Everything You Need To Know About Boiler Pressure

Have you ever noticed the pressure gauge on your boiler? It’s there for a very good reason. Boilers are a bit like Goldilocks – they’re quite picky. The pressure can’t be too high or too low – it’s got to be just right.


That brings up a lot of questions. Why are boilers so sensitive to pressure? Why does a boiler’s pressure change? What are the consequences of high and low pressure? How do I change my boiler’s pressure? The list goes on and on.


Fortunately, you’re in the right place if you want to learn about boiler pressure. We’re going to address all of those questions – and more – so that you know everything you need to know about boiler pressure. 

Why Are Boilers Under Pressure?

Boilers are under pressure for the same reason that you or I is under pressure – they don’t have enough time in their day to accomplish what they want to, they’re filled with a general sense of existential dread, their taxes are past due…


We’re kidding, of course. But it is true that all of us are under pressure all the time – air pressure! The same holds true for boilers. There’s pressure resulting from the forces of air and water in the system straining against their constraints. Fluids take up all the space that’s available to them, and that creates pressure against your boilers, pipes, and radiators.


Boilers are a closed system. This means that when everything is working well, no water or air needs to be added or taken from the system. In an ideal world, the pressure of a boiler would remain constant, fluctuating only with the temperature of the fluids moving through it (more on that later).


Unfortunately, we don’t live in a utopia for physics fanatics, and no system is truly closed. As such, the pressure of your boiler can go up and down. That can create problems. 

How Do I Check Boiler Pressure?

Boiler pressure is easy to check. There should be a gauge on the front of your boiler. On almost all modern boilers, this gauge is incredibly easy to read: If the needle is in the green, you’re in the clear, while if it’s in the red, there’s a pressure problem. The “green” range is usually from about 1 bar to 2 bars – “bars” here meaning the unit of measurement for pressure, not the literal red and green bars you may see. (Confusing, we know.)


What Should My Boiler’s Pressure Be When It’s Off?

When your boiler is off, the pressure should be between about 1 to 1.5 bars. Different manufacturers have different recommendations for exactly what pressure your boiler should have when it’s off, but optimal pressure is usually around 1.3 bars. 

What Should My Boiler’s Pressure Be When It’s On?

The fluids inside your boiler expand when they’re heated. That, of course, leads to pressure building up in the system as the fluids push against the interior of your boiler and pipes. When you turn on your boiler, you should expect the pressure to go up by about .5 bars. That means the pressure level of an active boiler should be from about 1.5 to 2 bars, with an optimal pressure around 1.8 bars. 

What Happens When Boiler Pressure Is Too High?

Back in the day, a boiler with too much pressure was a very dangerous thing indeed. You’ll still see most insurance policies will contain a clause regarding coverage for catastrophic boiler failure or boiler explosion. When the pressure is too high, and there’s nothing to alleviate it, boilers and pipes can burst right open.


Fortunately, in the modern age, your boiler will probably just stop working. There are safety mechanisms to stop boiler pressure from getting too high, including automatic pressure release valves. In other words, it’s more inconvenient than dangerous. 

What Happens When Boiler Pressure Is Too Low?

There’s basically no danger at all to having boiler pressure that’s too low – though if your pressure gets very low indeed, your system’s safety features might kick in to shut off your boiler. In the worst-case scenarios, it could mean there’s next to no water in your boiler, and you don’t want natural gas firing off and heating nothing.


The biggest problem with low pressure is that it can lower the efficiency of your boiler. The system is built with a certain pressure level in mind, including the pumps that move hot water through the pipes. Keeping a high enough level of pressure can help you increase the lifespan of your boiler, all while saving you money on your heating bills. 

How Do I Increase My Boiler’s Pressure?

To increase your boiler’s pressure, you’ll need to add water. There should be a length of braided hose that links two water pipes. This is known as a filling tube. You may not see a filling tube – if you don’t, consult the boiler manual to learn how to repressurize.


Before doing anything else, turn your boiler off. From there, make sure you have a line of sight of the pressure valve – you want to fill your boiler until the pressure valve points to the middle of the green lines (usually around 1.3 bars). Let your boiler cool if it is hot, so the valve is reading cold water pressure.


To fill the boiler, simply turn the two handles that are attached to the filling tube until they point in the same direction the tube is moving in. You should hear the sound of running water. Once the pressure reaches the right level, turn the handles in the opposite direction again.


Wait for a few minutes to see if the pressure seems constant. If it does, you can turn your boiler back on! 

Why does my boiler’s pressure keep dropping?

If your boiler’s pressure is constantly dropping, there could be a few problems with your system. Hopefully, it’s just your pressure gauge that’s faulty – but in some cases, you may have a leak. Either way, you’ll need to call someone for repairs. 

How Do I Decrease My Boiler’s Pressure?

You can decrease the pressure of your boiler by bleeding your radiators. Start by turning off your boiler, then ensuring that the valves connected to your filling tube are fully closed. 


From there, go from radiator to radiator with a pail and either a flathead screwdriver or a radiator key, depending on the kind of radiator you have. Place the pail under the spout below the valve, then open the bleed valve with your screwdriver or key. If water pours out, close the valve and move on. If you hear hissing, it means air is coming out. Wait until water starts flowing out, then close the valve and move on.


Do this to every radiator. By the end of the process, your boiler’s pressure should be around 1.3 bars. If it’s still too high, repeat the process. 

Why does my boiler’s pressure keep going up?

There are a few different reasons why your boiler’s pressure might be increasing. It’s possible that the valves connected to your filling tube aren’t fully closed, which can lead to an excess of water in your system. The filling tube may also be broken, allowing water in no matter what you do. There could also be problems with your pressure valve.


If you’ve tried adjusting the valves of your filling tube, and the pressure still seems to be going up, call us! We can help. 


Your boiler may be like Goldilocks, but there are good reasons for it. Getting the pressure just right can help you improve your boiler’s efficiency, all while reducing wear and tear. We service boilers in Winnipeg, so if you need the pressure of your boiler adjusted, or you need to troubleshoot pressure problems, give us a call.