Every now and then, when you look at your boiler, you may see that the pressure gauge is dropping, and you start to think, Why is my boiler losing pressure?’ This sort of situation is common, and it’s not always the end of the world. Knowing the reason and way to fix it can save you time, money, and headaches.
In this article, we will focus on the common reasons for a boiler losing pressure and how to identify it.
How Boiler Pressure Works
Before getting into the problem and how to fix it, it is important to understand boiler pressure and how it works.
Most newer models of boiler include either a combi boiler or a system boiler that has a sealed heating system. In simple terms, water is heated and then sent to the radiators to warm the house.
The pressure in the system is usually at 1 to 1.5 bar when the system is cold and should rise to a maximum of around 2 bar when the system is hot. It is important that the system is maintained at the correct pressure for the following reasons:
- Low pressure will mean that the boiler will not be able to operate effectively to heat your home.
- High pressure will lead to the damage of the heating system, which may result in leaks and pose possible safety concerns.
So, if your boiler is always losing pressure, that is a clear indication that something is wrong.
What are the Standard Boiler Pressure Ranges
| Condition | Pressure (bar) | Notes |
| Cold system (off) | 1 – 1.5 | Normal for most home boilers |
| System heating | 1.5 – 2 | Pressure rises as water heats |
| Maximum safe | 2.5 – 3 | Beyond this, safety valve opens |
What are the Most Common Causes of Boiler Pressure Loss
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System leaks
Boilers lose pressure for a number of reasons, and one of the most common is leaks. Even a tiny leak can result in a slow and steady loss of pressure over time.
Some indicators of a leak are:
- Water near the radiator
- Dripping from the pipes
- Water stains or patches on the walls or ceiling
For larger leaks, especially inside the boiler, you will need to call a qualified heating engineer.
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Broken Pressure Relief Valve (PRV)
The pressure relief valve is a safety device designed to drain water when the pressure in the boiler gets too high. If a PRV becomes stuck or otherwise damaged, it can also leak water, and that can cause pressure to drop.
Some signs that a PRV is failing are:
- Water under the boiler
- A whistle or hissing noise from the boiler
- The pressure is constantly dropping
Most of the time when a PRV is failing, you will need to have the valve professionally inspected and replaced.
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Pressure Drop after Bleeding Radiators
Bleeding radiators removes trapped air to improve heat efficiency. However, it also lets some water escape, which can result in a loss of boiler pressure.
After bleeding radiators, you will need to repressurise your system via the filling loop. Always check the pressure to make sure that it sits between the recommended levels.
When the system is cold, the recommended levels are usually between 1 and 1.5 bars.
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Defective Expansion Vessel
An expansion vessel is a small plastic tank that is located inside the boiler. It absorbs the changes in pressure that occur as the water gets heated. If the expansion vessel loses charge air or gets damaged, it might cause your boiler pressure to drop.
Some signs that your expansion vessel may be faulty are:
- Pressure drops when the heat is on, and pressure rises when the heat is off
- Gurgling or knocking in the pipes
An engineer will be able to check your expansion vessel and recharge or replace it if necessary.
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Air Locks or System Corrosion
If your heating system has air trapped in it, it may create air locks, which will make it difficult for water to flow and thus will lower the pressure. If there is corrosion in the pipes or in the radiator, it can also cause blockage that leads to pressure drops.
Some indicators of air locks or corrosion are:
- Radiators that heat unevenly
- Gurgling in pipes
- Pressure that drops after system refill
If the issue is corrosion, a professional may be required to do a power flush or chemical clean on the system.
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Filling Loop Not Closed Properly
When topping up your boiler using the filling loop, it is important to completely shut both valves. If one or both valves are left partially shut, water will still flow and cause the pressure to drop over time.
Always check the filling loop to see if it is actually leaking and ensure it is closed properly after topping up.
Easy Steps to Find Out How Much Pressure is in Your Boiler
Finding out how much pressure is in your boiler is really easy.
- Check your boiler’s pressure gauge.
- When the system is cold, the needle should show pressure between 1 and 1.5 bar.
- If the gauge shows pressure under 1 bar: the boiler is under-pressured.
- If the gauge shows pressure is above 2.5 bar: the boiler is over-pressured. This can also cause problems.
How to Re-Pressurize Your Boiler to Fix the low pressure problem
If you need to top up the pressure in your boiler, you may be able to do so yourself by using the filling loop.
- First, find the filling loop, which will typically be a silver, flexibly connecting hose under the boiler.
- Open the valves slowly to allow water to flow into the system.
- Keep an eye on the pressure gauge, and make sure the valve is closed once it reads about 1.5 bar.
- If the filling loop is detachable, remove it and secure it.
Note: Only top up the filling loop when your boiler is cold. If your boiler continues to lose pressure after you have done this, there is probably a leak or another underlying problem.
When to call a professional
Most pressure loss issues can be solved with simple solutions, but some need more skilled hands. Contact a certified heating engineer when:
- There are identifiable leaks in the system
- Constant pressure drops, even when it has been topped up
- unusual sounds like banging, gurgling, or whistling
- Water is being discharged by the pressure relief valve
Attempting to repair an expansion vessel or internal boiler leaks is risky and is best left to the professionals.
How To Prevent Your Boiler From Losing Pressure
Pressure loss in your boiler is more of a hassle to fix than it is to prevent. Here are some more preventative measures:
- Annual Servicing: Have your boiler serviced annually.
- Regular Pressure Checks: Make it a habit to check your boiler’s pressure.
- Radiator Bleeding: Recheck your pressure and repressurise afterwards.
- Spot Monitoring: Use your best detective skills to find leaves in odd places, moisture, and strange sounds
- PRV Checks: Make sure the valve is free of leaks and is not stuck.
Choose Air Hive UK to Fix low Boiler Pressure
If you live in the UK and have boiler pressure problems, get Air Hive’s boiler services to fix your boiler pressure problems so that your house is safe and warm. Services include:
- Boiler inspection and repair
- Diagnosing pressure loss
- Bleeding radiators and maintaining the system
- Replaces broken parts like PRVs or expansion vessels
Using Air Hive UK will help resolve your boiler pressure problems in the most professional manner
Conclusion
It’s common for a boiler to lose pressure. Most pressure drops happen due to leaks, radiator bleeding, malfunctioning PRVs, expansion vessel damage, air locks, or filling loops that may be open.
Leaks, damage, or other causes must be fixed by a professional to ensure that the boiler continues to operate safely and efficiently. Knowing the most common boiler pressure problems and how to fix them will relieve stress and save you money on repairs.
Frequently Asked Questions:
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What is the ideal pressure on a boiler?
Most boilers are designed to operate with pressure levels between 1 and 1.5 bar when cold and typically rise to 2 bar when hot.
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Can I top up my boiler myself?
Yes, with the filling loop you can raise the pressure if it is too low. However, make sure the system is cold, and be sure to close the valves completely afterward.
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Why does my boiler pressure drop after bleeding radiators?
When you release air, you also let water escape. After the bleeding process, you will have to repressurise the system to bring the pressure level back to normal.
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Is a leaking PRV dangerous?
Yes, a leaking pressure relief valve can definitely mean that there is a faulty component that will result in water damage.
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How often should my boiler be serviced?
You should definitely have your boiler serviced every year by a qualified engineer.
