Boiler Pressure: Everything you Need to Know November 20, 2023

Boiler Pressure: Everything you Need to Know

Boiler Pressure: Everything you need to know. WarmZilla blogs.
How do Water Cylinders Work in Homes?
Boiler Pressure: Everything you Need to Know. WarmZilla Blogs.

There’s nothing more frustrating than living in a home where you’re experiencing boiler pressure issues. Boilers are complex, with loads of working parts, so diagnosing its problem may seem daunting. But, understanding the fundamental concepts of boiler pressure will help.

The most important idea to grasp is that water pressure and boiler pressure are different measurements.

Before going further, let’s define the most basic concepts.

What Is Boiler Pressure?

It can be easy to confuse the two, but your boiler pressure measures the pressure of the hot water inside its system, while water pressure measures the rate of water flow coming out of your home’s faucets (usually the cold water).

Boiler pressure is measured by a pressure gauge inside or underneath the unit. The pressure is measured in bars (100,000 kPa) and is a metric unit (it’s also used to measure tyre pressure).

Now that we understand the fundamental concept of boiler pressure, how does pressure affect your boiler?

How Does Pressure Affect My Boiler?

Heating systems need to be pressurised to move water effectively around the home. Boilers require stable pressure to correctly and evenly heat the water.

Your boiler’s function is to heat cold water fed from the council mains and pump it out, when required, to outlets around the home. Maintaining a constant pressure means your boiler’s system is not under unnecessary strain.

If not appropriately circulated, heated water will expand your system piping and contract if allowed to cool. This continuous expansion and contraction can cause issues in the future, such as leaks.

Now that we understand the importance of pressure for your boiler, what is your correct pressure?

What’s The Correct Pressure For My Boiler?

Generally, regular boilers in the UK have a pressure between one and two bars. However, checking your boiler’s manual and pressure gauge is essential.

Most manufacturers will have colour-coded their meters to indicate whether or not the system is over or under-pressured.

According to Worcester-Bosch’s website, the pressure should read between one and one-and-a-half bars when your heating system is cool. This will most likely be reconfirmed if the gauge’s needle is sitting in the green section.

If your boiler is below half a bar or over 2.75 bars, your gauge will inform you that it is over or under-pressured, with the needle in the red.

It’s also important to note that some boilers will have a digital gauge. In this case, it will most likely indicate a problem with pressure via an error message, code, or symbol.

If, for some reason, you’re not able to find a gauge or meter, you can diagnose the issue from these signs.

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Common Signs of High Boiler Pressure

When a boiler is experiencing high pressure, exercise caution even though all boilers have multiple built-in safety features. Signs your boiler has a high-pressure issue include:

  • A leaking pressure relief valve, which is designed to release excess pressure from the system
  • Unusual noises from the boiler (such as gurgling, banging, or whistling)
  • The radiator is cold at the bottom, caused by air pressure preventing proper circulation
  • Boiler lockout, which is a built-in safety feature in modern units
  • Water leaks from the system
  • Dripping or discharge from pressure relief pipe
  • Increase in energy bills
  • And difficulty in bleeding the radiators.

If you suspect your boiler has a high-pressure issue, you must address it quickly. You can bleed excess pressure, but if this doesn’t work, consult a qualified heating engineer or boiler technician.

Common Signs of Low Boiler Pressure

A boiler with a low-pressure issue is less severe than one on the other end of the scale. While not as serious, it can quickly affect your comfort levels. Signs your boiler has a low-pressure issue include:

  • No running hot water
  • Your radiators are not heating up
  • Strange noises from the boiler (such as gurgling or banging)
  • Visible leaks from the system
  • Boiler locks you out (which is a safety mechanism in modern boilers)
  • Difficulty in igniting the boiler
  • And needing to reset the boiler.

Now that you can identify what a boiler with low pressure looks like, you can comfortably determine the issue with your boiler without a gauge.

But, your boiler’s pressure will continuously fluctuate when it is being used. So, what causes boilers to have high or low pressure?

Causes Of High Boiler Pressure

Maintaining the recommended pressure for your boiler is crucial for efficiency, safety, and the unit's longevity. As mentioned above, you can read the gauge to determine if the boiler system is running at a high or low pressure.

Or if you can’t find the gauge, you can diagnose it. Many of the diagnosis signs will also be the cause of the problem. Here are the most common causes of high-pressure issues.

Closed Water Supply Valve

As mentioned, if the water supply valve is partially or entirely closed, it can lead to a pressure increase in the system.

Faulty Expansion Vessel

The expansion vessel is designed to absorb excess pressure in the system. If it fails, pressure can rise beyond the normal range.

Incorrect Boiler Settings

Incorrectly set pressure controls or other boiler settings can result in higher-than-normal pressure.

Issues with Pressure Reducing Valve

A faulty pressure-reducing valve may fail to regulate incoming water pressure, causing an increase in the boiler pressure.

Now that you know the common causes of high-pressure issues, let’s focus on the other side of the scale.

Common Causes of Low Pressure

Here are seven common causes of low boiler pressure.

Water Leaks

Leaks in the boiler system or its components, such as pipes, radiators, or the boiler itself, can lead to water loss and lower pressure.

Bleeding Radiators

Allowing air to enter the system can reduce the water volume and lower pressure. Bleeding radiators are a standard procedure to release trapped air.

Faulty Pressure Relief Valve

A malfunctioning pressure relief valve might cause water to escape, leading to a gradual decrease in pressure.

Closed Water Supply Valve

A closed or partially closed water supply valve can result in low pressure.

Faulty Expansion Vessel

If the expansion vessel fails to maintain proper pressure, it can lead to low pressure in the boiler.

Loss of Water Due to System Bleeding

When bleeding the system to remove air, it's essential to replenish the lost water to maintain adequate pressure.

Issues with Pressure Reducing Valve

A malfunctioning pressure-reducing valve might not allow sufficient water into the system, causing low pressure.

If you’re confident enough to attempt it and it is safe to do so, you can increase your boiler’s pressure. But how would you do this, and why?

How to Increase Boiler Pressure And Why You Would

Increasing your boiler’s pressure should be attempted cautiously. As we’ve discussed, your boiler is designed to operate within a specific range.

The main reasons for increasing your boiler’s pressure are either due to a loss in pressure or if you’re planning on bleeding your radiators.

We’d recommend increasing your boiler’s pressure in steps, which are as follows.

Check your boiler's current pressure

As mentioned, your unit will most likely have an easy-to-read gauge. When ready, switch off your boiler. Making any adjustments to your boiler while it is operating can be dangerous.

Find and fill your filling loop

This can be a flexible hose or lever-operated valve that connects the boiler to the water mains. Read how to open the filling loop in the boiler’s manual and follow their instructions.

While you’re topping up the boiler via the filling loop, make sure to monitor the pressure gauge. You will increase pressure as more water is added. And when you’re happy with the pressure reading, it is time to close the filling loop.

Restart your boiler

After restarting the boiler, check for any signs of leaks. Contact a qualified heating engineer or boiler technician if there are any issues or if you want to get it serviced.

If your boiler’s readings are too high, here’s how to decrease it and why you’d be keen to do so.

How To Decrease Boiler Pressure And Why

A significant motive to decrease your boiler’s pressure is that its readings exceed the recommended range. Another reason is if your radiators are not heating properly.

As with increasing your boiler’s pressure, decreasing it should be done in steps. Like before, check your boiler’s gauge and turn it off before adjusting.

Bleed the radiators

To reduce the pressure, you can bleed the radiators, releasing trapped air in the system. Bleeding a radiator will require a radiator key to open the valve. Turn the key and release the trapped air until water drips out.

Open the pressure release valve

Another option is opening the pressure relief valve. Usually, this valve will be marked on the boiler. If you are unsure, refer to the boiler’s manual. Most pressure relief valves will have a manual lever.

Open it carefully to release water and gently reduce the pressure. Monitor your gauge to ensure you don’t release too much pressure. Allow some time for the boiler’s system to equalise.

Check the new pressure & restart the boiler

Once you’re happy with the pressure level, check for leaks and restart the boiler.