The easiest way to safely reduce boiler pressure is to bleed your radiators. This releases any trapped air and water from the heating system and should bring your boiler pressure back to safe levels. However, don’t let the pressure drop too low, as this can cause the boiler to lock out. At the same time, you shouldn’t dismiss high boiler pressure either, as this can damage the system.
In this guide, we explain how to reduce boiler pressure, why it’s important and when you may need a Gas Safe registered emergency boiler engineer.
How to reduce the pressure on a boiler at a glance
- Bleed a radiator to release trapped air
- Use the drain cock if bleeding a radiator doesn’t work
- Your boiler should read 1-1.5 bar when cold.
- Pressure should not exceed 2 bar when the boiler is running
- Pressure that is too low results in boiler lockout
- Pressure that is too high may stress parts and lead to leaks
- Call an engineer if bleeding the radiator or the drain cock doesn’t reduce pressure
What pressure should my boiler be?
Your boiler should normally read between 1 and 1.5 bar when cold. When it’s running, it will increase but shouldn’t really go above 2 bar. Check the gauge on your boiler to see if the current reading is too high or too low.
- Boiler pressure too low: The boiler locks out
- Boiler pressure too high: The pressure relief valve (PRV) may activate, parts may be put under intense stress, and the boiler may leak.
How can I tell if the boiler pressure is too high?
If the pressure gauge reads above 2 bar when the boiler is running, or above 1.5 bar when cold, the pressure in your boiler is too high. Of course, you won’t be looking at your boiler all the time, but luckily some other signs can indicate the pressure needs addressing.
- Noise: If your boiler is whistling, banging or gurgling, there could be a pressure issue.
- Cold spots: When your radiators are hot in some places and cold in others.
- Leaking PRV: You may see dripping from the PRV or water being discharged outside.
- Fault codes: If you don’t notice the pressure gauge but see an over-pressure or lockout error, you’ll need to check the current pressure levels.
What is causing my boiler pressure to get too high?
Several reasons could be causing your boiler pressure to be too high. You could have an overfilled system, but you may also have a faulty pressure reducing valve or an open filling loop, among other things.
Some issues are a quick DIY fix; others may need an engineer. It’s important to remember that any work that requires the boiler casing to be opened can only be carried out by a Gas Safe engineer. Doing it yourself is dangerous, and even if you fixed the issue correctly, you’ll likely invalidate any warranty you hold.
Common causes of high boiler pressure include:
- Overfilled system
- Filling loop left open
- Failed pressure reducing valve
- Failed expansion vessel
Overfilled system
An overfilled system is one of the most common and easily fixed causes of high boiler pressure. It happens when too much water enters the system, usually after topping up via the filling loop. Bleeding a radiator or draining a small amount of water through the drain cock is normally enough to bring the system back to safe pressure levels, and the good news is, you can do this fix yourself.
Filling loop left open
If the filling loop is left open after topping up, water will continue to enter the system and gradually increase the pressure. Check that both loops are closed fully. You’ll find them connecting the mains water supply to the boiler. Closing them and then bleeding the radiators should set the pressure back to its regular levels.
Failed pressure reducing valve
The pressure reducing valve controls how much mains water is allowed into the sealed heating system. If it develops a fault and starts passing water, it keeps topping up the system on its own, gradually putting the pressure above safe levels. This is not a DIY repair and requires a professional Gas Safe engineer to diagnose the issue and replace the part.
Failed expansion vessel
The expansion vessel absorbs the extra volume as water heats and expands, keeping pressure stable. If it fails, it cannot accommodate that expansion, so pressure spikes when the boiler is running, and drops back when it cools. Because this is also behind the boiler casing, any re-pressurising or replacement can only be carried out by a Gas Safe registered engineer.
Both the failed pressure reducing valve and expansion vessel require professional intervention. If you are unsure, call our team; we can dispatch gas engineers in London 24/7.
Is high boiler pressure dangerous?
It can be. On many modern boilers, you’ll find a boiler pressure relief valve, typically set to open at around 3 bar. This part directs excess water outside and keeps pressure under control, meaning that high pressure may not become an issue. However, it can falter, resulting in constant high pressure. A constant boiler pressure in excess of 2.5 bar can lead to weakened joints, leaks and total boiler failure, which can be expensive to repair.
Does bleeding a radiator reduce boiler pressure?
Yes, bleeding a radiator does reduce boiler pressure, and it’s one of the most common ways to get your boiler pressure back to safe levels. This is a quick job that you can do yourself without professional help.
By bleeding a radiator, you release trapped air and water from the heating system and should return your boiler to the safe 1 to 1.5 bar.
What you’ll need
Before you bleed a radiator, you’ll need a few items to ensure you can get the job done. You should already have many at home, but if not, you can pick them up in DIY stores for relatively low cost.
- Radiator bleed key
- A small bucket to collect the discharged water
- Some old cloths or towels to help protect the floor and collect any drips
- Gloves; the discharged water can be dirty and very hot
- Boiler manual to check pressure levels, safe use and fault codes
How to bleed a radiator to reduce boiler pressure
Once you have the correct equipment, you can reduce the boiler pressure by bleeding a radiator in just a few simple steps:
- Turn off the central heating and allow it to cool
- Lay towels or cloths underneath the radiator. Place them at the end where the bleed valve is.
- Apply the radiator bleed key to the small bleed valve at the top of the radiator.
- Turn it slowly, anti-clockwise
- You’ll hear some hissing and see water trickle out
- If someone else is with you, have them check the boiler to see if the gauge has dropped to between 1 and 1.5 bar.
- If you don’t have someone with you, bleed the radiator a little, then tighten the valve and check the boiler. Repeat until the desired pressure is reached.
- Once the pressure is at the safe levels, tighten the bleed valve back up.
What if bleeding the radiator doesn’t reduce boiler pressure?
If bleeding the radiator doesn’t reduce boiler pressure, you can attempt to reduce it by draining water directly from the system using the drain cock.
This is a quick way to reduce pressure, but it must be done slowly so you don’t cause the pressure to drop low enough to make the boiler shut down.
The drain cock is normally located near a ground-floor radiator, or at the lowest point of the system. You’ll need a hosepipe that you can attach to the drain cock and lead to an outside drain. The water can flow quickly, so add clips to prevent the hose from slipping and leaking water all over the floor.
Open the valve slowly; you’ll likely need a spanner, and have someone check the pressure gauge. Once the boiler reaches the correct pressure levels, close the valve tightly. If you have nobody with you, release some water, tighten the valve, and check the boiler. Repeat until your boiler shows the correct pressure reading.
When do I need an engineer to reduce boiler pressure?
If you have tried to bleed the radiators or attempted to reduce boiler pressure via the drain cock and neither has worked, you might need professional help. Constant high boiler pressure could point to more serious issues, such as a faulty pressure reducing valve, filling loop or expansion vessel. These are not DIY jobs and can only be conducted by a qualified professional. As touched upon earlier, do not open the boiler casing to investigate this yourself. By law, only a Gas Safe registered engineer can work on gas appliances or internal boiler parts.
What should I do if the pressure keeps dropping after I’ve reduced it?
If you have reduced the pressure to safe levels, and your boiler pressure continues to fall, the problems may lie with your radiator, pipes, valves or the boiler itself. You can conduct one check yourself, but two common boiler faults will need professional intervention.
- Check every radiator, valve and pipe joint for signs of a leak: Laying kitchen roll under them will be a good way to detect any signs of moisture.
- The expansion vessel in your boiler manages the changes in pressure: It could become faulty and fail to absorb fluctuations in pressure.
- Boiler pressure relief valve leaking: If the PRV starts to leak constantly, it’s developed a fault, as explained in the earlier common causes section.
Booking professional help to reduce boiler pressure
If you have been unable to reduce boiler pressure by bleeding radiators or using the drain cock, professional help is advised. Contact London Property Services today, and we can send out a fully certified, experienced Gas Safe engineer to your home or commercial property. We are on call 24/7, covering all areas of the city. We provide 12-month workmanship guarantees and attend round the clock to ensure your boiler is operational and, most of all, safe. Contact us today to book your engineer.
