home, steam is a warning sign your boiler system is not working properly. How are boilers supposed to work? Boilers heat water, which increases the pressure in the boiler. This pressure forces the heated water throughout your home via pipes. The water then transfers its heat to your home’s air through radiators or radiant floor systems.

If steam is escaping your boiler system, you’re wasting energy (and money!) heating water that’s not warming your home. While a decrease in efficiency is not ideal, energy loss is the least of your worries. Steam coming from your boiler can indicate components of your system are over pressurized, leaking, or broken.

Steam coming from your boiler can indicate one of three things:

In this blog, we’ll cover why your boiler is letting out steam as well as what to do to fix it.

When you notice steam around your boiler, the first thing to do is to identify where it’s coming from. Is the steam coming from the pressure relief valve on top of the boiler (also sometimes called the safety valve)? Skip to the next section to learn what to do.

If steam is coming from anywhere besides the pressure relief valve on top of the boiler, such as the body of your boiler or the surrounding pipes, you have a leak.

Your boiler uses the pressure generated during the heating process to distribute heat throughout your home. Steam should not escape the system—except through the pressure release valve (we’ll discuss this in more detail below). If you see steam coming from the boiler's body or pipes, it has a leak.

Leaking steam is scalding hot, so you should contact a professional to fix any leaks to prevent injuries. Trying to fix a leak yourself is dangerous and can sometimes result in severe burns.

Every boiler has a safety valve called the “pressure relief valve,” which releases steam when the pressure in the boiler gets too high. Without this essential valve, your boiler could explode.

How do you know if your boiler is over pressurized? Follow these two steps to check the pressure of your boiler:

• Step 1. Locate the pressure gauge on the side of your boiler.

• Step 2. Read the pressure listed on the gauge to check if it’s within the standard range. Typical boiler pressure should be between 12–15 pounds per square inch (psi). Psi is how pressure is measured.

If steam is coming from your pressure relief valve, but your boiler system is not over pressurized, then your pressure relief valve is broken. Like any component of an HVAC system, pressure relief valves are subject to wear and tear. Pressure relief valves can start to malfunction over time if the seal between the valve and your boiler becomes clogged with sediment.

Solution: Regular maintenance can help to prevent pressure relief valve problems. If your pressure relief valve seems like it is not working properly, contact an HVAC professional for a repair or replacement.

If steam is coming from the pressure relief valve and your boiler’s pressure is above the standard range, skip to the next section.

Is the pressure in your boiler system above the standard range of 12–15 psi? The good news is your safety valve is working! Pressure relief valves release steam once the pressure in your boiler is at 30 psi or above. The bad news is there is another problem with your boiler system.

While you should call a professional to determine why your boiler is specifically over pressurized, some common reasons include:

The pressuretrol is a component that controls the burners depending on pressure levels inside the system. When it’s working correctly, it turns off your burners once your boiler reaches a certain pressure to stop additional water from being heated.

A broken pressuretrol might not recognize when the boiler is pressurized. If it doesn’t turn off the boiler burners, water will continue to be heated, increasing the pressure in your boiler. Too much pressure can cause serious damage to the boiler system.

Solution: An HVAC professional can repair or replace your pressuretrol.

Your boiler’s water valve is what allows water into the system to be heated. If it is not working properly, it may allow too much water into the system, increasing pressure.

Solution: An HVAC professional can repair or replace your water valve.

Older boilers come with expansion tanks. These tanks absorb water when boiler pressure levels rise to keep the boiler system at the correct pressure.

Over time, expansion tanks can become full of excess water. Additionally, the drainage pipes to the expansion tanks can become clogged. Both of these scenarios prevent the expansion tank from reducing boiler pressure.

Solution: An HVAC professional can clean your drainage pipes and drain full expansion tanks.

If air pockets develop in your boiler, they will inadvertently increase the pressure in your system. Sometimes, boilers have air pockets because their water capacity is not full. Air can also enter your boiler system through leaks in your pipes.

Solution: HVAC professionals can release this excess air from your radiators to correct the pressure in your boiler.